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Human Digestive System – Organs, Structure, Functions

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Table of Contents

  • The human digestive system is a complex network of organs and processes that work together to break down food and extract nutrients for the body’s energy and nourishment. It consists of the gastrointestinal tract, which includes the mouth, esophagus, stomach, small intestine, large intestine, and rectum, as well as the accessory organs of digestion, such as the tongue, salivary glands, pancreas, liver, and gallbladder.
  • The digestive process can be divided into three main stages: the cephalic phase, the gastric phase, and the intestinal phase. The first stage, the cephalic phase, begins with the anticipation of food and the sensory stimulation of seeing and smelling it. This triggers the release of digestive enzymes from gastric glands and prepares the body for digestion. The mechanical breakdown of food begins in the mouth through chewing, while the chemical breakdown starts with the secretion of saliva, which contains enzymes like amylase and lingual lipase from the salivary glands. The food is then formed into a bolus and swallowed, passing through the esophagus and entering the stomach.
  • In the second stage, the gastric phase, the food is further broken down in the stomach. It mixes with gastric acid, which helps to kill bacteria and break down proteins. The stomach’s muscular contractions, along with the mixing action, further break down the food into a semi-liquid mass called chyme. Eventually, the chyme is released from the stomach and enters the duodenum, the first part of the small intestine.
  • The third stage, the intestinal phase, takes place primarily in the small intestine. The partially digested food mixes with enzymes produced by the pancreas, such as amylase, lipase, and proteases, which further break down carbohydrates, fats, and proteins, respectively. Bile, produced by the liver and stored in the gallbladder, is released into the small intestine to aid in the digestion and absorption of fats. The small intestine also plays a vital role in the absorption of nutrients, as its walls are lined with tiny finger-like projections called villi, which increase the surface area for nutrient absorption into the bloodstream.
  • Throughout the digestive process, various movements aid in the breakdown and movement of food. Chewing, carried out by the muscles of mastication, the tongue, and the teeth, mechanically breaks down food into smaller pieces, while peristalsis, the rhythmic contraction of muscles, helps propel food through the esophagus, stomach, and rest of the gastrointestinal tract. Segmentation, another type of movement, occurs primarily in the small intestine and helps mix the food with digestive enzymes and facilitate nutrient absorption.
  • Most of the digestion and nutrient absorption occur in the small intestine. Water and some minerals, along with any remaining undigested material, then pass into the large intestine or colon. In the colon, water and electrolytes are reabsorbed, resulting in the formation of feces. The waste products of digestion are stored in the rectum until they are eliminated from the body through the anus in a process called defecation.
  • The human digestive system is a remarkable and intricate system that allows our bodies to obtain the necessary nutrients for growth, energy, and overall health. Its coordinated actions and the involvement of various organs and processes ensure the efficient breakdown, absorption, and elimination of food, ultimately supporting our well-being.

How Does the Digestive System Work?

  • The digestive system is a complex network of organs and tissues that work together to break down food and absorb nutrients for the body’s growth, energy, and maintenance. It consists of the gastrointestinal (GI) tract and various glands, including the tongue, salivary glands, liver, gall bladder, and pancreas.
  • The GI tract, a long tube of varying diameter, begins at the mouth and ends in the anus. The process of digestion can be divided into three stages based on the position of food within the digestive tract: the oral phase, the gastric phase, and the intestinal phase.
  • The oral phase, also known as the mouth phase, is where digestion begins. It starts with the process of chewing and mixing food with saliva. The tongue helps move the food around the mouth, while the salivary glands secrete saliva that contains enzymes, such as amylase, which begins the breakdown of carbohydrates. The food then forms a small, soft mass called a bolus, which is ready to be swallowed.
  • Swallowing triggers the next phase of digestion, the gastric phase. The food bolus enters the stomach, where it is further broken down by stomach acid and digestive enzymes. The stomach muscles churn and mix the food, creating a semi-liquid mixture called chyme. The gastric phase primarily focuses on the breakdown of proteins with the help of the enzyme pepsin.
  • From the stomach, the partially digested chyme enters the small intestine, marking the beginning of the intestinal phase. The small intestine is where most of the digestion and absorption of nutrients occur. The chyme mixes with digestive enzymes from the pancreas and bile from the liver and gall bladder. The pancreas releases enzymes that break down proteins, fats, and carbohydrates, while bile emulsifies fats for better digestion. The small intestine has specialized structures called villi and microvilli, which increase the surface area for nutrient absorption. Nutrients, such as amino acids, glucose, vitamins, and minerals, are absorbed into the bloodstream and transported to various parts of the body for use.
  • After the small intestine, any remaining waste material moves into the large intestine, or colon. The colon absorbs water and electrolytes from the waste, forming solid stool. The waste is then eliminated from the body through the rectum and anus in the process of defecation.
  • It is worth noting that the digestive system begins to develop early in the human body, starting around the third week after fertilization. The primitive gut forms through invaginations of embryonic cells, extending from the buccopharyngeal membrane to the cloacal membrane. The mouth forms when the buccopharyngeal membrane breaks down, allowing the digestive tract to open to amniotic fluid. Throughout fetal development, the amniotic fluid is actively swallowed, preparing the digestive system for its crucial role after birth.
  • In summary, the digestive system is a remarkable mechanism that enables the body to break down food, extract nutrients, and eliminate waste. It involves the coordinated effort of various organs, glands, and enzymes throughout the GI tract. Understanding how the digestive system works can help us make informed choices about our diet, maintain our health, and support overall well-being.
Human Digestive System - Organs, Structure, Functions
Human Digestive System

Main Parts/Organs of Human Digestive System

There are present two main parts of human digestive system. The parts are: 1. Alimentary Canal 2. Digestive Glands.

digestive system
digestive system

Part 1. Alimentary Canal

The alimentary canal, also known as the digestive tract or gastrointestinal (GI) tract, is a long muscular tube that forms the core of the digestive system. It extends from the mouth to the anus and plays a vital role in the process of digestion, absorption, and elimination of food.

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The alimentary canal consists of several distinct regions, each with its own specialized functions. Let’s explore these regions in order:

  1. Mouth: The process of digestion begins in the mouth. Teeth break down food into smaller pieces through chewing, while the tongue helps mix the food with saliva. Salivary glands release saliva, which contains enzymes that initiate the digestion of carbohydrates.
  2. Pharynx and Esophagus: Once food is sufficiently chewed and moistened, it is formed into a bolus and pushed into the pharynx by the tongue. From there, it enters the esophagus, a muscular tube that connects the throat to the stomach. Peristaltic contractions of the esophageal muscles propel the bolus downward toward the stomach.
  3. Stomach: Upon reaching the stomach, the bolus is further broken down and mixed with gastric juices. The stomach’s muscular walls contract and churn the food, transforming it into a semi-liquid substance called chyme. Gastric juices, which include hydrochloric acid and the enzyme pepsin, aid in the digestion of proteins.
  4. Small Intestine: The small intestine is the longest section of the alimentary canal and is where the majority of digestion and absorption occur. It has three regions: the duodenum, jejunum, and ileum. In the duodenum, digestive enzymes from the pancreas and bile from the liver and gall bladder are released to break down proteins, fats, and carbohydrates. The small intestine also has numerous finger-like projections called villi, which increase the surface area available for nutrient absorption into the bloodstream.
  5. Large Intestine: After passing through the small intestine, the remaining undigested food and waste material enter the large intestine, or colon. The colon absorbs water and electrolytes from the waste, consolidating it into feces. The feces are stored in the rectum until elimination occurs through the anus during the process of defecation.

Throughout the journey of the alimentary canal, the muscles in the walls of the digestive tract undergo rhythmic contractions called peristalsis. These contractions help propel food along the entire length of the GI tract, allowing for efficient digestion and absorption.

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The alimentary canal is a crucial component of the digestive system, as it facilitates the breakdown of food into its nutrient components and enables the body to absorb these nutrients for energy, growth, and maintenance. Understanding how the alimentary canal works can provide valuable insights into maintaining a healthy digestive system and making informed dietary choices.

I. Mouth

  • The mouth, also known as the oral cavity, is an essential organ in the human body that serves multiple functions. Located at the upper end of the digestive system, it is the primary entry point for food and plays a crucial role in communication and expressing emotions. From a biological standpoint, the mouth is a complex structure comprising various tissues, muscles, and organs that work together seamlessly to carry out its diverse functions.
  • One of the primary functions of the mouth is the ingestion and initial processing of food. The mouth is equipped with specialized structures that aid in the mastication, or chewing, of food. The teeth, with their different shapes and sizes, are responsible for breaking down food into smaller, more manageable pieces. The tongue assists in the process of chewing by manipulating food within the mouth and pushing it towards the teeth. Additionally, the tongue helps in forming a bolus, a compact mass of food, which can be easily swallowed and transported down the esophagus to the stomach.
  • In addition to its role in digestion, the mouth plays a significant role in communication. It is an essential component in the production of speech sounds. The tongue, lips, teeth, and other structures within the oral cavity work together to create a wide range of sounds necessary for human language. Articulation, pronunciation, and modulation of voice are all facilitated by the intricate movements and coordination of these oral structures. Moreover, the mouth is instrumental in nonverbal communication as well, as facial expressions and gestures can convey a wealth of emotions and intentions.
  • The mouth is also involved in the sense of taste, which greatly contributes to the overall enjoyment of food. Taste buds, located on the tongue and other parts of the oral cavity, allow us to discern between different flavors such as sweet, sour, salty, bitter, and umami. This sensory information is transmitted to the brain, which helps us appreciate the intricate nuances of taste and contributes to our overall eating experience.
  • Furthermore, the mouth is a crucial site for maintaining oral health. Regular oral hygiene practices such as brushing, flossing, and rinsing help to prevent the buildup of bacteria, plaque, and food debris that can lead to various dental issues. The saliva produced by the salivary glands within the mouth also plays a significant role in maintaining oral health. It helps in lubrication, digestion, and the prevention of tooth decay by neutralizing acids and providing minerals that strengthen tooth enamel.
  • In summary, the mouth is a remarkable organ with a multitude of functions. It serves as the gateway to our digestive system, aids in communication and expression, enables the perception of taste, and plays a vital role in oral health. Its intricate design and coordination of various structures allow us to perform essential activities for survival and enjoy the pleasures of eating and speaking. Understanding the significance of the mouth emphasizes the importance of taking care of this remarkable organ to ensure overall well-being.
 oral digestive system
oral digestive system

Human mouth consists of two parts.

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(a) Vestibule

  • The vestibule is a narrow, slit-like space that can be found within the oral cavity. It is bordered externally by the lips and cheeks, while internally, it is defined by the gums and teeth. This area plays a significant role in supporting the functions of the mouth and contributes to overall oral health.
  • Located between the lips or cheeks and the teeth and gums, the vestibule serves as a transition zone or passageway. It acts as a barrier, separating the inner oral cavity from the outer environment, helping to protect the delicate structures within the mouth. The lips and cheeks provide a protective covering to the vestibule, safeguarding the teeth, gums, and other oral tissues from potential injuries and external elements.
  • The vestibule also plays a role in the proper functioning of the oral cavity. During the process of chewing and swallowing, the cheeks and lips help to keep the food within the oral cavity, preventing it from falling out or escaping prematurely. This allows for efficient mastication and the breakdown of food particles, promoting digestion.
  • Maintaining good oral hygiene is crucial in the vestibule area. Regular brushing and flossing help to remove food debris and plaque that may accumulate within this space. It is important to pay attention to the inner surfaces of the cheeks and lips when cleaning, as these areas contribute to the overall cleanliness of the oral cavity.
  • Additionally, the vestibule is closely associated with the gums and teeth. The gums, or gingiva, form a protective seal around the necks of the teeth, helping to anchor them in place and provide stability. The vestibular extension of the gums creates a tight seal around the teeth, preventing food particles and bacteria from entering the spaces between the teeth and gums. Proper oral hygiene, including regular brushing and flossing, is essential in maintaining the health of the gums and preventing gum disease.
  • Overall, the vestibule plays a vital role in supporting the functions and health of the oral cavity. Its location between the lips or cheeks and the gums and teeth helps to protect the delicate structures within the mouth and maintain a proper oral environment. By understanding the significance of the vestibule, we can appreciate its role in oral health and hygiene, ensuring the well-being of our entire oral cavity.

(b) Oral Cavity (Buccal Cavity)

The oral cavity, also known as the buccal cavity, is the inner portion of the mouth that encompasses various important structures. Let’s explore some of its key components:

1. Palate:

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The roof of the oral cavity is called the palate. It can be divided into two distinct regions. The anterior part is known as the hard palate, which is made up of bone and covered by a specialized mucous membrane. It features transverse ridges called rugae, which aid in food manipulation during chewing. The posterior part is known as the soft palate, which is composed of soft tissue. It lacks rugae and has a smooth surface. The free posterior end of the soft palate forms a small hanging flap called the uvula.

2. Tongue:

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The tongue is a muscular organ located in the floor of the oral cavity. It is attached to the mouth floor by a fold of tissue called the lingual frenulum. The upper surface of the tongue can be divided into two parts by an inverted V-shaped furrow known as the sulcus terminalis. The front part of the tongue, called the anterior oral part, is involved in taste perception, manipulation of food, and speech articulation. The posterior part, known as the posterior pharyngeal part, extends towards the throat and assists in swallowing. At the apex of the sulcus terminalis, there is a small depression called the foramen caecum.

Tongue
Tongue

Papillae

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Papillae are small projections found on the upper surface of the tongue that contribute to the sensory perception of taste and tactile sensations. Let’s delve into the different types of papillae and their characteristics:

  1. Vallate Papillae or Circumvallate Papillae: Vallate papillae are the largest among the four types and are typically found at the back of the tongue. Usually numbering between 8 to 12, they are circular in shape and have a trench-like groove surrounding them. Each vallate papilla can contain up to 100 taste buds. These papillae play a significant role in taste perception, particularly for bitter flavors.
  2. Filiform Papillae: Filiform papillae are the most abundant and smallest among the papillae. They cover most of the upper surface of the tongue, especially towards the center. Unlike other papillae, filiform papillae do not contain taste buds. Instead, they have tactile (touch) receptors, which help in detecting textures and manipulating food during chewing.
  3. Fungiform Papillae: Fungiform papillae are rounded and slightly larger than filiform papillae, but smaller than vallate papillae. They are scattered across the upper surface of the tongue, with a higher concentration near the tip. Each fungiform papilla typically houses around five taste buds. These papillae contribute to the perception of various taste sensations, including sweet, sour, salty, and bitter.
  4. Foliate Papillae: Foliate papillae are not well-developed in the human tongue compared to other mammals. They are leaf-like structures situated at the sides of the base of the tongue. Each border of the foliate papillae has four or five vertical folds. However, in humans, the taste buds in these papillae tend to degenerate during early childhood, reducing their significance in taste perception.
Cross section of circumvallate papilla showing arrangement of nerves and taste buds
Cross section of circumvallate papilla showing arrangement of nerves and taste buds

The arrangement and distribution of these papillae on the tongue contribute to our sense of taste. Taste buds, located within these papillae, contain specialized cells that enable the detection and interpretation of different flavors. The human tongue is generally divided into taste areas associated with sweet, salty, sour, and bitter tastes. It’s important to note that the areas of sweet and salt tastes can overlap, meaning that some taste buds may respond to both sweet and salty stimuli.

In conclusion, papillae on the tongue play a crucial role in taste perception and tactile sensations. Vallate papillae, filiform papillae, fungiform papillae, and foliate papillae each have their distinct characteristics and contribute to our ability to discern flavors. Understanding the different types of papillae helps us appreciate the intricacies of our taste perception and how it enhances our overall sensory experience of food.

Functions of the Tongue

The tongue serves several important functions in the human body. Let’s explore these functions in more detail:

  1. Chewing: The tongue plays a crucial role in the initial process of digestion by aiding in the chewing of food. It helps move food around the mouth, positioning it between the teeth for effective mastication. The tongue helps break down food into smaller, more manageable pieces, making it easier to swallow and further digest.
  2. Swallowing: Once food is adequately chewed, the tongue assists in the process of swallowing. It propels the chewed food towards the back of the throat and into the esophagus, allowing it to enter the digestive system. The coordinated movements of the tongue help push the food bolus along the correct path during the swallowing process.
  3. Cleaning the Teeth: The tongue acts as a natural “brush” for the teeth. As it moves around the mouth during chewing and speaking, it helps remove food particles and debris from the teeth and gums. This action contributes to maintaining oral hygiene and prevents the buildup of plaque and bacteria, reducing the risk of dental issues such as tooth decay and gum disease.
  4. Speech: The tongue is a vital organ for speech production. It works in conjunction with other structures in the mouth, such as the lips, teeth, and palate, to articulate sounds and form words. The tongue’s ability to move rapidly and precisely allows us to produce a wide range of speech sounds, enabling effective communication and expression of thoughts and ideas.
  5. Taste Sensation: The tongue is an essential organ of taste. It contains taste buds, which are specialized sensory receptors that detect different flavors. The taste buds perceive taste sensations such as sweet, salty, sour, bitter, and umami (savory). These taste sensations provide us with information about the quality and nature of the food we consume, enhancing our overall gustatory experience.

Overall, the functions of the tongue are diverse and crucial for various aspects of digestion, oral health, communication, and sensory perception. Its role in chewing, swallowing, cleaning the teeth, speech production, and taste sensation highlights its significance in our daily lives. Understanding and appreciating the multifaceted functions of the tongue underscore its importance in maintaining overall well-being.

These structures within the oral cavity are essential for various functions:

  • Chewing and Manipulation of Food: The hard palate, with its transverse ridges, aids in breaking down food into smaller pieces during the chewing process. The tongue helps in manipulating the food bolus, pushing it towards the teeth and mixing it with saliva to facilitate swallowing.
  • Taste Perception: The tongue contains numerous taste buds that allow us to perceive different flavors. These taste buds are responsible for detecting sweet, sour, salty, bitter, and umami tastes. The information gathered by taste buds is relayed to the brain, contributing to our overall sensory experience of food.
  • Speech Articulation: The tongue, along with other structures within the oral cavity, plays a crucial role in speech production. It assists in forming various sounds and syllables by contacting different areas of the mouth, teeth, and palate. Movements and positions of the tongue help produce distinct sounds required for language and communication.
  • Swallowing: The tongue is involved in the process of swallowing. It pushes the chewed food towards the back of the throat, triggering a series of muscular contractions that propel the food into the esophagus and down into the digestive system.

Understanding the components of the oral cavity, such as the palate and tongue, provides insights into their functions and how they contribute to essential processes like chewing, taste perception, speech, and swallowing. These structures work together harmoniously to support our overall oral health and enable crucial daily activities.

Teeth

Teeth are vital structures in the oral cavity that serve multiple functions, including chewing, biting, and grinding food. Let’s explore some key aspects of teeth:

Muscles of larynx. Posterior view.
Muscles of larynx. Posterior view.
  1. Characteristics: Humans have diphyodont dentition, meaning they develop two sets of teeth during their lifetime. The first set is the milk or deciduous teeth, which are eventually replaced by the permanent teeth. Teeth are thecodont, meaning they are embedded in sockets within the jaw bones. Humans also have heterodont dentition, with four types of teeth:
  • Incisors: Located at the front of the mouth, incisors are specialized for cutting food.
  • Canines: Positioned next to the incisors, canines are also involved in cutting and tearing food.
  • Premolars and molars: These teeth, often referred to as cheek teeth, are broad and sturdy, designed for crushing and grinding food. The third molars, commonly known as wisdom teeth, are vestigial in humans and may not fully erupt or be present at all.
  1. Number: The milk teeth, also called deciduous or temporary teeth, consist of 20 teeth, with 10 in the upper jaw and 10 in the lower jaw. Milk teeth begin to erupt around 6 months of age and are usually all present by the end of 24 months. The permanent teeth start to replace the milk teeth around the age of 6. There are 32 permanent teeth in total, typically fully developed between the ages of 18 and 25.
  2. Dental Formulae: The dental formula gives a representation of the number and types of teeth in one-half of the mouth. For milk teeth, the dental formula is 212/212 x 2, indicating 8 incisors, 4 canines, and 8 molars (no premolars). As the milk teeth are shed, they are replaced by permanent teeth. The dental formula for permanent teeth is 2123/2123 x 2, representing 8 incisors, 4 canines, 8 premolars, and 12 molars.
  3. Structure: A typical tooth consists of three main regions:
  • Crown: The crown is the part of the tooth that projects above the gums and is visible in the mouth.
  • Neck: The neck of the tooth is the area that is surrounded by the gum tissue.
  • Root: The root is the part of the tooth that is embedded in the bone of the jaw.

Teeth can have different numbers of roots depending on their type. Incisors and canines usually have one root, while upper first premolars have two roots. Upper second premolars and lower premolars typically have one root. Upper molars have three roots, while lower molars have two roots.

The structure of a tooth includes several components:

  • Enamel: Enamel is the outermost layer of the tooth crown and is the hardest substance in the human body. It protects the underlying dentin.
  • Dentin: Dentin forms the bulk of the tooth structure and is located beneath the enamel. It consists of tiny canaliculi that radiate from the pulp cavity towards the enamel.
  • Cement: Cement covers the root of the tooth and helps anchor it within the socket in the jaw bone.
  • Periodontal Ligament: The periodontal ligament is composed of collagen fibers and surrounds the cement. It provides support and fixation of the tooth within its socket.
  • Pulp Cavity: The pulp cavity is located within the dentin and contains connective tissue, blood vessels, and nerves. The pulp is vital for the tooth’s vitality and sensation. Root canals, narrow extensions of the pulp cavity, run through the roots of the tooth.

Furthermore, teeth are formed by specialized cells. Odonoblasts are responsible for dentin formation, while ameloblasts contribute to enamel formation.

In summary, teeth are remarkable structures with diverse functions and characteristics. They aid in the mastication of food, contribute to speech, and provide support to the facial structure. Understanding the structure and composition of teeth helps us appreciate their importance in our overall well-being and oral health.

II. Pharynx (Throat)

The pharynx, also known as the throat, is a muscular tube that connects the nasal and oral cavities to the esophagus and larynx. It is divided into three parts for descriptive purposes:

  1. Nasopharynx: The nasopharynx is located behind the nasal cavities and extends from the base of the skull to the soft palate. It serves as a passageway for air and is connected to the middle ear through the Eustachian tube. The nasopharynx plays a crucial role in equalizing pressure between the middle ear and the atmosphere.
  2. Oropharynx: The oropharynx is situated behind the oral cavity, extending from the soft palate to the hyoid bone. It serves as a common passage for both food and air. The oropharynx is involved in the initial stages of digestion and respiration, as it allows the passage of food and directs it towards the esophagus while also enabling the passage of air into the larynx.
  3. Laryngopharynx: The laryngopharynx is the most inferior portion of the pharynx, located behind the larynx. It extends from the hyoid bone to the esophagus. The laryngopharynx acts as a pathway for food to enter the esophagus during swallowing and for air to pass through the larynx during respiration.

Functionally, the pharynx serves as a common pathway for food and air. When we swallow, the muscles of the pharynx contract and propel the food from the oral cavity to the esophagus, ensuring that it enters the digestive system. Additionally, the pharynx plays a vital role in vocalization and speech production by acting as a resonating chamber for sound.

The pharynx contains a ring of lymphatic tissues known as Waldeyer’s ring or the Waldeyer’s lymphatic ring. This ring includes several lymphoid structures, including:

  • Pharyngeal Tonsil (Adenoids): Attached to the posterior wall of the nasopharynx, the pharyngeal tonsil can become enlarged in children, causing obstruction and difficulty in breathing.
  • Tubal Tonsils: Situated around the opening of the Eustachian tube, these tonsils help protect the middle ear.
  • Palatine Tonsils: Located on both sides of the back of the oral cavity, the palatine tonsils can become infected and inflamed (tonsillitis), causing a sore throat. In some cases, enlarged palatine tonsils may require surgical removal (tonsillectomy).
  • Lingual Tonsil: Found at the base of the tongue in the pharyngeal part, the lingual tonsil contributes to the immune response.

These lymphoid tissues within Waldeyer’s ring play a significant role in the production of immunoglobulin A (IgA), an antibody that helps defend against infections. They form an important part of our immune system, protecting us from pathogens that enter through the respiratory and digestive tracts.

In summary, the pharynx serves as a vital passage for both food and air, allowing us to swallow, breathe, and produce speech. It also houses important lymphoid tissues that contribute to our immune response, safeguarding our respiratory and digestive systems.

III. Oesophagus

The esophagus, also known as the food pipe or gullet, is a muscular tube that plays a crucial role in the digestive system. Here are some key points about the esophagus:

  1. Anatomy: The human esophagus is approximately 25 centimeters long. It is situated behind the trachea (windpipe) and the heart. It consists of three parts: the cervical part, located in the neck; the thoracic part, located in the thorax (chest); and the abdominal part, located in the abdomen. The esophagus passes through an opening in the diaphragm called the esophageal hiatus and connects to the stomach.
  2. Structure: The walls of the esophagus are made up of layers of smooth muscle, which help propel food downward through rhythmic contractions known as peristalsis. The inner lining of the esophagus is composed of mucous membrane that secretes mucus to aid in the smooth passage of food.
  3. Function: The primary function of the esophagus is to transport food from the pharynx (throat) to the stomach. When we swallow, food is propelled through the esophagus by peristaltic waves. The upper esophageal sphincter, a muscular ring at the junction of the esophagus and pharynx, relaxes to allow food to enter the esophagus. The lower esophageal sphincter, located at the junction of the esophagus and stomach, opens to allow food to enter the stomach and then closes to prevent the backflow of stomach acid into the esophagus.
  4. Protection: The esophagus has mechanisms in place to protect its delicate lining from damage. The lining is coated with a layer of mucus that acts as a barrier between the esophageal tissue and the acidic contents of the stomach. Additionally, the lower esophageal sphincter helps prevent acid reflux, which is when stomach acid flows back into the esophagus, causing discomfort and potentially leading to conditions such as gastroesophageal reflux disease (GERD).
  5. Disorders: The esophagus can be affected by various disorders. Gastroesophageal reflux disease (GERD) occurs when the lower esophageal sphincter is weakened or malfunctioning, leading to frequent acid reflux and heartburn. Other conditions that can affect the esophagus include esophagitis (inflammation of the esophageal lining), esophageal strictures (narrowing of the esophagus), and esophageal cancer.

In summary, the esophagus serves as a conduit for transporting food from the pharynx to the stomach. It relies on peristaltic contractions to propel food downward, and its structure and protective mechanisms help prevent damage from stomach acid. Understanding the function and anatomy of the esophagus is essential for maintaining a healthy digestive system.

IV. Stomach (= Gaster)

The stomach, also known as the gaster, is a vital organ in the digestive system. Here are some key points about the stomach:

  1. Anatomy: The stomach is the widest organ of the alimentary canal and has a distinctive J-shaped structure. The lesser curvature of the stomach is located on its posterior surface, while the greater curvature is on the anterior surface. Attached to the greater curvature is a fold of peritoneum known as the greater omentum, which stores fat. The stomach can be divided into four parts: the cardiac part, fundus, body, and pyloric part.
  2. Cardiac Part: The cardiac part, also known as the cardia, is located near the heart. It is the region where the esophagus connects to the stomach. The gastroesophageal sphincter, also called the cardiac sphincter, is found at this junction. Although it is not a true valve, it functions as a sphincter, regulating the passage of food from the esophagus into the stomach.
  3. Fundus: The fundus is the uppermost part of the stomach and is often filled with air or gas. It can expand to accommodate food and fluids.
  4. Body: The body of the stomach is the main central region where food is stored and undergoes digestion. It contains specialized cells and glands that produce gastric juice, which aids in the breakdown of food.
  5. Pyloric Part: The pyloric part of the stomach is located at the posterior end. It consists of two parts: the pyloric antrum and the pyloric canal. The pyloric canal connects to the duodenum, the first part of the small intestine. The pyloric sphincter, a muscular ring, controls the passage of partially digested food from the stomach into the duodenum.
  6. Functions: The stomach serves several important functions in the digestive process. It acts as a storage reservoir for food, allowing for controlled release into the small intestine. The stomach contracts and churns the food, mixing it with gastric juices to initiate the digestion of proteins and fats. The gastric juice contains enzymes like pepsinogen, which is converted into pepsin for protein digestion, and gastric lipase and gastric amylase for partial digestion of fats and carbohydrates, respectively. The stomach also produces a glycoprotein called Castle’s intrinsic factor, necessary for the absorption of vitamin B12 in the intestines. Additionally, the stomach secretes the hormone gastrin, which regulates the production of gastric juice.
  7. Absorption: While the stomach is primarily responsible for digestion, it also has limited absorption capabilities. Small amounts of alcohol, aspirin, lipid-soluble drugs, moderate amounts of sugar, and water can be absorbed through the stomach wall.
Areas of the stomach
Areas of the stomach

V. Small Intestine

The small intestine is a vital component of the digestive system, responsible for completing digestion and absorbing nutrients into the bloodstream. Here are some key points about the small intestine:

Layers of the small intestine
Layers of the small intestine
  1. Anatomy: The small intestine derives its name from its relatively small diameter. It is the longest part of the alimentary canal, measuring approximately 6.25 meters in length. It consists of three parts: the duodenum, jejunum, and ileum.
  2. Duodenum: The duodenum, named because it is about the length of 12 fingers, is the shortest and widest part of the small intestine. It has a somewhat C-shaped structure and spans approximately 25 cm. The hepatopancreatic ampulla, also known as the ampulla of Vater, opens into the duodenum. This ampulla receives both the bile duct from the liver and the main pancreatic duct from the pancreas. The duodenum is where iron is primarily absorbed.
  3. Jejunum: The jejunum is the middle part of the small intestine and has a diameter of about 4 cm. Its wall is thick and highly vascular, giving it a redder appearance. It is approximately 2.5 meters long and plays a significant role in nutrient absorption.
  4. Ileum: The ileum is the longest part of the small intestine, measuring around 3.5 meters in length. It has a diameter of 3.5 cm and its wall is thinner compared to the jejunum. Both the jejunum and ileum are characterized by extensive coiling. They are suspended within the abdominal cavity by a fold of peritoneum known as the mesentery. Along the entire length of the small intestine, small nodules of lymphatic tissue, called Peyer’s patches or lymph nodules, can be found. These patches are particularly concentrated in the ileum and play a role in immune function.
  5. Villi and Circular Folds: The small intestine features finger-like projections of the mucosa called villi. These villi greatly increase the surface area for nutrient absorption. Each villus is covered with epithelial cells and contains a lymph capillary called a lacteal, as well as blood capillaries. The small intestine also has circular folds of the mucous membrane known as plicae circulares or “valves of Kerkring.” These folds are more prominent in the jejunum and further enhance the absorptive surface area.
  6. Functions: The small intestine carries out several essential functions in the digestive process. It completes the digestion of proteins, carbohydrates, fats, and nucleic acids through the action of digestive enzymes. It is the primary site for nutrient absorption, where digested nutrients are absorbed into the bloodstream and lymphatic system. The small intestine also secretes various hormones, including cholecystokinin, secretin, enterogastrone, duocrinin, enterocrinin, and villikinin. These hormones regulate digestive processes and aid in the coordination of digestion and absorption.
Illustration of small intestine
Illustration of small intestine
Liver and gall bladder
Liver and gall bladder

VI. Large Intestine

The large intestine, as its name suggests, has a larger diameter compared to the small intestine. It plays a crucial role in the final stages of digestion and the elimination of waste. Here are the key features and functions of the large intestine:

  1. Anatomy: The large intestine is approximately 1.5 meters long and can be divided into three main parts: the caecum, colon, and rectum.
    • Caecum and Vermiform Appendix: The caecum is a pouch-like structure that is around 6 centimeters long. Attached to the caecum is the vermiform appendix, a small, slightly coiled blind tube that measures about 8 centimeters long. The appendix contains prominent lymphoid tissue, and although its exact function is not fully understood, it is considered a vestigial organ. Inflammation of the appendix is known as appendicitis.
    • Colon: The caecum connects to the colon, which can be further divided into four regions: the ascending colon, transverse colon, descending colon, and sigmoid colon. The ascending colon is the shortest segment of the colon. The colon is characterized by three longitudinal bands called taeniae coli and small pouches known as haustra. The haustra and taeniae coli give the colon its distinctive appearance.
    • Rectum: The sigmoid colon leads into the rectum, which comprises the last 20 centimeters of the digestive tract. The rectum terminates in the 2-centimeter long anal canal, and its opening is called the anus. The anus consists of an internal anal sphincter, composed of smooth muscle fibers, and an external anal sphincter, composed of voluntary (striped) muscle fibers. Swollen veins in the anal canal or anus can result in a condition called hemorrhoids or piles.
  2. Functions: The large intestine performs several important functions in the digestive process.
    • Water Absorption: The primary function of the large intestine is to absorb water from the indigestible food residues that pass through it. This absorption helps in the formation of solid feces.
    • Waste Elimination: The large intestine eliminates solid waste from the body in the form of feces. It consolidates and stores fecal matter until it is ready to be expelled during bowel movements.
    • Bacterial Activity: Beneficial bacteria residing in the large intestine play a role in the synthesis of certain vitamins, including vitamin K and portions of the vitamin B complex. These vitamins are produced by the bacterial fermentation of undigested food materials.
    • Formation of Feces: As water is absorbed, the large intestine concentrates and compacts the remaining waste, forming feces. Feces primarily consist of undigested food residues, bacteria, and water.

The large intestine is a vital component of the digestive system, responsible for reabsorbing water and electrolytes, forming and storing feces, and facilitating the elimination of waste from the body. Its interaction with beneficial bacteria also contributes to the production of certain vitamins. Understanding the structure and functions of the large intestine is crucial for maintaining digestive health and overall well-being.

Lower GI tract - 3) Small intestine; 5) Cecum; 6) Large intestine
Lower GI tract – 3) Small intestine; 5) Cecum; 6) Large intestine

Histology of Human Gut (Alimentary Canal)

The histology of the human gut, or alimentary canal, involves the study of its four basic layers that make up its wall. Each layer serves specific functions in the process of digestion and absorption. Here are the key features of each layer:

  1. Visceral Peritoneum (Serosa): The outermost layer of the gut wall is known as the visceral peritoneum or serosa. It consists of squamous epithelium and areolar connective tissue. The serosa is continuous with the mesentery, a fold of peritoneum that attaches and supports the abdominal organs. However, the esophagus, which lies outside the coelom, is not covered by serosa but by a layer of dense elastic fibrous connective tissue called adventitia external.
  2. Muscularis (Muscular Coat): The muscularis layer is responsible for the movement and contraction of the gut. It consists of two layers of smooth muscle fibers:
    • Outer Longitudinal Muscle Fibers: These fibers run parallel to the long axis of the gut and provide the force for peristaltic movements.
    • Inner Circular Muscle Fibers: These fibers encircle the gut and play a role in mixing and propelling the contents through the digestive tract. In the stomach, there is an additional layer of oblique muscle fibers that aids in churning and mechanical digestion.
    Between the layers of muscle fibers, there is a network of nerve cells and parasympathetic nerve fibers known as Auerbach’s plexus or myenteric plexus. This plexus controls the peristaltic movements of the gut.
  3. Submucosa: The submucosa is a layer of loose connective tissue that lies beneath the muscularis layer. It is richly supplied with blood and lymphatic vessels and may contain glands in certain areas. The submucosa also houses Meissner’s plexus or submucosal plexus, which consists of nerve cells and sympathetic nerve fibers. This plexus controls the secretion of intestinal juice.
  4. Mucosa (Mucous Membrane): The innermost layer lining the lumen of the gut is called the mucosa or mucous membrane. The mucosa plays a vital role in absorption and secretion. It consists of three layers:
    • Muscularis Mucosa: This layer contains outer longitudinal and inner circular muscle fibers, similar to the muscularis layer, but in a more compact arrangement.
    • Lamina Propria: The lamina propria is a layer of loose connective tissue that contains blood vessels, glands, and some lymphoid tissue. It provides support and nourishment to the overlying epithelium.
    • Epithelium: The epithelium is the innermost layer of the mucosa and directly interacts with the gut contents. It varies in structure depending on the region of the gut. In the stomach, it forms gastric glands, while in the small intestine, it features finger-like projections called villi, as well as intestinal glands. The epithelium secretes mucus to lubricate the gut lining and assists in absorption and digestion.

It is worth noting that in the upper one-third of the esophagus, both Auerbach’s and Meissner’s plexuses are absent.

Understanding the histology of the human gut provides insight into the specialized structures and functions of each layer, contributing to the overall digestive process and the absorption of nutrients.

Part 2. Digestive Glands

I. Salivary Glands

The salivary glands play a crucial role in the production and secretion of saliva into the oral cavity. In humans, there are three pairs of salivary glands: parotid glands, sublingual glands, and submandibular glands. Let’s explore each of them:

  1. Parotid Glands: The parotid glands are the largest salivary glands and are located near the ears. They have ducts known as Stenson’s ducts, which open into the oral cavity near the upper second molars. The parotid glands primarily secrete a significant amount of salivary amylase or alpha-amylase, which is an enzyme involved in the initial digestion of carbohydrates.
  2. Sublingual Glands: The sublingual glands are the smallest salivary glands and are situated beneath the tongue. They have ducts called sublingual ducts or ducts of Rivinus that open into the floor of the oral cavity. These glands secrete both salivary amylase and mucus. The mucus helps lubricate the oral cavity and aids in swallowing.
  3. Submandibular Glands: The submandibular (or submaxillary) glands are medium-sized salivary glands located at the angles of the lower jaw. They have ducts known as Wharton’s ducts, which open into the oral cavity near the lower central incisors. The submandibular glands secrete a combination of salivary amylase, mucus, and serous fluids.

Saliva, the fluid secreted by the salivary glands, has several components and functions. It is slightly acidic, with a pH of around 6.8. On average, 1,000-1,500 ml of saliva is produced per day. Saliva consists of water, electrolytes (such as sodium, potassium, chloride, and bicarbonate ions), mucus, serous fluids, and various enzymes and substances:

  • Salivary Amylase (Ptyalin): Saliva contains the enzyme salivary amylase or ptyalin, which begins the digestion of starches and carbohydrates in the mouth.
  • Lysozyme: Saliva also contains lysozyme, an antibacterial agent that helps protect the oral cavity against harmful microorganisms.
  • Thyocyanate Ions: Ions of thyocyanate can be found in saliva, contributing to its composition.

The production of saliva and its components are essential for the proper lubrication, digestion, and protection of the oral cavity. Saliva helps moisten food, making it easier to chew and swallow. It also aids in the initial breakdown of carbohydrates and provides a protective mechanism against oral bacteria.

Certain conditions, such as viral infections like mumps, can affect the salivary glands, particularly the parotid glands. These infections may cause swelling and discomfort in the affected glands.

In summary, the salivary glands secrete saliva, which is a combination of water, electrolytes, enzymes, mucus, and antibacterial agents. Each pair of salivary glands has its specific location and contributes to the overall production and composition of saliva, playing a vital role in oral health and digestion.

The main salivary glands
The main salivary glands

II. Gastric Glands

Gastric glands are microscopic, tubular glands found in the epithelium of the stomach. They consist of three main types of cells, each with distinct functions:

  1. Chief Cells or Peptic Cells: Chief cells, also known as peptic cells or zymogenic cells, are located primarily at the base of the gastric glands. Their main role is the secretion of gastric digestive enzymes in the form of pro-enzymes or zymogens. The chief cells secrete pepsinogen, which is converted to the active enzyme pepsin in the acidic environment of the stomach. They also produce pro-rennin, an enzyme involved in the digestion of milk protein in young mammals. In addition, chief cells secrete small amounts of gastric amylase and gastric lipase, although their contribution to digestion is limited.
  2. Oxyntic Cells or Parietal Cells: Oxyntic cells, also referred to as parietal cells, are larger in size and are most abundant on the side walls of the gastric glands. These cells stain strongly with eosin, hence their name oxyntic cells. They lie against the basement membrane of the stomach lining. Oxyntic cells are responsible for the secretion of hydrochloric acid (HCl), which helps create the highly acidic environment of the stomach. They also produce Castle intrinsic factor, a glycoprotein necessary for the absorption of vitamin B12 in the small intestine.
  3. Mucous Cells or Goblet Cells: Mucous cells, also known as goblet cells, are dispersed throughout the epithelium of the gastric glands. Their primary function is to secrete mucus, which forms a protective layer on the inner surface of the stomach. Mucus acts as a lubricant, protecting the stomach lining from the corrosive effects of gastric acid and providing a barrier against digestive enzymes.

The secretions of these cells combine to form gastric juice, which has a very acidic pH ranging from 1.5 to 2.5. Gastric juice contains pro-enzymes such as pepsinogen and pro-rennin, as well as enzymes like gastric lipase and gastric amylase. It also contains mucus for protection and hydrochloric acid for maintaining acidity.

In addition to the three main types of cells, the epithelium of the gastric glands also contains two other types of cells:

  • Endocrine Cells: These cells are typically located in the basal parts of the gastric glands. Argentaffin cells produce serotonin, somatostatin, and histamine. Serotonin acts as a vasoconstrictor and stimulates smooth muscle contraction. Somatostatin suppresses the release of hormones from the digestive tract, while histamine dilates blood vessels. Gastrin cells, also known as G-cells, are found in the pyloric region and secrete and store the hormone gastrin, which stimulates the gastric glands to release gastric juice.
  • Stem Cells: Stem cells are undifferentiated cells present in the epithelium of the gastric glands. They have the ability to multiply and replace other cells. When the gastric epithelium is damaged, such as in the case of gastric ulcers, these stem cells increase in number and play a crucial role in the healing process.

In summary, the gastric glands in the stomach consist of chief cells, oxyntic cells, and mucous cells that secrete enzymes, hydrochloric acid, and mucus, respectively. The combined secretions form gastric juice, contributing to the digestion of food. Additionally, endocrine cells produce various hormones that regulate gastric function, while stem cells play a role in epithelial regeneration and repair.

III. Liver (= Hepar)

  • The liver, also known as the hepar, is the largest gland in the human body. It is located in the upper right side of the abdominal cavity, just below the diaphragm. The liver is divided into two main lobes, the right and left lobes, which are separated by the falciform ligament. The right lobe consists of the right lobe proper, the quadrate lobe, and the caudate lobe on the posterior surface.
  • Internally, the liver is composed of structural and functional units called hepatic lobules. These lobules contain hepatic cells, or hepatocytes, arranged in the form of cords. Each lobule is covered by a thin connective tissue sheath called Glisson’s capsule, which is a characteristic feature of mammalian liver. Kupffer cells, which are phagocytic cells, can be found in the liver and play a role in removing worn-out white blood cells, red blood cells, and bacteria. Additionally, fat storage cells are present in the liver.
  • The liver cells secrete bile, a substance that aids in digestion. Bile is produced by hepatocytes and enters bile canaliculi, which are a network of tubular spaces between the liver cells. From there, bile flows into small Hering’s canals lined by cuboidal epithelium. These canals then empty into interlobular bile ducts, which are lined by columnar epithelium.
  • Attached to the posterior surface of the liver is a pear-shaped sac-like structure called the gallbladder. It serves as a storage organ for bile produced by the liver. However, it should be noted that not all animals, such as rats and horses, possess a gallbladder.
  • The bile duct system consists of the right and left hepatic ducts, which join to form the common hepatic duct. The common hepatic duct then combines with the cystic duct, originating from the gallbladder, to form the bile duct. The bile duct descends posteriorly and joins the main pancreatic duct to form the hepatopancreatic ampulla, also known as the ampulla of Vater. The ampulla opens into the duodenum, and the opening is guarded by the sphincter of Oddi. The sphincter of Boyden surrounds the opening of the bile duct before it joins with the pancreatic duct.
  • The liver receives blood from two sources. Oxygenated blood is supplied by the hepatic artery, which branches from the aorta. Deoxygenated blood, containing newly absorbed nutrients from the intestine, is carried by the hepatic portal vein. The liver has a remarkable regenerative capacity, allowing it to repair and regenerate damaged tissue.
  • Bile, the fluid secreted by the liver, has several functions. It neutralizes the acidic chyme from the stomach with its sodium bicarbonate content. Bile salts, such as sodium glycocholate and sodium taurocholate, aid in the emulsification of fats, breaking down large fat droplets into smaller ones. Bile salts also facilitate the absorption of fat and fat-soluble vitamins (A, D, E, and K) in the small intestine. Bile pigments, bilirubin, and biliverdin are excretory products found in bile. The alkaline nature of bile helps prevent food decomposition by inhibiting bacterial growth. Bile also stimulates peristalsis in the intestines and activates the enzyme lipase.
  • Obstruction of the hepatic or bile duct, commonly caused by gallstones or other factors, can lead to a condition known as obstructive jaundice. In this condition, bile is absorbed into the bloodstream instead of reaching the duodenum, resulting in the yellowing of the eyes and skin.
  • In summary, the liver is the largest gland in the body, divided into lobes and consisting of hepatic lobules. Its functions include bile production, detoxification, storage, metabolism, and synthesis of various substances vital for proper bodily function. The liver plays a crucial role in digestion and the elimination of waste products from the body.

IV. Pancreas

  • The pancreas is a vital organ nestled within the abdominal cavity, positioned just posterior to the stomach. This remarkable gland, weighing approximately 60 grams, exhibits a unique soft texture and a beautiful greyish-pink hue. With a width of about 2.5 centimeters and a length ranging from 12 to 15 centimeters, the pancreas plays a crucial role in maintaining the body’s overall health and well-being.
  • Anatomically, the pancreas showcases a lobulated structure, divided into distinct lobes and lobules that facilitate its numerous functions. It is intricately connected to the surrounding organs and blood vessels, making it an integral part of the digestive and endocrine systems. This multifaceted organ holds remarkable significance in both nutrient absorption and hormone regulation.
  • One of the primary responsibilities of the pancreas is the production and secretion of digestive enzymes. These enzymes, including amylase, lipase, and protease, play a vital role in breaking down carbohydrates, fats, and proteins respectively. By facilitating the digestion and absorption of nutrients from the food we consume, the pancreas aids in the body’s energy production and overall nourishment.
  • Another crucial aspect of the pancreas lies in its endocrine function, specifically the production of insulin and glucagon. These hormones are secreted by specialized cells called the islets of Langerhans, scattered throughout the pancreas. Insulin and glucagon work in tandem to regulate the body’s blood glucose levels. Insulin promotes the uptake of glucose from the bloodstream into cells, facilitating energy utilization and storage, while glucagon stimulates the release of stored glucose, ensuring a stable blood sugar balance.
  • The pancreas also acts as a sentinel, sensing and responding to changes in blood glucose levels. It maintains a delicate equilibrium by releasing the appropriate amount of insulin or glucagon based on the body’s needs. This dynamic regulation prevents extreme fluctuations in blood sugar, safeguarding against conditions like hyperglycemia (high blood sugar) or hypoglycemia (low blood sugar).
  • Despite its critical functions, the pancreas is not exempt from ailments. Diseases such as pancreatitis, pancreatic cancer, and diabetes mellitus can afflict this vital organ, leading to severe health complications. Pancreatitis refers to the inflammation of the pancreas, causing debilitating pain and digestive disturbances. Pancreatic cancer, although relatively rare, is a highly aggressive form of cancer that often presents late with limited treatment options. Diabetes mellitus, a chronic metabolic disorder, occurs due to inadequate insulin production or impaired insulin function, resulting in uncontrolled blood glucose levels.
  • Understanding the intricacies of the pancreas helps shed light on its significance in maintaining overall health and wellness. From aiding digestion through enzyme secretion to regulating blood sugar levels through hormone production, the pancreas serves as a vital player in the intricate symphony of our body’s functions. It highlights the importance of taking care of our bodies, adopting a balanced diet, and leading a healthy lifestyle to support optimal pancreatic function and overall well-being.
Pancreas, duodenum and bile duct
Pancreas, duodenum and bile duct

External Structure of Pancreas

  • The external structure of the pancreas encompasses distinct regions known as the head, neck, body, and tail, each with its specific location within the abdominal cavity. These divisions help define the pancreas’s position in relation to neighboring organs and highlight its importance in various physiological processes.
  • Starting with the head of the pancreas, it resides in the curve of the duodenum, a section of the small intestine. This strategic positioning allows for close interaction between the pancreas and the duodenum, facilitating the secretion of digestive enzymes into the gastrointestinal tract. The head is intricately involved in the process of nutrient breakdown and absorption, playing a significant role in the digestion of carbohydrates, fats, and proteins.
  • Continuing from the head, the neck of the pancreas follows in succession. It serves as a connecting region between the head and the subsequent portions of the pancreas. This transition area allows for the smooth flow of pancreatic secretions and hormones, ensuring their efficient distribution throughout the body.
  • Moving further, the body of the pancreas is situated behind the stomach. This location allows the pancreas to maintain close proximity to the gastric environment, enabling prompt responses to the presence of food and the release of appropriate digestive enzymes. The body of the pancreas is responsible for synthesizing and secreting the digestive enzymes necessary for breaking down complex food components into more manageable forms for absorption and utilization by the body.
  • Lastly, the tail of the pancreas extends toward the spleen, situated in front of the left kidney. This elongated region plays a vital role in maintaining the overall balance and coordination of the digestive and endocrine functions of the pancreas. It contributes to the production and release of hormones such as insulin and glucagon, which regulate blood sugar levels and ensure metabolic homeostasis.
  • When examining the pancreatic duct system, the main pancreatic duct, also known as the duct of Wirsung, plays a significant role in transporting pancreatic secretions. It is formed from smaller ducts within the pancreas, collecting and merging their contents. Ultimately, the main pancreatic duct opens into a structure called the hepatopancreatic ampulla, also known as the ampulla of Vater. This region represents a crucial junction where the pancreatic duct merges with the common bile duct, facilitating the release of pancreatic enzymes and bile into the duodenum, further aiding in digestion.
  • Additionally, the pancreas possesses an accessory pancreatic duct known as the duct of Santorini. This secondary duct provides an alternative pathway for pancreatic secretions, allowing for additional flexibility in the distribution and delivery of digestive enzymes. The accessory duct opens directly into the duodenum, providing an independent route for the release of pancreatic juices.
  • Understanding the external structure of the pancreas, along with its ductal system, provides insights into the complex architecture and functional dynamics of this organ. The precise arrangement of the head, neck, body, and tail, coupled with the interconnected ducts, ensures the efficient synthesis, transport, and release of digestive enzymes and hormones critical for maintaining proper digestion, nutrient absorption, and metabolic regulation.

Internal Structure of Pancreas

The pancreas is a vital organ in the human body, responsible for both exocrine and endocrine functions. Its internal structure consists of two main parts: the exocrine part and the endocrine part.

(i) Exocrine part

  • The exocrine part of the pancreas plays a crucial role in the digestion of food. It is composed of rounded lobules, known as acini, which are responsible for secreting pancreatic juice. This pancreatic juice is alkaline in nature, with a pH of 8.4. On average, the pancreas secretes about 500-800 ml of pancreatic juice per day.
  • The main pancreatic duct serves as a conduit for the pancreatic juice to reach the small intestine. It carries the pancreatic juice into the duodenum, the first part of the small intestine, through a structure called the hepatopancreatic ampulla.
  • In addition to the main pancreatic duct, there is another duct called the accessory pancreatic duct. This duct directly pours pancreatic juice into the duodenum. This dual duct system provides an alternative pathway for the pancreatic juice to enter the small intestine.
  • Pancreatic juice is a complex fluid containing various components that aid in the digestion of different macromolecules. It contains sodium bicarbonate, which acts as a buffer and helps neutralize the acidic chyme from the stomach. This alkaline environment created by sodium bicarbonate is essential for the optimal functioning of digestive enzymes.
  • The pancreatic juice also contains pro-enzymes, which are inactive precursor forms of enzymes. These include trypsinogen, chymotrypsinogen, and procarboxypeptidase. In the duodenum, these pro-enzymes are activated by enzymes present on the surface of the intestinal lining. Once activated, they become trypsin, chymotrypsin, and carboxypeptidase, respectively. These enzymes play a crucial role in the breakdown of proteins into smaller peptides and amino acids.
  • Furthermore, the pancreatic juice contains other enzymes such as elastase, pancreatic alpha-amylase, DNase, RNase, and pancreatic lipase. Elastase aids in the digestion of elastin, a protein found in connective tissues. Pancreatic alpha-amylase is responsible for breaking down starches into simpler sugars like maltose. DNase and RNase are enzymes that degrade DNA and RNA, respectively. Pancreatic lipase is involved in the breakdown of fats or lipids into fatty acids and glycerol.
  • Overall, the pancreatic juice secreted by the exocrine part of the pancreas plays a vital role in the digestion of various macromolecules, including starches, proteins, fats, and nucleic acids. Its alkaline nature, along with the presence of digestive enzymes, helps in the efficient breakdown and absorption of nutrients in the small intestine.

(ii) Endocrine part


The endocrine part of the pancreas is made up of clusters of cells called the islets of Langerhans. In the human pancreas, there are approximately one million islets, with the highest concentration found in the tail of the pancreas. Each islet of Langerhans contains different types of cells that secrete hormones into the bloodstream.

  1. Alpha cells (α-cells): These cells are primarily located towards the periphery of the islet and make up about 15% of the islet of Langerhans. They produce the hormone glucagon. Glucagon plays a crucial role in regulating blood glucose levels by stimulating the breakdown of glycogen stored in the liver into glucose. As a result, glucagon is considered a diabetogenic hormone.
  2. Beta cells (β-cells): Beta cells are predominantly found towards the middle of the islet and constitute approximately 65% of the islet of Langerhans. They are responsible for producing the hormone insulin. Insulin plays a vital role in glucose metabolism by facilitating the uptake of glucose from the bloodstream into cells. It also promotes the conversion of glucose into glycogen, which is stored in the liver and muscles. Insulin deficiency leads to a condition known as diabetes mellitus.
  3. Delta cells (δ-cells): Delta cells are also located towards the periphery of the islet of Langerhans and make up about 5% of the islet. These cells secrete the hormone somatostatin (SS). Somatostatin acts as an inhibitory hormone by suppressing the secretion of glucagon by alpha cells and the release of insulin by beta cells. Additionally, somatostatin slows down the absorption of nutrients from the gastrointestinal tract. It is also secreted by the hypothalamus in the brain, where it inhibits the release of growth hormone (somatotropin) from the anterior lobe of the pituitary gland. Hence, somatostatin is referred to as a growth inhibitory hormone.
  4. Pancreatic polypeptide cells (PP cells or F-cells): Apart from the three main cell types mentioned above, the pancreas also contains pancreatic polypeptide (PP) cells. These cells constitute about 15% of the islet of Langerhans. PP cells secrete pancreatic polypeptide, which acts to inhibit the release of pancreatic juice.

In summary, the endocrine part of the pancreas, consisting of the islets of Langerhans, is responsible for producing and releasing several hormones into the bloodstream. These hormones include glucagon, insulin, somatostatin, and pancreatic polypeptide. Each hormone plays a distinct role in regulating various physiological processes such as glucose metabolism, nutrient absorption, and growth hormone regulation.

V. Intestinal Glands

Intestinal glands are structures formed by the surface epithelium of the small intestine and are responsible for the secretion of various substances important for digestion and protection of the intestinal wall. There are two types of intestinal glands: crypts of Lieberkuhn and Brunner’s glands.

  1. Crypts of Lieberkuhn: These are simple, tubular structures that are present throughout the small intestine, situated between the villi. The crypts secrete digestive enzymes and mucus. Goblet cells, specialized mucous-secreting cells, produce mucus, while enterocytes, cells lining the intestinal crypts, secrete water and electrolytes. At the base of the crypts, there are two types of cells:a) Paneth cells: These cells are predominantly found in the duodenum and are located at the bottom of the crypts. Paneth cells contain acidophilic granules and are rich in zinc. Although their exact function is not fully understood, they are believed to secrete lysozyme, an antibacterial substance. Paneth cells also have phagocytic capabilities.b) Argentaffin cells: These cells synthesize the hormone secretin and 5-hydroxytryptamine (5-HT), also known as serotonin.
  2. Brunner’s glands: These glands are found exclusively in the duodenum and are located in the submucosa, a layer beneath the mucosa. Brunner’s glands secrete a small amount of enzyme and mucus. The mucus serves to protect the duodenal wall from digestion. Most of the digestion of nutrients takes place in the duodenum with the help of various enzymes. Brunner’s glands open into the crypts of Lieberkuhn.

The secretion produced by intestinal glands is called intestinal juice or succus entericus, which has a pH of 7.8. Approximately 2,000-3,000 ml of intestinal juice is secreted per day. Intestinal juice contains a variety of enzymes, including maltase, isomaltase, sucrase, lactase, α-dextrinase, enterokinase, aminopeptidases, dipeptidases, nucleotidases, nucleosidases, and intestinal lipase. These enzymes are involved in the further breakdown of carbohydrates, proteins, nucleotides, and fats to facilitate their absorption.

It is worth noting that throughout the entire alimentary canal, there are also mucous glands that produce mucus. The mucus serves to lubricate the digestive tract and food, facilitating its movement and protecting the lining of the digestive system.

In addition to the intestinal glands, the human digestive system includes several accessory organs, such as the tongue, salivary glands, liver, gallbladder, and pancreas. These organs play important roles in the digestion and absorption of nutrients.

Swallowing or Deglutition

Swallowing, also known as deglutition, is the process by which food is transferred from the oral cavity to the stomach. It involves the coordinated activity of various structures, including the tongue, soft palate, pharynx, and esophagus.

The process of swallowing can be divided into three stages:

  1. The Voluntary stage: During this stage, the tongue blocks the mouth, and the bolus of food, which is a mass of chewed and mixed food with saliva, is pushed from the oral cavity into the oropharynx. This stage is under voluntary control.
  2. The Pharyngeal stage: Once the bolus reaches the pharynx, the involuntary pharyngeal stage of swallowing begins. The soft palate closes off the nasal passage, and the epiglottis, a flap of tissue located at the base of the tongue, seals off the glottis of the larynx. This action prevents food from entering the nasal cavity and the airway, respectively. Breathing is temporarily interrupted during this stage. The bolus is propelled through the pharynx and into the esophagus.
  3. The Esophageal stage: This stage represents the involuntary stage of swallowing. The bolus passes through the laryngopharynx and enters the esophagus within 1 to 2 seconds. The esophagus is a muscular tube that connects the pharynx to the stomach. Contractions of the esophageal muscles, known as peristalsis, propel the bolus downward toward the stomach. The respiratory passage reopens, allowing breathing to resume.

Swallowing is controlled by a swallowing center located in the medulla oblongata and lower pons varolii of the brain. This center coordinates the complex sequence of muscle contractions and relaxation required for successful swallowing.

In summary, swallowing is a multi-stage process that involves the voluntary movement of the bolus from the oral cavity to the pharynx, followed by the involuntary passage through the pharynx into the esophagus. The coordination of various structures and the involvement of the swallowing center in the brain ensure the smooth and efficient transfer of food from the mouth to the stomach.

Peristalsis

Peristalsis is an important physiological process involved in the movement of food through the digestive system, particularly during the oesophageal phase of swallowing.

During peristalsis, involuntary contractions occur in the circular muscles located just above and around the top of the bolus in the oesophagus. At the same time, the longitudinal muscles around the bottom of and just below the bolus also contract. These coordinated muscle contractions create a wave-like motion that propels the bolus forward through the oesophagus.

When the longitudinal muscles contract, they shorten the lower part of the oesophagus, pushing its walls outward. This allows the oesophagus to expand and receive the bolus of food. Following this, the circular muscles of the oesophagus relax. This relaxation allows the bolus to be propelled forward.

The contractions and relaxations of the muscles occur in a sequential wave-like pattern. The wave of contraction moves down the oesophagus, pushing the food ahead of it and propelling it towards the stomach. This wave-like movement ensures the smooth and efficient passage of the bolus through the oesophagus.

It’s worth noting that peristaltic movement is least pronounced in the rectum of humans. The rectum is the final part of the large intestine where feces are stored before elimination. Peristalsis in the rectum is relatively weaker compared to other parts of the digestive system, as the primary function of the rectum is to store feces until defecation occurs.

In summary, peristalsis is the involuntary muscular movement that facilitates the propulsion of food through the oesophagus during swallowing. The coordinated contractions and relaxations of the circular and longitudinal muscles create a wave-like motion, pushing the bolus towards the stomach. However, peristaltic movement is less prominent in the rectum.

Absorption of Digested Food

  • The process of absorption of digested food occurs primarily in the ileum, which is the final section of the small intestine. The wall of the ileum is lined with finger-like projections called villi, which greatly increase the surface area available for absorption.
  • The villi play a crucial role in absorption by providing an extensive surface area for nutrient exchange. Each villus contains a network of blood vessels, including capillaries, that are responsible for absorbing the digested nutrients. The absorbed nutrients include carbohydrates, proteins, fats, vitamins, and minerals.
  • As the digested food passes over the surface of the villi, nutrients are transported across the epithelial lining of the small intestine and into the blood capillaries within the villi. This transfer of nutrients from the intestinal lumen to the bloodstream is facilitated by various mechanisms, such as active transport, facilitated diffusion, and osmosis, depending on the specific nutrient being absorbed.
  • Once the absorbed materials enter the blood capillaries of the villi, they are carried away from the small intestine. The absorbed nutrients are then transported by veins to the liver through the hepatic portal system. The liver plays a vital role in processing and metabolizing the absorbed nutrients. It regulates the nutrient levels in the blood, stores certain nutrients, and detoxifies harmful substances.
  • From the liver, the nutrients are further distributed to different parts of the body via the bloodstream. The heart pumps the nutrient-rich blood to various organs and tissues, providing them with the necessary energy and building blocks for their functioning and growth.
  • In summary, the absorption of digested food occurs in the ileum, where the villi increase the surface area for efficient absorption. The absorbed nutrients enter the bloodstream through the villi’s blood capillaries and are then transported to the liver for processing and distribution to different parts of the body. This process ensures that the body receives the necessary nutrients for its functioning and maintenance.

Assimilation of Digested Food

  • Assimilation refers to the process by which the cells of the body take in and utilize the digested food. Once the nutrients have been absorbed into the bloodstream, they are transported to various cells and tissues where they are used for different purposes.
  • One of the primary uses of digested food is to obtain energy through the process of cellular respiration. Carbohydrates, such as glucose, are broken down further within the cells to release energy that is necessary for the functioning of cellular processes. Any excess glucose beyond immediate energy needs is converted into glycogen and stored in the liver and muscles for later use.
  • Amino acids, the building blocks of proteins, are another essential component of digested food. These amino acids are utilized by cells for the synthesis of proteins. Proteins are crucial for the growth, repair, and maintenance of tissues in the body. They are involved in various functions, including enzyme production, immune response, and structural support.
  • The digested fats, in the form of glycerol and fatty acids, have multiple fates within the body. They can be used as an immediate source of energy by cells. Alternatively, they can be reconverted into fats and stored in adipose tissue located below the skin or around organs. These stored fats serve as an energy reserve for the body and also help in insulation and protection.
  • In addition to energy production and protein synthesis, the absorbed food is also utilized for the formation of new cells and tissues. This process is particularly important during periods of growth and development, such as childhood and adolescence. The nutrients obtained from digested food provide the necessary building blocks for the body to generate new cells and tissues, allowing for growth, repair, and overall development.
  • Overall, the assimilation of digested food involves the utilization of nutrients for energy production, protein synthesis, storage of excess energy in the form of glycogen or fats, and the formation of new cells and tissues. This process ensures that the body can effectively utilize the nutrients obtained from food, supporting its energy needs, growth, and maintenance.

Types of Digestive Juice

There are five different digestive juices that play important roles in the process of digestion. These include saliva, gastric juice, pancreatic juice, succus entericus (intestinal juice), and bile. These juices are produced by specific glands located throughout the digestive system, and they are released into the alimentary canal at different points, starting from the mouth and progressing towards the end of the digestive tract.

The mechanism of secretion refers to two aspects. Firstly, it involves how the glands respond to stimuli and initiate the process of secretion. This includes understanding the factors that trigger glandular activity and the regulation of their flow and composition. Secondly, it encompasses the ability of the glands to modify their secretions in terms of both flow and composition in response to various types of stimuli and the specific site of stimulation within the digestive system.

Types 1. Salivary Secretion

  • Salivary secretion is primarily a reflex process that involves two types of reflexes: conditioned and unconditioned. The conditioned reflex is evident from the fact that even the sight and smell of food can stimulate salivary secretion. Additionally, other conditioned stimuli can be established to trigger salivary flow.
  • The stimulus for the unconditioned reflex primarily originates in the mouth. However, it can also arise from other areas such as the oesophagus, leading to the oesophago-salivary reflex, or from the stomach, resulting in the gastro-salivary reflex. Furthermore, stimuli from other visceral organs, such as the gravid uterus, can also induce salivary secretion.
  • Overall, salivary secretion is a complex process involving reflex mechanisms that can be triggered by various stimuli. These reflexes play a crucial role in preparing the oral cavity for the digestion and swallowing of food by ensuring an adequate production of saliva.

Types 2. Gastric Secretion

Gastric secretion is a complex process that involves different phases and is influenced by various stimuli. Here is a breakdown of the information provided:

Experiments:

Two experiments have been conducted to study gastric secretion:

i. Sham feeding: This involves the process of chewing and swallowing without actually consuming food.

ii. Pavlov’s pouch: This refers to the surgical creation of an isolated pouch in an animal’s stomach for studying gastric secretion.

Phases:

Gastric secretion occurs in three phases:

(a) Cephalic (or nervous) phase:

This phase begins immediately after food intake and is a reflex process involving both conditioned and unconditioned reflexes. The juice secreted during this phase is called appetite juice. It has a constant composition and does not vary with the nature of food. Although the amount is small in humans, it plays an important role.

(b) Gastric phase:

This phase starts approximately thirty minutes after food enters the stomach. The stimulus for this phase is chemical, and a substance called gastrin is produced by the pyloric mucous membrane. Gastrin is released into the bloodstream and stimulates the gastric glands to secrete gastric juice independently of neural input. The gastric phase is characterized by the largest quantity of gastric juice secretion. The quality and quantity of gastric juice vary depending on the nature of the food. Proteins increase the amount and the hydrochloric acid (HCl) content of the gastric juice, while fats inhibit both. Bread stimulates the secretion with the highest digestive power, while water, coffee, and spices also contribute to stimulation.

(c) Intestinal phase:

This phase begins when food enters the duodenum (the first part of the small intestine). It is a smaller amount of secretion and is independent of neural stimulation. The stimulus for this phase is chemical, although the exact nature of the stimulus is not yet fully understood. The presence of fat in the duodenum inhibits gastric secretion. This inhibition is believed to be caused by the release of an inhibitory hormone called enterogastrone from the intestine.

The interrelation of the phases:

The three phases of gastric secretion are closely interconnected. The cephalic phase initiates the secretion of appetite juice, which begins the digestion of proteins. From the products of protein digestion, gastrin is produced, initiating the gastric phase. Once gastric digestion has progressed to the desired stage, the stomach empties its contents into the duodenum, signaling the start of the intestinal phase. Each phase sets the stage for the next.

Additional information:

Even in the fasting state, the stomach continues to secrete gastric juice at a rate of 10-60 ml per hour. The exact cause of this secretion is not known. It is believed to serve as an antiseptic, protecting against pathogenic bacteria that may be ingested with saliva when the stomach is empty.

Types 3. Pancreatic Secretion

  • Pancreatic secretion is a vital process in the digestive system that involves the release of pancreatic juice from the pancreas into the small intestine. The secretion of pancreatic juice occurs in two distinct phases: the nervous phase and the chemical phase.
  • The nervous phase of pancreatic secretion begins approximately 1-2 minutes after food is consumed. Unlike gastric juice, the reflex for pancreatic secretion is purely unconditioned, meaning it is not a learned response. The stimulus for this phase originates from two sources. Firstly, during the process of chewing, the stimulation in the mouth triggers pancreatic secretion. Secondly, after food is swallowed, the stomach also contributes to the stimulus for pancreatic secretion.
  • Following the nervous phase, the chemical phase of pancreatic secretion commences when the stomach empties its contents into the duodenum, the first part of the small intestine. This phase is regulated by hormone-like substances called secretin and pancreozymin. Secretin is released in response to the acidic chyme (partially digested food) entering the duodenum from the stomach. It is released into the bloodstream and travels to the pancreas, where it stimulates the secretion of pancreatic juice. The secretin fraction of pancreatic juice stimulates the secretion of water, alkali, and other salts. On the other hand, pancreozymin increases the enzyme content of the pancreatic juice.
  • The pancreatic juice stimulated by secretin is characterized by being rich in alkali but poor in enzymes. In contrast, the secretion induced by the stimulation of the vagus nerve (part of the nervous system) is low in alkali but high in enzymes. Therefore, the composition of pancreatic secretion can vary depending on the type of food consumed. For instance, meat consumption stimulates the secretion of a “secretin” type of juice, while fat consumption stimulates the “vagus” type. When bread is consumed, it elicits a mixed type of pancreatic secretion, containing both alkali and enzymes.
  • In summary, pancreatic secretion occurs in two phases: the nervous phase and the chemical phase. The nervous phase is initiated by stimuli in the mouth and stomach, while the chemical phase is triggered by secretin and pancreozymin. These hormones regulate the composition of pancreatic juice, with secretin promoting the secretion of alkali and pancreozymin increasing the enzyme content. The type of food consumed can influence the composition of pancreatic secretion, with different foods eliciting different types of pancreatic juice.

Types 4. Bile Secretion

  • Bile secretion is an essential process carried out by the liver, involving the production and release of bile into the digestive system. The mechanisms and stimuli involved in bile secretion can be understood through various experiments and factors.
  • One experimental method used to study bile secretion is altercursive intubation, where alternate intubation of the bile duct and duodenum is performed. Another method is triple intubation, which involves simultaneous intubation of the bile duct, duodenum, and stomach. These experiments help in understanding the dynamics of bile secretion.
  • Bile secretion by the liver is an active and continuous process, with the total amount ranging from 800 to 1,000 milliliters per day, or approximately 15 milliliters per kilogram of body weight. The rate of secretion increases about one hour after consuming food and remains elevated for 2 to 5 hours before gradually declining. At the end of intestinal digestion, bile secretion returns to fasting levels. In the process of bile secretion, only chemical stimuli play a significant role, while nerves have limited practical importance.
  • The chemical stimuli that regulate bile secretion include bile salts, foodstuffs (such as fats and proteins), humoral factors, and the succus entericus (intestinal juice).
  • Bile salts act as chief cholagogues, substances that promote the flow of bile. Fats and proteins in the diet stimulate bile secretion, with a noted increase in secretion about one hour after a meal. This elevated secretion levels persist for 2 to 5 hours before decreasing.
  • Humoral factors involved in bile secretion include cholecystokinin, which stimulates bile secretion by contracting the gallbladder, and hepatocrinin, which stimulates the liver cells to secrete bile.
  • The succus entericus, or intestinal juice, also contributes to bile secretion. The Thiry-Vella fistula experiment has been used to study the succus entericus. During fasting, no bile secretion occurs. However, approximately one hour after consuming food, the flow of bile starts, reaching its maximum in the third hour, and then gradually declining.
  • The mechanism of bile secretion involves both mechanical and chemical factors. The mechanical factor is related to the mechanical irritation of the intestinal mucosa, which triggers secretion. This process is independent of the vagus nerve and sympathetic nervous system but relies on local nerve plexuses. The presence of food in the intestine acts as a normal stimulus for bile secretion.
  • Chemical stimuli also play a crucial role in bile secretion. Some products of digestion, particularly proteins, as well as enterocrinin and duocrinin, act as important stimuli. Enterocrinin, a hormone produced by the intestinal mucosa, is believed to play a significant role in the normal process of intestinal juice secretion. These factors are independent of nerves and contribute to the normal functioning of bile secretion during the process of digestion and absorption in the intestine. After the completion of this period, typically lasting 2 to 5 hours, the flow of bile ceases.

Secretions in the Gastro-Intestinal Tract

Secretions in the gastrointestinal tract are regulated by various humoral factors. These chemical factors play a crucial role in controlling the different secretions that occur in the digestive system. Let’s take a closer look at the factors responsible for the secretions in each part of the gastrointestinal tract.

  1. Gastric Secretion:
  • i. Gastrin: Gastrin is a hormone that is primarily responsible for the gastric phase of secretion in the stomach. It is released by specialized cells called G cells in the stomach lining. Gastrin stimulates the secretion of gastric acid and promotes the production of pepsin, an enzyme that aids in protein digestion.
  • ii. Hormones or Secretagogues: During the intestinal phase of gastric secretion, various chemical substances, including hormones or secretagogues, come into play. These substances trigger the release of gastric secretions, facilitating the continued digestive process.
  • iii. Enterogastrone: Enterogastrone is a hormone that acts to inhibit gastric secretion. It is released in response to the presence of chyme (partially digested food) in the duodenum. Enterogastrone helps regulate the movement of food from the stomach into the small intestine, allowing for efficient digestion and absorption.
  1. Pancreatic Secretion:
  • i. Secretin: Secretin is the main chemical stimulus for pancreatic secretion. It is released by the S cells in the duodenum when acidic chyme enters the small intestine. Secretin stimulates the pancreas to secrete bicarbonate-rich pancreatic juice, which helps neutralize the acidity of chyme.
  • ii. Pancreozymin: Also known as cholecystokinin (CCK), pancreozymin is another hormone involved in pancreatic secretion. It is released by the intestinal mucosa in response to the presence of fats and proteins in the small intestine. Pancreozymin stimulates the pancreas to release enzymes that aid in the digestion of fats and proteins.
  • iii. Foodstuffs: The composition of the food being digested can also act as a stimulus for pancreatic secretion. Certain components of food, such as amino acids and fatty acids, can directly stimulate the release of pancreatic enzymes.
  1. Succus Entericus:
  • i. Products of Protein Digestion: The breakdown products of protein digestion, such as peptides and amino acids, act as stimuli for the secretion of succus entericus. Succus entericus, also known as intestinal juice, contains enzymes that further break down peptides into individual amino acids for absorption.
  • ii. Enterocrinin: Enterocrinin is believed to be the chief stimulus for succus entericus secretion. It is released by the enteroendocrine cells in the small intestine in response to the presence of partially digested proteins and peptides.
  • iii. Duocrinin: Duocrinin is another humoral factor involved in the secretion of succus entericus. It is released in response to the presence of fats and carbohydrates in the small intestine.
  1. Bile Secretion:
  • i. Bile Salts: Bile salts are the chief stimulus for bile secretion. They are synthesized in the liver from cholesterol and stored in the gallbladder. When fatty food enters the small intestine, bile salts are released, aiding in the emulsification and absorption of fats.
  • ii. Fat and Protein Food: The presence of fat and protein in the small intestine stimulates the release of bile. Bile helps in the digestion and absorption of dietary fats and fat-soluble vitamins.
  • iii. Hepatocrinin: Hepatocrinin is a humoral factor that acts on liver cells, stimulating them to secrete bile.
  • iv. Cholecystokinin: Cholecystokinin (CCK) not only plays a role in pancreatic secretion but also stimulates the contraction of the gallbladder. This contraction leads to the release of stored bile into the small intestine, facilitating fat digestion.

Main Phases of Gastric Secretion

Gastric secretion has divided into four phases: 1. Nervous Phase 2. Gastric Phase 3. Intestinal Phase 4. Interdigestive Phase.

1. Nervous Phase

The “Nervous Phase” of gastric secretion involves the stimulation of the vagus nerve, sympathetic nerves, and the influence of the hypothalamus on gastric secretion. This phase is also associated with conditioned reflexes and the secretion of what is known as the “appetite juice” or psychic juice. Let’s delve into the details of this phase based on the provided content.

  1. Vagal Stimulation: Stimulation of the vagus nerve leads to gastric secretion. The vagus nerve influences the gastric cells, causing them to release acetylcholine, which stimulates the gastric cells to secrete pepsin, hydrochloric acid (HCl), and some mucus. Additionally, vagal stimulation may lead to the release of histamine, which further stimulates the parietal cells responsible for acid secretion. Vagal stimulation also promotes vasodilation of the gastric mucosa. However, under certain conditions, vagal stimulation can decrease or even inhibit gastric secretions. Vagal stimulation also enhances the release of gastrin and increases the response of stomach cells to other stimuli.
  2. Sympathetic Stimulation: Stimulation of the sympathetic nerves supplying the stomach causes vasoconstriction of the gastric blood vessels. However, the effects of sympathetic stimulation on gastric secretion are not consistent.
  3. Influence of the Hypothalamus: The hypothalamus plays a significant role in gastric secretion. Stimulation of the hypothalamus increases gastric secretion by augmenting vagal activity. Hypoglycemia, a condition of low blood sugar, can also stimulate gastric secretion through the same mechanism. Experimental lesions of the hypothalamus have been found to result in gastric hemorrhages, erosions, and even perforations. It is suggested that such lesions may be associated with the development of gastric ulcers.
    1. Unconditioned Reflex: The initial phase of gastric secretion is considered a reflex process known as the “appetite juice” or psychic juice. It involves two types of reflexes: unconditioned reflex and conditioned reflex. The unconditioned reflex is triggered by sensory stimuli arising in the mouth during chewing and swallowing of food. The sensory nerves involved are the fifth (trigeminal), seventh (facial), and ninth (glossopharyngeal) cranial nerves. The motor nerve responsible for this reflex is the vagus nerve.
    1. Conditioned Reflex (Psychic Reflex): Conditioned reflexes are established when stimuli such as the sight or smell of food, even without actual contact with food in the mouth, can stimulate gastric secretion. These stimuli involve the special senses of vision, smell, and hearing. Once conditioned reflexes are established, they can elicit gastric secretion through vagal stimulation.

    Characteristics of Appetite Juice:

    The appetite juice or psychic juice, associated with the nervous phase of gastric secretion, has the following characteristics:

    • It is rich in pepsin, has an acidic pH, and contains mucus.
    • The composition of appetite juice remains constant and does not vary with the type of food.
    • The quantity of appetite juice secreted varies with the intensity of appetite.
    • The secretion of appetite juice can be inhibited by factors such as shock, fear, and anxiety.
    • In animals, appetite juice constitutes a significant portion of total gastric secretion, but in humans, its quantity is likely much less and not essential.
    • Its primary importance lies in its role in initiating the second phase of gastric secretion.

    In conclusion, the nervous phase of gastric secretion involves the stimulation of the vagus nerve, conditioned and unconditioned reflexes, and the secretion of appetite juice. These processes are regulated by various neural pathways and can be influenced by factors such as the hypothalamus, sympathetic nerves, and conditioned stimuli.

    2. Gastric Phase (Hormonal)

    The “Nervous Phase” of gastric secretion is characterized by the response of the stomach to the presence of food and involves local and vagal reflexes, as well as the release of the hormone gastrin. This phase is responsible for the secretion of gastric juice and is influenced by various factors. Let’s explore the details of this phase based on the provided content.

    1. Gastric Phase and Gastrin: During the nervous phase, gastric secretion occurs in response to the entry of food into the stomach. This phase is mediated by local and vagal reflex responses to distention, as well as by the release of gastrin, a hormone produced by the mucosa of the pyloric area. When the stomach is completely denervated, this secretion is not affected, indicating that it is independent of nervous reflexes. Instead, it is triggered by a chemical stimulus, namely gastrin.

    Experimental evidence supporting the role of gastrin includes the following:

    • The acid extract of the pyloric mucosa, when injected, stimulates gastric secretion.
    • A substance that can excite gastric secretion is found in the venous blood of the stomach during the peak of gastric secretion.
    • Grafting a small pouch from the body of the stomach to a highly vascular area (such as the mammary region in a pig) results in the secretion of gastric juice in the grafted pouch. As there is no nervous connection between the stomach and the grafted pouch, the secretion in the pouch must be due to a chemical excitant carried through the bloodstream.
    • A protein derivative isolated from the pyloric mucosa has a strong stimulating effect on gastric secretion.
    • Resection of the pyloric part of the stomach greatly reduces this phase of gastric secretion.
    1. Stimulants and Inhibitors of Gastric Secretion: Different food substances have varying effects on gastric secretion:
    • Meat increases both the quantity and the hydrochloric acid (HCl) content of gastric secretion.
    • Bread stimulates a secretion with high digestive power.
    • Fat inhibits gastric secretion in terms of quality and quantity. This inhibitory effect may be due to a chemical substance called enterogastrone, and it is more pronounced from the duodenum than from the stomach.
    • Water, tea, coffee, spices, condiments, and vegetable juices stimulate gastric secretion.
    • Mechanical distention of the stomach, such as from gas or aerated waters, also stimulates gastric secretion and movements.
    1. Nature and Action of Gastrin: Gastrin is a polypeptide hormone, and two forms of gastrin, gastrin I and gastrin II, with differing amino acid sequences, have been isolated. Gastrin stimulates gastric secretion that is rich in acid but poor in pepsin. It also stimulates bile secretion and has a slight stimulatory effect on pancreatic secretion. The gastric phase of secretion constitutes the main part of gastric juice and typically lasts for about three hours. Unlike the psychic juice (appetite juice), the gastric phase varies in quality and quantity depending on the type of food ingested.

    In summary, the nervous phase of gastric secretion involves local and vagal reflexes, as well as the release of gastrin, in response to the presence of food in the stomach. Gastrin acts as a chemical stimulus, stimulating gastric secretion that is essential for digestion. The secretion varies based on the type of food and can be influenced by stimulants and inhibitors, as well as mechanical distention of the stomach.

    3. Intestinal Phase

    The “Intestinal Phase” of gastric secretion refers to the stimulation and inhibition of gastric secretion that occurs due to the presence of certain substances in the small intestine. This phase has a latent period of 2-3 hours and can continue for 8-10 hours. It is characterized by the effect of chemical stimulants or hormones absorbed from the intestine.

    Key points regarding the Intestinal Phase of gastric secretion are as follows:

    1. Stimulation of Gastric Secretion: Certain food substances, such as water, meat extract, peptone, and partly digested proteins, when present in the small intestine, can stimulate gastric secretion. This can occur during the normal process of digestion or by directly introducing these substances into the duodenum through a duodenal fistula. Importantly, denervation of the relevant parts does not affect this phase of gastric secretion, indicating that it is primarily due to a chemical stimulus absorbed from the intestine. The exact nature of the stimulus responsible for this phase is not known, but it is believed to be a hormone or secretagogue that is present in the food and absorbed into the bloodstream from the intestine.
    1. Inhibition of Gastric Secretion: The presence of certain substances in the duodenum can inhibit gastric secretion:
    • a) Introduction of alkali directly into the duodenum inhibits gastric secretion.
    • b) The presence of fats in the duodenum inhibits gastric secretion in both the gastric and intestinal phases. This inhibitory effect is mediated by the release of an intestinal hormone called enterogastrone. Enterogastrone not only inhibits gastric secretion but also reduces gastric motility. It has been detected in the blood of animals fed a high-fat diet and has been extracted from the intestinal mucosa.

    Additionally, urogastrone is another inhibitory substance, similar to but not identical to enterogastrone. It has been isolated from the urine of normal males and pregnant women. Urogastrone specifically inhibits gastric secretion and has been recommended for therapeutic use in the treatment of gastric ulcers. Its precise role in the normal process of gastric secretion is not yet fully understood.

    In summary, the Intestinal Phase of gastric secretion is characterized by the stimulation and inhibition of gastric secretion in response to the presence of certain substances in the small intestine. The exact nature of the chemical stimulants or hormones responsible for this phase is not known, but they are absorbed from the intestine and can either stimulate or inhibit gastric secretion.

    4. Interdigestive Phase

    The “Interdigestive Phase” refers to the period between meals when the gastrointestinal tract is not actively engaged in the digestion and absorption of food. Despite fasting, the interdigestive phase involves regular intervals of hydrochloric acid secretion in both humans and dogs. This phase is influenced by both hormonal and nervous mechanisms, with the nervous component primarily mediated through the vagus nerve.

    Key points regarding the Interdigestive Phase are as follows:

    1. Regular Acid Secretion: During the interdigestive phase, there are periodic episodes of hydrochloric acid secretion, even in the absence of food intake. These episodes occur at regular intervals and have been observed in fasting individuals and animals. The exact mechanisms triggering this acid secretion are not fully understood, but it is known that they involve stimulation of the nucleus of the vagus nerve.
    2. Involvement of Hormonal and Nervous Mechanisms: The interdigestive phase of acid secretion is thought to be influenced by both hormonal and nervous mechanisms. Hormones likely play a role in regulating this process, but the specific hormones involved have not been identified. On the other hand, the nervous component is mediated through the vagus nerve, which carries signals from the brain to the gastrointestinal tract.
    3. Relationship to Intestinal Phase: There is a close relationship between the interdigestive phase and the intestinal phase of gastric secretion. It is believed that the interdigestive phase is actually a part of the intestinal phase, which encompasses the overall activity of the gastrointestinal tract during the fasting period. In other words, the interdigestive phase represents a specific aspect of the larger intestinal phase.
    4. Spontaneous Salivary Secretion: One potential contributing factor to the interdigestive phase is the spontaneous secretion of saliva. Saliva is produced by the salivary glands, and its secretion can occur independent of food intake. The exact role of saliva in the interdigestive phase and its impact on acid secretion during this period are not yet fully understood.

    In summary, the interdigestive phase is characterized by the periodic secretion of hydrochloric acid in the gastrointestinal tract during fasting. This phase involves both hormonal and nervous mechanisms, with the vagus nerve playing a prominent role. The interdigestive phase is closely related to the intestinal phase and may be influenced by factors such as spontaneous salivary secretion. Further research is needed to fully elucidate the mechanisms and significance of the interdigestive phase in gastrointestinal physiology.

    Gastrointestinal Hormones and Digestive Secretions

    Gastrointestinal hormones play a crucial role in regulating digestive secretions and processes. Here are the ten important gastrointestinal hormones and their functions:

    1. Gastrin: Secreted by gastrin cells in the pyloric region of the stomach, gastrin stimulates gastric glands to secrete and release gastric juice. It also promotes gastric mobility.
    2. Enterogastrone (Gastric Inhibitory Peptide – GIP): Secreted by the duodenal epithelium, enterogastrone inhibits gastric secretion and motility. It slows down gastric contractions, earning it the name gastric inhibitory peptide.
    3. Secretin: Secretin, the first hormone discovered, is produced by the epithelium of the duodenum. It stimulates the release of bicarbonates in pancreatic juice, increases bile secretion, and decreases gastric secretion and motility.
    4. Cholecystokinin Pancreozymin (CCK-PZ): Secreted by the epithelium of the small intestine, CCK-PZ stimulates the gallbladder to release bile and the pancreas to secrete digestive enzymes in pancreatic juice.
    5. Duocrmin: Produced by the duodenal epithelium, duocrmin stimulates the Brunner’s glands to release mucus and enzymes into the intestinal juice.
    6. Enterocrinin: Secreted by the epithelium of the entire small intestine, enterocrinin stimulates the crypts of Lieberkuhn to release enzymes into the intestinal juice.
    7. Vasoactive Intestinal Peptide (VIP): VIP is secreted by the epithelium of the entire small intestine. It dilates peripheral blood vessels in the gut and inhibits gastric acid secretion.
    8. Villikinin: Produced by the epithelium of the entire small intestine, villikinin accelerates the movement of villi, aiding in nutrient absorption.
    9. Somatostatin (SS): Somatostatin, secreted by the Delta cells of the pancreatic islets of Langerhans, inhibits the secretion of glucagon by alpha cells and insulin by beta cells. Somatostatin produced by argentaffin cells of gastric and intestinal glands suppresses the release of hormones from the digestive tract.
    10. Pancreatic Polypeptide (PP): Pancreatic polypeptide is secreted by the pancreatic polypeptide cells (also known as PP cells or F-cells) of the islets of Langerhans. It inhibits the release of pancreatic juice from the pancreas.
    Action of digestive hormones
    Action of digestive hormones

    These gastrointestinal hormones play vital roles in regulating the secretion of digestive juices such as gastric juice, pancreatic juice, and bile. They also influence gastric motility and other digestive processes, ensuring effective digestion and absorption of nutrients.

    FAQ

    What is the digestive system?

    The digestive system is a complex network of organs and tissues that work together to process food, extract nutrients, and eliminate waste from the body. It includes organs such as the mouth, esophagus, stomach, small intestine, large intestine, liver, gallbladder, and pancreas.

    What happens in the stomach during digestion?

    In the stomach, food is mixed with gastric juices containing enzymes and hydrochloric acid. This helps break down proteins and kill bacteria present in the food. The stomach’s muscular walls also aid in churning and mixing the food.

    What is the role of the small intestine in digestion?

    The small intestine is the primary site for nutrient absorption. Here, the partially digested food from the stomach is further broken down by enzymes from the pancreas and bile from the liver. The small intestine absorbs nutrients, such as carbohydrates, proteins, fats, vitamins, and minerals, into the bloodstream.

    What does the large intestine do?

    The large intestine, also known as the colon, absorbs water and electrolytes from the remaining undigested food. It also houses beneficial bacteria that help ferment indigestible fibers and produce certain vitamins. The waste material, known as feces, is formed in the large intestine and eliminated from the body through the rectum and anus.

    How does the liver contribute to digestion?

    The liver plays a vital role in digestion by producing bile, which is stored in the gallbladder and released into the small intestine when needed. Bile helps break down fats into smaller droplets, making them easier to digest and absorb.

    How does the digestive process begin?

    The digestive process begins in the mouth, where food is chewed and mixed with saliva. The enzymes present in saliva start breaking down carbohydrates.

    What is the function of the pancreas in digestion?

    The pancreas produces digestive enzymes that are released into the small intestine. These enzymes break down carbohydrates, proteins, and fats into simpler molecules that can be absorbed by the body. The pancreas also secretes insulin and glucagon, hormones that regulate blood sugar levels.

    How long does the digestion process take?

    The duration of digestion varies depending on the type of food consumed. On average, it takes about 24 to 72 hours for food to pass through the entire digestive system.

    What are some common digestive disorders?

    Common digestive disorders include gastroesophageal reflux disease (GERD), irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), peptic ulcers, gallstones, and constipation.

    How can I maintain a healthy digestive system?

    To maintain a healthy digestive system, it is important to eat a balanced diet rich in fiber, drink plenty of water, exercise regularly, manage stress, and avoid excessive alcohol consumption and smoking. It is also recommended to have regular check-ups with a healthcare provider and seek medical attention if any digestive symptoms or concerns arise.

    References

    • Image Source: https://en.wikipedia.org/wiki/Human_digestive_system
    • https://www.medicinenet.com/digestive_disease_myths_pictures_slideshow/article.htm
    • https://www.brainkart.com/article/Human-Digestive-System_39912/
    • https://www.turito.com/blog/biology/human-digestive-system
    • https://www.biologydiscussion.com/essay/human-digestive-system-with-diagram/1521
    • https://www.biologydiscussion.com/human-physiology/human-digestive-system/main-parts-of-human-digestive-system-with-diagram/52385
    • https://courses.lumenlearning.com/suny-ap2/chapter/overview-of-the-digestive-system/
    • https://basicmedicalkey.com/structure-and-function-of-the-digestive-system/
    • https://www.kenhub.com/en/library/anatomy/the-digestive-system
    • https://biologydictionary.net/digestive-system/
    • https://www.labxchange.org/library/items/lb:LabXchange:644a2c8b:html:1
    • https://www.saintlukeskc.org/health-library/anatomy-digestive-system

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