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Home » Circulatory System of Frog Parts and Functions

Circulatory System of Frog Parts and Functions

  • Zoology

In this circulatory system of frog parts and functions post external structure of circulatory system of frog, internal structure of circulatory system of frog, and working of the circulatory system of frog.

Circulatory system of frog is closed. The heart, blood arteries, blood, and lymphatic systems are all included. The primary role of this system is to distribute digested food and oxygen to various regions of the body, releasing energy for daily activities and transporting excretory and gaseous wastes to elimination organs, such as the kidneys and lungs.

Circulatory System of Frog


The heart is a muscular pumping organ that pushes blood into the circulatory system of frog, which is closed.

Circulatory System of Frog

Circulatory System of Frog: Heart 

External Structure

Circulatory system of frog heart is a dark red conical muscular organ located ventrally to the liver in the pericardial cavity at the level of the forelimbs along the mid-ventral line.

(a) Pericardium

The heart is surrounded by a sac made up of two membranes, an outer pericardium and an inner epicardium, which closely surrounds the heart. A serous or pericardial fluid is found between these two membranes, which prevents friction and keeps the heart moist. It also shields the heart from external shocks.

(b) Chambers

The heart appears to be an angular structure with a broad anterior portion and a narrow posterior portion. The heart has three chambers. The larger part of the heart has two atriums or auricles, while the smaller part has a single ventricle. The atrium, also known as the auricle, is located anterior to the ventricle. A faint longitudinal inter-auricular groove separates both auricles on the outside.

A transverse coronary sulcus or auriculo-ventricular groove, however, clearly separates the two auricles from the ventricle. The ventricle is the most visible and noticeable part of the heart. It has a conical shape and thick muscular walls. Aside from the auricles and ventricles, the heart has two additional chambers: the truncus arteriosus and the sinus venosus.

Truncus arteriosus arises as a cylindrical structure from the ventral upper right side of the ventricle, extends forward obliquely across the right auricle, and finally divides into two branches or trunks, each of which divides into three arches carotid, systemic, and pulmocutaneous. The sinus venosus is located on the heart’s dorsal surface. It is a dark-colored, thin-walled chamber formed by the union of three caval veins, two anterior, precaval, and one posterior, postcaval.

Circulatory System of Frog

Circulatory System of Frog: Heart External Structure

Internal Structure

Internally the heart is three-chambered with two auricles and one ventricle. In the circulatory system of frog blood flows only in one direction through various chambers. Their openings are guarded by valves.

(a) Auricles

The right and left auricles are separated by a thin vertical inter-auricular septum. The left ear is smaller than the right. The sinu-auricular aperture is a transverse oval opening in the right auricle near the septum through which blood enters the auricle from the sinus-venous system. It is protected by two lip-like sinu-auricular valves, one on the dorsal edge and one on the ventral.

These valves allow blood to flow freely into the right auricle but prevent it from flowing backward. A small pulmonary vein opening with no valve is located in the left auricle, slightly anterior to the sinu-auricular aperture but close to the septum. An auriculo-ventricular aperture connects the two auricles to form a single ventricle. This aperture is bounded by two pairs of auriculo-ventricular valves, one arising from the dorsal edge and the other from the ventral edge.

(b) Ventricle

The ventricle is a prominent triangular chamber of the heart that has muscular walls. Its inner surface has irregular ridges, the columnae carnae, with deep pockets between them, which help to prevent blood from the two auricles from mixing. These ridges narrow the ventricle lumen. Auriculo-ventricular valve flaps are attached to the ventricle wall by thread-like chordae tendinae.

Circulatory System of Frog

Circulatory System of Frog: Heart Internal Structure 

(c) Truncus Arteriosus

A tubular truncus arteriosus emerges from the ventricle’s upper right side. Its opening is protected by three semilunar valves with edges pointing towards the truncus. When the ventricle contracts, these valves are pushed apart, allowing blood to flow freely from the ventricle into the truncus while preventing blood from flowing back into the ventricle.

The truncus arteriosus is made up of a thick-walled conus arteriosus at the base and a thin-walled ventral aorta at the distal end. The conus arteriosus portion next to the ventricle is known as pylangium, and the distal ventral portion is known as synangium. Pylangium is a short tubular structure, whereas synangium is formed simply by joining the basal parts of the arteries.

A row of semilunar valves marks the boundary line of the pylangium and synangium at the distal end of the pylangium or the conus arteriosus. One of these valves has been modified to form a large spirally twisted spiral valve that partially divides the conus arteriosus cavity into two passages, a dorsal and left cavum pulmocutaneum and a ventral and right cavum aorticum, both of which communicate with the ventral aorta.

The truncus arteriosus divides into two halves in front of the ventral aorta, with each half further subdivided into a carotid arch, a systemic arch, and a pulmonary arch. An aperture leading to the pulmocutaneous arch is located immediately anterior to the spiral valve. This aperture is protected by two pulmocutaneous valves, one dorsal and one ventral.

Soon after the bifurcation of the truncus arteriosus are two apertures leading into systemic arches and more anteriorly are two apertures of carotid arches.

Working of the Heart

A pacemaker called the sinu-auricular node is located in the wall of the sinus venosus and is responsible for causing the heart to contract. Because the muscles of the auricles are continuous with those of the ventricle, the wave of excitation or contraction begins at the sinus venosus and ends at the truncus, causing these chambers to contract sequentially.

When the sinus venosus contracts, impure blood flows into the right auricle via the sinu-auricular aperture. Simultaneously, the left auricle receives blood from the lungs via the pulmonary veins. Both auricles contract almost simultaneously, allowing blood to enter the ventricle via the auriculo-ventricular aperture.

(a) Older View

The left side of the ventricle receives oxygenated blood from the left auricle, according to an old theory proposed by Brucke (1851) and confirmed by Sabatier (1873), while the right side receives deoxygenated blood from the right auricle. Because of the viscous nature of blood and the spongy texture of the ventricle due to the presence of columnae cameae, mixing of the blood is avoided in the ventricle.

Thus, there is impure blood on the right side, pure blood on the left side, and mixing of two bloods in the middle. When the ventricle contracts, impure blood from the right side enters the truncus arteriosus first.

The spiral valve in the conus arteriosus is positioned so that deoxygenated blood can enter the pulmocutaneous arches via their common opening and be carried to the lungs and skin for oxygenation. The spiral valve then closes the pulmo-cutaneous arches’ common opening.

Now, mixed blood from the ventricle’s middle is pushed through the cavum aorticum into the systemic arches, and oxygenated blood from the ventricle’s left side is pushed into the carotid arches. Thus, the spiral valve in the truncus arteriosus directs blood flow into various arches. This explanation for blood flow from the ventricle into the arches is no longer accepted.

(b) Modern View

According to a new theory proposed by Vandervael (1933) and Foxon (1953), all blood entering the auricles, whether from the lungs or the sinus venosus, is oxygenated because blood oxygenation occurs not only in the lung but also in the skin and the buccal cavity. It means that the blood entering the sinus venosus from the skin and buccal cavity is equally, if not more, oxygenated.

When the two auricles contract, blood flows into the ventricle, where it becomes completely mixed up. This mixed blood is distributed throughout the body via the carotid, systemic, and pulmocutaneous arches.

The spiral valve does not turn the blood; instead, it supports the conus arteriosus and keeps it from collapsing. The carotid labyrinth does not raise blood pressure, but it is sensory and detects oxygen pressure changes in the blood. The above explanation of blood flow in the heart is now widely accepted.

According to DeLong (1962), the carotid arches receive oxygenated blood, the pulmocutaneous arches receive less oxygenated blood, and the systemic arches receive mixed blood. The precise mechanism is unknown.

The flow and pressure are controlled by the heart’s extrinsic nerves. Vagus nerve fibres slow down the rate of the heartbeat, whereas sympathetic nerve fibres speed up the rate of the heartbeat.

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