Circulatory System of Frog

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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

The heart functions as a muscle organ for pumping blood through the closed circulatory system.

Circulatory System of Frog
Circulatory System of Frog | Image Source:

1. External Structure of Frog’s Heart

  • Heart is located mid-ventrally in the region of the anterior trunk.
  • The pectoral girdle protects it.
  • It’s a reddish color.
  • It’s a bit triangular or conical in its shape.
  • It is characterized by its broad base that is directed to the anterior as well as the narrower apex is directed posteriorly.

a. Pericardium

  • The heart is housed in the sac that is made up of two membranes: an outer pericardium as well as an inner epicardium that covers the heart.
  • In between these membranes, a serous fluid, also known as pericardial fluid, is discovered, which helps to prevent friction, and keeps the heart humid.
  • It also shields the heart from shocks that come from outside.

b. Chambers of Heart

  • The heart appears to be an angular structure , with an anterior portion that is larger and an elongated posterior. The heart has three chambers.
  • The larger portion of the heart has two auricles or atriums while the posterior has one ventricle.
  • The auricle or atrium lies in front of the ventricle.
  • Both auricles are externally separated by a thin longitudinal groove inter-auricular groove.
  • The two auricles are clearly distinguished from the ventricle through the transverse coronary sulcus, or the auriculoventricular groove.
  • The ventricle is one of the most visible and prominent area within the heart. It’s a conical form with muscular walls that are thick.
  • Alongside the ventricles and auricles the heart also has two additional chambers: the sinus venosus and the truncus arterios.
    • Truncus arteriosus: Truncus arteriosus arises from the ventral upper right side of the ventricle as a cylindrical structure, which extends forward obliquely across the right auricle and finally divides into two branches or trunks, each further divides into three arches–carotid, systemic and pulmocutaneous.
    • Sinus venosus: Sinus veins are located on the posterior side that surrounds the heart. It is a thin, dark-colored, thin-walled somewhat triangular-shaped chamber made up of joining three veins of the caval, two anterior precavals, and one posterior postcaval.
study of circulatory system of frog
study of circulatory system of frog | Image Source:

2. Internal Structure

  • The heart’s internal structure is three-chambered, with 1 ventricles and 2 auricles.
  • The blood only flows in one direction, through different chambers.
  • The openings are protected by valves.

a. Auricles

  • Both auricles right and left are separated from one the other by a vertical thin inter-auricular septum.
  • The left auricle is smaller that the one on the left. Within the right auricle, close to the septum is an oval transverse opening known as sinu-auricular aperture , through which blood flows into the auricle via the sinus-venous. It is protected by two lip-like sinu-auricular valves, one of them arising from the dorsal side and the other one from the ventral.
  • The valves allow for the flow of blood to flow exclusively into the right auricle, but they block the backflow of blood.
  • The left auricle is, just in front of the sinu-auricular opening however close to the septum is a tiny opening in the pulmonary vein that has no valve.
  • The two auricles merge into a single ventricle through an auriculoventricular aperture that is divided by two auriculo-ventricular valves. One of them is located on the dorsal edge , and another from the ventral edge of the aperture.

b. Ventricle

  • The ventricle is the very prominent triangular chamber of the heart, with walls of muscle.
  • The inner surface of the ventricle has irregular ridges. The columnae carnae, with deep pockets in between that, to a certain extent, hinder the mixing of blood of both auricles. The ridges decrease the size of the ventricle’s lumen.
  • The flaps of the auriculoventricular valves are fixed to the ventricle’s wall through cordae tendinae.
circulatory system of frog diagram
circulatory system of frog diagram

c. Truncus Arteriosus

  • The upper right side of the ventricle is where a tubular truncus arteriosus originates from the heart. The entry is guarded by three semilunar valves, all of which have their edges facing in the direction of the truncus.
  • These valves are forced apart by the contraction of the ventricle, which allows blood to flow freely from the ventricle into the truncus but prevents blood from flowing back into the ventricle.
  • The conus arteriosus and the ventral aorta are the two components that make up the truncus arteriosus. The conus arteriosus is located at the base of the truncus arteriosus. Pylangium is the name given to the segment of the conus arteriosus that is located next to the ventricle, while synangium is the name given to the distal ventral portion.
  • In contrast to the synangium, which is produced by simply uniting the basal parts of the arteries, the pylangium is a very brief tubular construction.
  • The distal end of the pylangium, also known as the conus arteriosus, is equipped with a row of semilunar valves. These valves serve the purpose of delineating the line that separates the pylangium from the synangium.
  • The cavity of the conus arteriosus is incompletely divided into two passages: a dorsal and left cavum pulmocutaneum and a ventral and right cavum aorticum, both of which interact with the ventral aorta. One of these valves has been changed to produce a big spirally twisted spiral valve.
  • In front of the ventral aorta, the truncus arteriosus splits in half, and then each half splits again into a carotid arch, a systemic arch, and a pulmonary arch. These three arches make up the pulmonary arch.
  • An opening that leads to the pulmocutaneous arch can be found directly anterior to the spiral valve in the lung. This opening is protected by a pair of pulmocutaneous valves, one of which is dorsal and the other of which is ventral.
  • A short distance beyond the bifurcation of the truncus arteriosus, there are two apertures that go into systemic arches. Further anteriorly, there are two apertures that lead into carotid arches.
  • Sharma (1957) noted that Rana tigrina has just one common entrance for the systemo-carotid arches, and this opening is protected by a pair of systemo-carotid valves.

Working of the Heart

  • The sinu-auricular node, located in the wall of the sinus venosus, is a pacemaker that triggers cardiac contractions.
  • All four chambers of the heart contract in unison, beginning with the sinus venosus and ending with the truncus, since the muscles of the auricles are continuous with those of the ventricle.
  • The moment the sinus venosus contracts, the impure blood it contains is pushed through the sinu- auricular opening and into the right auricle.
  • Concurrently, the left auricle is supplied with blood from the pulmonary veins.
  • At nearly the same time, both auricles contract, forcing blood through the auriculo-ventricular opening and into the ventricle.
  • One theory a and one theory b can account for the circulation of blood via the heart.

The flow of blood in the heart is explained by two theories

a. Older View

  • Left ventricle receives oxygenated blood from the left auricle, whereas the right ventricle receives deoxygenated blood from the right auricle, as suggested by an ancient idea proposed by Brucke (1851) and supported by Sabatier (1873).
  • Due to the viscous nature of blood and the spongy texture of the ventricle caused by the existence of columnae cameae, mixing of the blood is prevented in the ventricle.
  • This means that there is a mingling of bloods in the centre, where the right side has impure blood while the left side has pure blood.
  • When the ventricle contracts, blood from the right side, where it is more contaminated, is pumped into the truncus arteriosus first.
  • Deoxygenated blood is delivered to the lungs and skin via the pulmo-cutaneous arches via their shared entrance, thanks to the strategic placement of the spiral valve in the conus arteriosus.
  • After then, the pulmo-cutaneous arches’ shared aperture is sealed up by a spiral valve.
  • The cavum aorticum then forces the oxygenated blood from the left side of the ventricle into the carotid arteries, while the deoxygenated blood from the right side of the ventricle is pumped into the systemic arteries.
  • In this way, the truncus arteriosus’s spiral valve controls blood flow to the numerous arches.
  • This theory is no longer accepted as to how blood travels from the ventricle to the aortic arch.

b. Modern View

  • Vandervael (1933) and Foxon (1953) proposed a novel idea positing that oxygenation of blood occurs not only in the lung but also in the skin and the buccal cavity, making it such that all blood entering the auricles is oxygenated.
  • This indicates that the blood entering the sinus venosus from the epidermis and the buccal cavity is as oxygenated as the blood entering from the lungs.
  • When the two auricle squeeze together, blood flows into the ventricle and is thoroughly mixed up.
  • The pulmocutaneous, systemic, and common carotid arteries all carry this mingled blood throughout the body.
  • The conus arteriosus relies on the spiral valve for support, although the valve itself plays no role in blood rotation.
  • Although it does not increase blood pressure, the carotid labyrinth serves a sensory function by picking up on shifts in oxygen saturation levels in the arterial circulation.
  • Most people now agree with the preceding description of how blood moves through the heart.
  • The carotid arches receive highly oxygenated blood, the pulmocutaneous arches receive less oxygenated blood, and the systemic arches receive mixed blood, as described by DeLong (1962).
  • There is a lack of insight about the particular process.
  • Pressure and blood flow are controlled by the heart’s peripheral nervous system.
  • Fibers in the vagus nerve act to reduce the heart rate, whereas sympathetic nerve fibres have the opposite effect and speed it up. 

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

It is a short vessel that moves forward and outward. It splits into two parts 

  1. External
  2. internal carotids.

a. External carotid

  • It also goes by the name lingual.
  • Blood gets to the tongue and other nearby parts through the smaller inner branch.

b. Internal carotid

  • It is the larger branch on the outside.
  • At its base, it makes a small lump called the carotid labyrinth or gland.
  • Its lumen is turned into a labyrinth by folding the walls.
  • The inside of the carotid labyrinth is made up of a network of small blood vessels that give it a sponge-like structure.
  • It’s likely a sense organ.
  • It controls the blood pressure in the carotid artery inside the neck.
  • The internal carotid artery splits into three different blood vessels:
    1. A palatine to the roof of mouth.
    2. A cerebral to the brain
    3. An ophthalmic to the eye

2. Systemic arch

  • This arch is the longest of the three.
  • It has the most blood flowing through it.
  • The dorsal aorta is made up of the two systemic arches that curve around the oesophagus and meet behind the heart.
  • Each systemic arch has 3 arteries coming out of it:
    • Oesophageal: It is a small artery that goes to the oesophagus.
    • It could be caused by the occipito-vertebral.
    • Occipito-vertebral: It sends an occipital branch to the back of the head (the occiput) and a vertebral branch to the spine and spinal cord right away.
    • Subclavian: This is a large artery that runs from the neck to the shoulder and into the arm as the brachial artery.
  • These arteries are bilaterally symmetrical.
  • But the left systemic arch has an extra branch called the oesophageal that doesn’t exist on the right side.

Dorsal aorta

The dorsal aorta is made when the two systemic arches join together. It goes back and lies in the middle of the back, just below the spinal column. The following branches come off of it:

  • Coeliaco-mesentric:  It is an artery with only one layer. It comes from where the two systemic arches meet. There are two main parts:
    1. Coeliac disease: to the liver, pancreas, and stomach
    2. Anterior mesenteric: to the spleen and intestine
  • Gonadial: It is a pair of short arteries that lead to the gonads. In male frogs, it is called spermatic, and in female frogs, it is called ovarian.
  • Renal:  As the dorsal aorta goes between the two kidneys, it sends 5 or 6 pairs of small renal arteries into each kidney.
  • Posterior mesenteric: It starts at the back end of the dorsal aorta or sometimes at the front of the mesenteric artery. It goes to the rectum or large intestine.
  • Common iliacs: In the end, the dorsal aorta splits in the back into two common iliacs. Each iliac nerve sends a femoral nerve to the hip and upper thigh, a sciatic nerve to the lower leg, and an epigastric nerve to the ventral body wall.

3. Pulmocutaneous arch

It’s made up of two parts: the Pulmonary artery and the Cutaneous artery.

  • Pulmonary artery: It brings deoxygenated blood from different parts of the body to the lungs.
  • Cutaneous artery: It brings blood with oxygen to the skin. 

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

  • Blood that has been oxygenated by the lungs travels through the right and left pulmonary veins.
  • The left dorsal auricle receives blood from this group of veins via the common pulmonary vein.

2. Caval veins

Three major veins, the two anterior venae cavae and the one posterior vena cava, all of which empty into the sinus venosus, carry deoxygenated blood from the rest of the body back to the heart.

  • Anterior venae cavae or precavals: Blood from the anterior region of the body is drained into the right and left precavals, also known as the anterior venae cavae. It is the simultaneous meeting of three main veins that creates each precaval:
  • External jugular: The external jugular is made up of two thin, winding structures: the lingual from the tongue and the mandibular from the outside of the jaw.
  • Innominate: lingual from the tongue and the mandibular from the outside of the jaw. Internal jugular from the skull and orbits and the subscapular from the shoulder and back of the arm meet to produce the innominate.
  • Subclavian: created by the musculocutaneous side of the head and body and the brachial forearm.
  • Posterior vena cava or postcaval: The solitary, massive, and dark-colored postcaval is part of the superior vena cava system. It’s located below the aortic arch, or dorsally. There are 5-6 pairs of renal veins that drain blood into its posterior end, which is created between the two kidneys. Another pair of veins supply it, this time from the gonads (spermatic in males and ovarian in females) or the anterior pair of renal veins. The postcaval then continues onward, dorsally, toward the liver. It opens into the posterior angle of the sinus venosus after taking in two short, stocky hepatic veins.

3. Renal portal system

  • Frog’s portal systems are well made:
    1. renal portal
    2. hepatic
  • These portal systems have a strange link between them.
  • The renal portal system is made up of the veins that carry blood to the capillary system in the kidneys.
  • Each back leg has two veins that collect blood: the femoral vein on the outside and the sciatic vein on the inside.
  • The femoral splits into a dorsal renal portal and a ventral pelvic vein as it goes into the abdominal cavity.
  • The renal portal and the sciatic nerve come together.
  • It gets blood from the lumbar area through a dorso-lumbar vein, which runs along the outside edge of the kidney on its side.
  • The renal portal vein comes into the kidney through several branches that split into capillaries.
  • When the veins in both sides of the pelvis come together, they make a median ventral vein or an anterior abdominal vein.
  • It gets blood from the urinary bladder and the wall of the lower abdomen.
  • The blood then goes to the liver, where it splits into capillaries.
  • Before going into the liver, the anterior abdominal and hepatic portal veins are linked by a small loop. 

4. Hepatic portal system

  • Branches from the stomach, intestine, spleen, and pancreas come together to make a large portal vein in the liver.
  • It takes the blood from the digestive tract, which is full of digested food, to the liver, where it is broken up into capillaries.
  • In the area of the liver, where the liver is, the anterior abdominal vein is linked to the hepatic portal vein. 

Blood of frog

  • Blood is the main fluid that moves through the body.
  • In reality, it is a liquid connective tissue.
  • The clear liquid inside is called plasma.
  • Blood corpuscles are different kinds of free cells that float in plasma.


  • Plasma makes up almost two-thirds of the blood.
  • It is mostly water (90%) that dissolves mineral salts, absorbed foods (sugars, proteins), wastes from the body (urea), hormones, and other soluble substances.


Most blood cells are made in the bone marrow and spleen. The spleen also kills old cells. There are three main types of blood cells, or corpuscles:

  1. Erythrocytes or red blood corpuscles (RBC)
  2. Leukocytes or white blood corpuscles (WBC)
  3. Thrombocytes or blood platelets
circulatory system of frog diagram
circulatory system of frog diagram | Image Source:

a. Erythrocytes or red blood corpuscles (RBC)

  • These are 14 by 23 micrometres, oval, flattened, nucleated, and biconvex.
  • Between 250,000 and 450,000 of them live in every cubic millimetre of blood.
  • They have the red blood cell pigment haemoglobin in them.
  • Hemoglobin is a protein that is yellow to red and has iron in it.
  • Hemoglobin’s job is to carry oxygen to tissues through chemical bonds.

b. Leukocytes or white blood corpuscles (WBC)

  • They have no colour, a nucleus, and mostly look like amoebae.
  • There are at least five kinds of it. On average, there are 5,000 to 7,000 of them per cubic millimetre.
  • Most of them are phagocytic, which means they eat bacteria and other foreign particles that are in the blood.
  • They also take out old or dead cells.
  • There are lymphocytes, monocytes, and granulocytes in a frog’s white blood cells.
  • There are three kinds of granulocytes: neutrophils, eosinophils, and basophils.

c. Thrombocytes or blood platelets

  • These are small spindle cells with a nucleus.
  • They are very important to the clotting process.
  • When a blood vessel is hurt, thrombocytes break apart and release an enzyme called thrombin.
  • It changes soluble fibrinogen in the blood plasma into insoluble fibrin. This makes the clot that seals the wound and stops any more blood loss.

Lymphatic system of Frog

  • Frogs have a more basic lymphatic system than higher vertebrates. It is made up of lymph, lymph vessels, lymph hearts, lymph spaces, and the spleen.

1. Lymph

  • Confined to the veins and arteries, the blood never meets the cells and tissues of the body face to face.
  • Tissue fluid, also known as lymph, is formed when blood is regularly filtered through capillaries into intercellular spaces.
  • Lymph washes across the tissues and acts as a natural lubricant for many of the body’s organs.
  • As a byproduct of blood processing, it mimics leukocyte-rich plasma in many ways.
  • However, it does not contain red blood cells or other blood components. It acts as a go-between, transporting nutrients to cells and waste products back to the circulatory system.
  • Lymph is the fluid that continuously drains out of our tissues through our lymphatic system.

2. Lymph vessels

  • Lymph vessels, also called lymphatics, are networks of fine-walled, fragile blood vessels that range in size but are often invisible to the naked eye.
  • When lymph capillaries join together, they produce bigger vessels that flow into the venous system, bringing the lymph back into the bloodstream.

The peritoneum has some tiny holes that allow lymph vessels to pass through.

3. Lymph hearts

  • At frogs, lymph arteries drain into veins in four different locations.
  • Lymph vessels, often known as lymph hearts, originate at each of these orifices.
  • The lymph heart is a tiny, contracting sac that gently pumps lymph into veins.
  • For this reason, frogs have a total of four lymph hearts.
  • One set of veins leads into the subscapular veins, which are located anteriorly below the scapulae.
  • The femoral veins are reached by a second set of veins that lie behind the urethra, on each side of the urostyle’s tip.

4. Lymph spaces

  • Some lymph veins in frogs and toads get dilated to produce enormous lymph channels or sinuses, which are not seen in other animals.
  • Having large dorsal, lateral, abdominal, etc. subcutaneous gaps makes the skin flimsily linked.
  • There are septa made of connective tissue separating these areas. Sub-vertebral lymph space above kidneys is a particularly crucial one.

5. The spleen

  • The spleen is a tiny, spherical, and dark crimson gland. It is close to the rectum in the mesentery.
  • Lymphatic tissues are concentrated mostly there.
  • It eliminates old red blood cells and replaces them with antibodies, fresh RBCs, and phagocytic lymphocytes.


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