Frogs are amphibians that live on land and in water. Amphibians are the earliest group of chordates to live outside of water. Frogs are coldblooded vertebrate tetrapods.

Classification of Rana tigrina (Frog)

• Phylum – Chordata
• Subphylum – Gnathostomata
• Superclass- Tetrapoda
• Class- Amphibia
• Order- Anura
• Genus- Rana
• Species- tigrina

Habits & Habitats in frogs

They are multi-cultural in distribution. They are typically found in freshwater ponds, ditches, rivers under-stones and humid areas, except for desert areas of the country. They are active in the seasons of rain and spring and then become inactive in the summer.

Indian Frog is also known by the name of “Indian bullfrog” due to its huge size and has a loud voice.

Characteristics of Rana tigrina

Frogs typically have smooth skin, robust hind legs that can leap with webbed feet. They reproduce mostly in water, by laying eggs that grow to larvae (tadpoles). The following behaviors are observed:

• Resting:  The majority of frogs rest on the ground in a squatting posture. in this posture, they keep their forelimbs, which are short, up straight, and their hind limbs, which are longer , are folded over the body. If they encounter any disturbance, they jump into the water with their hind limbs in order to get away from their adversaries.
• Feeding habit: The frog is a carnivore animal, Their food sources are earthworms, insects as well as snails, spiders and Tadpoles. Tadpoles feed on aquatic plants because they are herbivores.
• Aestivation (Summer sleep):  As frogs are cold-blooded Their body temperature changes depending on the conditions and, therefore, they are unable to survive in harsh conditions. Frogs remain in Aestivation (summer rest) to get through the difficult conditions. They lie in the soft mud, and then become lazy and inactive.
• Hibernation (Winter sleep): The winter months, frogs sink themselves in deep mud and take rest. This is known as hibernation or winter sleep.
  • In winter, metabolism goes slow , and the temperature drops to a minimum.
  • Because of a drop in temperature and a slow body activity, the frogs are lazy and inactive.
  • Frogs don’t respire through the lungs, they respire via the skin.
  • The general vital functions slow down in order to conserve energy and prolong the life.
  • In this period the frogs don’t eat food , but instead use the energy stored in glycogen in the body.

Circulatory system of Frog

• The blood vascular system consists of three chambers in the heart as well as blood vessels and blood. The heart is protected by a membrane with two walls known as pericardium.
• There are two thin-walled anterior chambers, known as auricles (Atria) and one deep posterior chamber with a thick wall called ventricle.
• Sinus venosus can be described as a massive thin walled triangular chamber that is located on the dorsal part of the heart.
• Truncus arteriosus is a large walled , cylindrical structure that is located obliquely on the side of the heart’s ventral. It originates from the ventricle and is divided into left and right the aortic trunk. It is later divided in three arches, including carotid, systemic, and the pulmo-cutaneous.
• The Carotid trunk is a source of blood for the anterior part of the body.
• The Systemic trunk on each side is connected laterally to form the dorsal Aorta. They provide liquid to the anterior region in the human body.
• The Pulmo-cutaneous trunk is a source of blood for the lungs and the skin.
• Sinus venosus is able to receive the deoxygenated blood of the body’s parts via two anterior precaval veins as well as one post-caval vein. It transfers the liquid to the right auricle simultaneously; the left auricle is receiving oxygenated blood via the vein called the pulmonary.
• The blood is composed of plasma (60%) and blood cells (40%)which include white blood cells and platelets. RBCs are stuffed with red pigment. They are nucleated, with an oval shape. Leukocytes are nucleated and circular in form.

Frog’s Heart Structure and physiology

1. External Structure of Frog’s Heart

The heart of a frog is a vital organ responsible for pumping blood throughout its body. Unlike mammalian hearts, the frog’s heart has some unique external features that make it distinct. Here’s a detailed and sequential breakdown of its external structure, characteristics, and functions:

1. Introduction to the Frog’s Heart:
   • The heart is centrally located in the anterior trunk region of the frog.
   • It is safeguarded by the pectoral girdle and exhibits a reddish hue.
2. General External Features:
   • The heart’s shape is somewhat conical or triangular.
   • It possesses a broad base directed anteriorly and a narrow apex pointing posteriorly.
3. Pericardium:
   • The heart is encased within the pericardium, a thin, transparent, two-layered sac.
   • The outer layer is known as the parietal pericardium.
   • The inner layer, which closely envelops the heart, is termed the visceral pericardium.
4. Chambers of the Heart:
   • The frog’s heart comprises three primary chambers.
     • Two Atria or Auricles:
       • These are dark-colored and positioned anteriorly.
       • A faint longitudinal inter-auricular groove externally demarcates the two auricles.
     • One Ventricle:
       • This chamber is pink-colored, conical, and situated posteriorly.
       • A distinct transverse auriculo-ventricular groove or coronary sulcus separates the auricles from the ventricle.
   • Besides these primary chambers, two additional chambers are present:
     • Sinus Venosus:
       • A dark-colored, thin-walled, triangular chamber attached dorsally to the heart.
     • Truncus Arteriosus:
       • A tubular chamber originating anteriorly from the ventricle’s right ventral side.
       • It bifurcates into two branches, each further dividing into three arches: carotid, systemic, and pulmocutaneous.

2. Internal structure of frog’s heart

The heart of a frog, while externally similar to other vertebrates, possesses a unique internal structure that facilitates its amphibious lifestyle. This expository breakdown provides a detailed look into the intricacies of the frog’s heart, emphasizing its functions and characteristics.

1. Auricles:
   • The heart comprises two auricles: left and right.
   • These auricles are thin-walled and separated by a vertical inter-auricular septum.
   • The left auricle is notably smaller than the right.
   • The sinus venosus connects to the right auricle’s dorsal wall via the sinu-auricular aperture, which is guarded by flap-like valves.
   • Similarly, the common pulmonary vein connects to the left auricle near the septum.
2. Ventricle:
   • The ventricle boasts a thick, muscular, and spongy wall.
   • Its inner surface features columnae carnae or trabeculae and fissures, which significantly reduce the ventricle’s cavity.
   • Chordae tendineae, thread-like structures, anchor the flaps of the auriculo-ventricular valves to the ventricle’s wall.
3. Truncus Arteriosus:
   • The ventricle’s opening into the truncus arteriosus is protected by three semilunar valves, preventing blood backflow.
   • The truncus arteriosus has a spirally twisted cavity, divided by another set of semilunar valves into:
     • Conus Arteriosus or Pylangium: A long, basal section with thick walls.
     • Bulbus Aorta or Synangium: A shorter, distal section with thin walls.
   • The conus or pylangium’s cavity is further divided by a large, twisted longitudinal spiral valve into a left dorsal cavum pulmocutaneum and a right ventral cavum aorticum.
   • The spiral valve is dorsally attached, with its ventral edge free.
   • Various openings in the conus and synangium connect to different arches and are equipped with valves for protection.

Working of the Heart

The heart, a vital organ in the circulatory system, plays a pivotal role in pumping blood throughout the body. Its intricate workings, regulated by the nervous system, ensure the continuous and efficient flow of blood. This expository piece delves into the heart’s functioning, emphasizing its structure, sequence, and the role of various chambers.

1. Heart’s Mechanism:
   • The heart’s contraction phase is termed “systole,” while its relaxation phase is called “diastole.”
   • The heart’s chambers contract in a systematic sequence, with valves ensuring unidirectional blood flow.
   • The sinus venosus contracts, pushing non-oxygenated venous blood into the right auricle.
   • Simultaneously, oxygenated blood from the lungs enters the left auricle.
2. Conventional View:
   • The right side of the ventricle contains deoxygenated blood from the right auricle.
   • The left side holds oxygenated blood from the left auricle, with some mixed blood centrally.
   • Blood mixing is minimized due to its viscosity and the ventricle’s spongy nature.
   • The ventricle’s sponginess is attributed to the columnae carneae network.
   • As the ventricle contracts, deoxygenated blood flows into the truncus arteriosus, directed by the spiral valve to the pulmocutaneous arches for oxygenation.
3. Modern Perspective:
   • Recent studies suggest that the ventricle and truncus contain completely mixed blood.
   • Blood from the skin and buccal cavity is more oxygenated than that from the lungs.
   • The inter-auricular septum and spiral valve in frogs might be functionless.
   • However, some findings indicate varying oxygenation levels in different arches, leaving the exact mechanism still debated.
4. Regulation of Heart Function:
   • Extrinsic nerves regulate blood flow and pressure in the heart.
   • Vagus nerve fibers slow down the heart’s beat, while sympathetic nerve fibers accelerate it.

Arterial system of frog

• All parts of the body get oxygenated blood from the heart through the arteries.
• Truncus arterious sends out two branches: the right aortic trunk and the left aortic trunk. Each of these two trunks sends out three branches:
  1. The Carotid Arch
  2. Arching system
  3. Pulmocutaneous arch

1. Common carotid arch

The common carotid arch of the frog is a crucial component of its circulatory system, playing a pivotal role in the transportation of blood to vital regions of the amphibian’s body. This short vessel, characterized by its forward and outward trajectory, bifurcates into two distinct branches: the external carotid and the internal carotid.

External Carotid: Also referred to as the lingual, the external carotid is the smaller inner branch. Its primary function is to transport blood to the tongue and its adjacent parts. Therefore, it is integral for the frog’s oral functions and overall health.

Internal Carotid: The internal carotid, on the other hand, is the larger outer branch. At its base, it exhibits a slight swelling, known technically as the carotid labyrinth or gland. An intriguing feature of this carotid is the transformation of its lumen into a labyrinthine structure due to the folding of its walls. This intricate design results in the formation of a network of tiny vessels within the inner cavity of the carotid labyrinth, giving it a spongy appearance. Besides its structural intricacy, the carotid labyrinth is believed to serve as a sense organ. Its primary function is to regulate blood pressure within the internal carotid artery, ensuring optimal blood flow.

Then, as we delve deeper into the anatomy of the internal carotid artery, we find that it further divides into three branches, each with its specific function:

1. Palatine Branch: This branch is responsible for supplying blood to the roof of the mouth. Its role is vital for the frog’s oral health and functionality.
2. Cerebral Branch: As the name suggests, this branch directs blood to the brain, ensuring the frog’s neurological functions remain optimal.
3. Ophthalmic Branch: Catering to the visual needs of the frog, this branch channels blood to the eye, playing a pivotal role in maintaining the frog’s visual acuity.

2. Systemic arch

• Introduction: The systemic arch is a vital component of the frog’s circulatory system. It is responsible for the extensive distribution of blood throughout the body.
• Structure and Distribution:
  • The systemic arch is the longest among the three arches present in the frog’s heart.
  • The two systemic arches curve dorsally around the oesophagus and converge behind the heart to form the dorsal aorta.
  • Each systemic arch gives rise to three primary arteries:
    1. Oesophageal: A small artery directed towards the oesophagus. Occasionally, it may originate from the occipito-vertebral artery.
    2. Occipito-vertebral: This artery bifurcates into the occipital branch, supplying the posterior part of the head, and the vertebral branch, catering to the vertebral column and spinal cord.
    3. Subclavian: A significant artery that provides blood to the shoulder region and extends into the forelimb as the brachial artery.
• Dorsal Aorta:
  • Formed by the union of the two systemic arches, the dorsal aorta runs posteriorly, positioned mid-dorsally beneath the vertebral column.
  • It gives off several branches, including:
    • Coeliaco-mesentric: A singular artery branching into the coeliac (supplying the stomach, pancreas, and liver) and the anterior mesenteric (directed to the spleen and intestine).
    • Gonadial: A pair of short arteries, termed spermatic in males and ovarian in females, supplying the gonads.
    • Renal: As the dorsal aorta passes between the kidneys, it gives off 5-6 pairs of renal arteries to both kidneys.
    • Posterior mesenteric: Originating from the dorsal aorta’s posterior end or sometimes from the anterior mesenteric, it supplies the large intestine or rectum.
    • Common iliacs: The dorsal aorta eventually bifurcates into two common iliacs, each supplying the epigastric (ventral body wall), recto-vesicular (rectum and urinary bladder), femoral (hip and upper thigh), and sciatic (lower leg) arteries.

3. Pulmocutaneous arch

• Introduction: The pulmocutaneous arch is an integral component of the frog’s circulatory system. It is specifically designed to cater to the respiratory needs of the frog, given its unique respiratory mechanisms involving both lungs and skin.
• Bifurcation and Distribution:
  • The pulmocutaneous arch divides into two primary arteries:
    1. Pulmonary Artery: This artery is directed towards the lungs. Given that frogs have simple lungs, this artery ensures the efficient supply of deoxygenated blood for oxygenation.
    2. Cutaneous Artery: This artery is responsible for supplying blood to the skin. Frogs have a unique capability to respire through their skin, especially when they are underwater or in damp environments. The cutaneous artery ensures that the skin receives an adequate blood supply to facilitate this cutaneous respiration.
• Further Branching:
  • The cutaneous artery further divides into several branches, including:
    • Auricularis: This artery supplies blood to various structures such as the tympanum (eardrum), thymus gland, lower jaw, pharynx, and the hyoid apparatus. These structures play vital roles in the frog’s auditory and respiratory systems.
    • Dorsalis: As the name suggests, this artery is responsible for supplying blood to the skin on the dorsal (back) side of the frog. The dorsal skin is essential for cutaneous respiration, especially when the frog is submerged in water.
    • Lateralis: This artery caters to the skin on the lateral (side) parts of the frog’s body. Like the dorsalis, it plays a crucial role in ensuring the skin receives an adequate blood supply for respiration.

The venous system of the frog

Veins, or the system of blood vessels via which blood travels back to the heart, are a part of the venous system. There are four distinct components to examine while looking at the frog:

1. Pulmonary veins
2. Caval veins
3. Renal portal veins
4. Hepatic portal veins

1. Pulmonary veins

The pulmonary veins are integral components of the circulatory system, playing a pivotal role in transporting oxygenated blood from the lungs to the heart. Here’s a detailed and sequential breakdown of their structure, characteristics, and functions:

1. Introduction to Pulmonary Veins:
   • Pulmonary veins are specialized blood vessels responsible for carrying oxygen-rich blood.
   • Their primary function is to transport blood that has been oxygenated in the lungs back to the heart.
2. Structural Characteristics:
   • There are two main pulmonary veins: the right and left pulmonary veins.
   • These veins are tasked with collecting oxygenated blood from the two lungs.
   • They are strategically positioned to ensure efficient blood flow from the lungs to the heart.
3. Formation and Connection to the Heart:
   • The right and left pulmonary veins converge.
   • This union results in the formation of a common pulmonary vein.
   • This common pulmonary vein has a direct opening into the left auricle.
   • Significantly, this opening is located on the dorsal side of the left auricle.
4. Function and Significance:
   • The primary function of the pulmonary veins is to ensure that oxygen-rich blood is efficiently transported from the lungs to the heart.
   • This process is vital for the overall oxygenation of the body, ensuring that oxygenated blood is available for distribution to various body tissues and organs.
   • Therefore, the pulmonary veins play a crucial role in maintaining the oxygenation balance and overall health of the organism.

2. Caval veins

The caval veins are essential components of the circulatory system, responsible for transporting deoxygenated blood back to the heart. Their intricate structure and strategic positioning ensure the efficient return of blood from various parts of the body. Here’s a detailed and sequential breakdown of their structure, characteristics, and functions:

1. Introduction to Caval Veins:
   • Caval veins are specialized vessels that carry deoxygenated blood.
   • Their primary function is to transport this blood from different parts of the body back to the heart.
2. Structural Characteristics:
   • Deoxygenated blood from the body is channeled towards the heart through three significant vessels: two anterior venae cavae and a single posterior vena cava.
   • All three vessels eventually open into the sinus venosus.
3. Anterior Venae Cavae or Precavals:
   • The right and left precavals gather venous blood from the body’s anterior part.
   • Each precaval is birthed from the union of three major veins:
     • External Jugular: Formed by the slender lingual from the tongue and the mandibular from the outer margin of the lower jaw.
     • Innominate: Originates from the internal jugular (serving the cranial cavity and orbit) and the subscapular (catering to the shoulder and back of the arm).
     • Subclavian: This vein is the result of the union of the brachial (from the forelimb) and the musculo-cutaneous (from the side of the body and head).
4. Posterior Vena Cava or Postcaval:
   • The postcaval is a singular, prominently dark-colored vein.
   • Positioned ventrally to the dorsal aorta, its posterior end is situated between the kidneys, drawing blood through 5-6 pairs of renal veins.
   • Additionally, it receives blood from a pair of genital veins. In males, these are termed spermatic, while in females, they are called ovarian. These veins can either connect directly or through the anterior pair of renal veins.
   • As the postcaval progresses forwards, it runs dorsally to the liver. Before its culmination into the sinus venosus, it is joined by a pair of stout hepatic veins.

In conclusion, the caval veins, with their intricate network and specific structure, play a pivotal role in the circulatory system. Their primary function of transporting deoxygenated blood back to the heart is vital for maintaining the body’s physiological balance and overall health.

3. Renal portal system

The renal portal system is a specialized component of the circulatory system in frogs, playing a pivotal role in the transportation of blood to the kidneys. This system, along with other portal systems, showcases an intricate network of interconnected veins. Here’s a detailed and sequential breakdown of its structure, characteristics, and functions:

1. Introduction to the Renal Portal System:
   • Frogs possess well-developed portal systems, notably the renal and hepatic systems.
   • These systems are intriguingly interconnected, ensuring efficient blood flow.
2. Structural Characteristics:
   • The renal portal system is defined by the veins that transport blood to a capillary system within the kidneys.
   • Blood from each hindleg of the frog is gathered by two primary veins:
     • Femoral (Outer): Upon entering the abdominal cavity, it bifurcates into a dorsal renal portal and a ventral pelvic vein.
     • Sciatic (Inner): This vein merges with the renal portal.
3. Formation and Pathway:
   • The renal portal, while traversing along the kidney’s outer border, receives blood from the lumbar region through a dorso-lumbar vein.
   • This renal portal vein then penetrates the kidney, branching out into multiple capillaries.
   • The pelvic veins from both sides converge to form a median ventral or anterior abdominal vein.
   • This vein collects blood from the urinary bladder and the ventral abdominal wall.
   • Subsequently, it advances towards the liver, where it disperses into capillaries.
4. Interconnection with Hepatic Portal System:
   • Before entering the liver, the anterior abdominal and hepatic portal veins form a small loop, establishing a connection.
   • This interconnected system ensures that the blood is efficiently channeled to the liver for further processing.
5. Function and Significance:
   • The primary function of the renal portal system is to transport blood to the kidneys for filtration.
   • This system plays a crucial role in maintaining the physiological balance of the frog by ensuring that waste products are efficiently filtered out.
   • Additionally, the interconnected nature of the renal and hepatic portal systems underscores their collective importance in the frog’s overall circulatory dynamics.

4. Hepatic portal system

The hepatic portal system is a specialized component of the circulatory system, playing a pivotal role in transporting nutrient-rich blood from the alimentary canal to the liver. This system ensures that the liver receives blood loaded with digested foodstuffs for further processing and detoxification. Here’s a detailed and sequential breakdown of its structure, characteristics, and functions:

1. Introduction to the Hepatic Portal System:
   • The hepatic portal system is a unique vascular network.
   • Its primary function is to channel blood from the alimentary canal to the liver.
2. Structural Characteristics:
   • A prominent hepatic portal vein is formed through the convergence of multiple branches.
   • These branches originate from various organs, including the stomach, intestine, spleen, and pancreas.
3. Formation and Pathway:
   • The hepatic portal vein is the result of the union of several branches.
   • This vein is responsible for transporting blood that is heavily laden with digested foodstuffs.
   • Upon reaching the liver, the hepatic portal vein disperses into a network of capillaries.
4. Connection with the Anterior Abdominal Vein:
   • A significant connection exists between the hepatic portal vein and the anterior abdominal vein.
   • This connection is established in the liver’s region, ensuring a seamless flow of blood between the two systems.
5. Function and Significance:
   • The primary function of the hepatic portal system is to transport nutrient-rich blood to the liver.
   • The liver then processes these nutrients, synthesizes essential proteins, and detoxifies potential harmful substances.
   • Therefore, the hepatic portal system plays a crucial role in maintaining the body’s metabolic balance and overall health.

Blood of frog

The blood of a frog is a vital circulatory fluid, playing a central role in transporting nutrients, oxygen, and waste products throughout the body. This complex fluid comprises various components, each with its unique structure and function. Here’s a detailed and sequential breakdown of the composition, characteristics, and functions of frog blood:

1. Composition of Blood:
   • Blood consists of a clear liquid known as plasma.
   • Suspended within the plasma are various types of cells, referred to as blood corpuscles.
2. Plasma:
   • Plasma constitutes approximately 2/3rd of the blood volume.
   • It is predominantly water (about 90%) and contains dissolved substances such as mineral salts, absorbed nutrients (like sugars and proteins), waste products (like urea), hormones, and other solubles.
3. Corpuscles:
   • Blood corpuscles are primarily of three types:
     • Erythrocytes (RBCs):
       • Oval, flattened, nucleated, and biconvex in shape.
       • Size ranges from 14 by 23 micrometers.
       • Their count varies between 250,000 to 450,000 per cubic millimeter of blood.
       • They contain the respiratory pigment called hemoglobin, which is yellow to red and iron-rich.
       • Hemoglobin’s primary function is to transport oxygen to tissues.
     • Leucocytes (WBCs):
       • Colorless, nucleated, and mostly amoeboid cells.
       • They exist in at least five distinct types.
       • Their count averages between 5,000 to 7,000 per cubic millimeter.
       • Many are phagocytic, consuming bacteria and other foreign particles in the blood.
       • They also help in the removal of dead or old tissue cells.
       • Types of WBCs in frogs include lymphocytes, monocytes, and granulocytes (neutrophilic, eosinophilic, and basophilic).
     • Thrombocytes:
       • Small, nucleated, spindle-shaped cells.
       • They play a pivotal role in blood coagulation.
       • When a blood vessel is injured, thrombocytes release an enzyme called thrombin, which transforms the soluble fibrinogen in blood plasma into insoluble fibrin, forming a clot to prevent further blood loss.
4. Production and Regulation:
   • Blood cells are primarily produced in the bone marrow and spleen.
   • The spleen also plays a role in eliminating worn-out cells.
5. Lymphatic System:
   • The frog’s lymphatic system is more rudimentary compared to higher vertebrates.
   • It comprises lymph, lymph vessels, lymph hearts, lymph spaces, and the spleen.

Lymphatic system of Frog

The lymphatic system in frogs plays a pivotal role in maintaining the fluid balance within the body and facilitating the transportation of nutrients and waste products. This system comprises various components, each with its unique structure and function. Here’s a detailed and sequential breakdown of the composition, characteristics, and functions of the frog’s lymphatic system:

1. Introduction to the Lymphatic System:
   • The lymphatic system is essential for ensuring that blood, confined within vessels, interacts indirectly with body cells and tissues through the medium of lymph.
2. Lymph:
   • Lymph originates as a filtrate from blood capillaries, accumulating in intercellular spaces as tissue fluid.
   • This fluid bathes tissues and lubricates many internal organs.
   • Lymph closely resembles plasma but lacks erythrocytes and certain blood proteins.
   • It serves as an intermediary, delivering nutrients and oxygen to cells and transporting waste materials to the blood.
3. Lymph Vessels:
   • These are delicate, thin-walled vessels of varying sizes that form intricate networks.
   • Lymph capillaries merge to form larger vessels, which eventually open into the venous system, reintroducing lymph back into the blood.
   • Some peritoneal openings communicate directly with lymph vessels.
4. Lymph Hearts:
   • Lymph vessels open into veins at four distinct locations in frogs, forming what are termed as lymph hearts at each opening.
   • These hearts are rhythmically contractile sacs that methodically pump lymph into veins.
   • Frogs possess two pairs of these lymph hearts: one anterior pair below the scapulae (opening into subscapular veins) and another posterior pair near the urostyle’s tip (opening into femoral veins).
5. Lymph Spaces:
   • In frogs, certain lymph vessels expand to form extensive lymph channels or sinuses.
   • There are vast subcutaneous spaces beneath the skin, separated by connective tissue septa, making the skin loosely attached.
   • A notable lymph space is the sub-vertebral lymph space located above the kidneys.
6. Spleen:
   • The spleen is a small, round, dark-red gland situated in the mesentery near the rectum.
   • It houses the most substantial lymphatic tissue mass in the body.
   • The spleen has multiple functions: it destroys old erythrocytes, produces antibodies, generates new erythrocytes, and produces phagocytic lymphocytes.