22 Biology -- Frog

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Circulatory System of Frog

Circulatory System

 Circulatory system helps to carry oxygen and other required nutrients.

The Circulatory System of frogs are closed type (inside blood vessels).

 It mainly includes: Blood vascular system (Heart, Blood vessels and Blood) and Lymphatic system.

The prime function of this system is to distribute the digested food and oxygen to different parts of the body in order to release energy to carry out life activities and also to bring the excretory and gaseous wastes to organs of elimination i.e. kidney and lungs.

Blood Vascular System


The heart is a muscular pumping organ that pushes the blood into the closed circulatory system. It can be studied under these topics:

  • External structure      -     Internal structure                           -     Mechanism

Heart of the frog is three-chambered. Coronary sulcus separates the auricle and ventricle. Two auricles are separated by an inter-auricular groove. All the veins carry deoxygenated blood except pulmonary veins.

External structure:

Externally, the heart of frog is a dark red colored conical muscular organ situated ventrally to the liver in the pericardial cavity along the mid ventral line at the level of fore limbs.


The heart is enclosed within a sac formed of two membranes, an outer pericardium and an inner epicardium which encloses the heart.

Between these two membranes, a serous or pericardial fluid is found which prevents friction and also keeps the heart moist.


1. Protects the heart from outer shocks.

2. Keeps the heart moist.

 3. Helps to keep the heart in its proper position.

Chambers of Heart:

Heart is 3-chambered, triangular. Its anterior part is broad and posterior is narrower. The broader part of the heart contains two atriums or auricles, whereas the posterior part has a single ventricle. Both the auricles are externally demarcated by a faint longitudinal inter-auricular groove. The two auricles, however, are clearly separated from the ventricle by a transverse coronary sulcus or auricular-ventricular groove. The ventricle is the most conspicuous and prominent part of the heart. It is conical in shape with thick muscular walls. Besides auricles and ventricles, the heart has two more chambers-truncus arteriosus and sinus venosus.

Note:  Due to the presence of truncus arteriosus and Sinus venosus, the heart seems to have five chambers but actually it is only three-chambered.

Truncus arteriosus:

Truncus arteriosus arises from the ventral upper right side of the ventricle as a cylindrical structure. It 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 venosus lies on the dorsal surface of the heart It is a thin walled dark-colored somewhat triangular chamber which is formed by the onion of three caval veins, two anterior precavals and one posterior postcaval. 

Pulmonary veins:

A pair of pulmonary veins (right and left) coming from lungs unite together and form a common pulmonary vein. The common pulmonary vein then directly opens into the left auricle on the dorsal side.

Q. Describe the internal structure of the heart and its mechanism.

 The internal structure of the heart can be studied by cutting a vertical section of the heart. The wall heart is made of three layers: the outer epicardium, the middle mesocardium and the inner endocardium. Internally the heart is three-chambered with two auricles and one ventricle. However, the ventral news of the internal structure of the heart shows two auricles, one ventricle, truncus arteriosus and the valves to keep the blood flowing in one direction.


              The two auricles right and left are thin walled and completely separated from each other by a thin vertical partition called interauricular septum. We can see the right auricle of the heart larger than the left. In the right auricle, close to the septum there is a transverse oval opening called sino-auricular aperture through which blood enters into the auricle from the sinus venosus. It is guarded by two lip-like sinu-auricular valves, one arising from the dorsal edge and the other from the ventral. These valves allow the free flow of blood only into the right auricle but prevent the backward flow of the blood.

In the left auricle, slightly anterior to the sinu-auricular aperture but close to the septum there is a small opening of the pulmonary vein which has no-valve. The two auricles open into a single ventricle by an auriculo- ventricular aperture which is bounded by two pairs of auriculo ventricular valves, one arising from the dorsal edge and the other from the ventral edge of this aperture.


             The ventricle is a most conspicuous triangular chamber of the heart with thick muscular walls. Its inner surface has an irregular ridge, the columnae carneae with deep pockets between them which to some extent prevents the mixing of the blood from the two auricles. These ridges reduce the lumen of the ventricle by thread like chordae tendineae.

Truncus Arteriosus

         From the upper right side of the ventricle arises a tubular truncus arteriosus. The opening of the ventricle into the truncus arteriosus is guarded by three semilunar valves. On contraction of the ventricle, these valves are pushed apart and they make a free passage for the blood to move from the ventricle into truncus but they prevent the backward flow of blood into the ventricle. The spirally twisted cavity of truncus arteriosus is divided unequally by another set of 3 semilunar valves into a long basal thick-walled conus arteriosus or pylangium and a short distal thin walled bulbous aorta or synangium. A large twisted longitudinal spiral value further incompletely divides the pylangium into a left dorsal cavum pulmo cutaneum and a right ventral cavum aorticum. The spiral valve is attached dorsally while its ventral edge is free.

Working of heart:

Heard is a muscular organ which constantly beats during life under neural control to pump blood into the circulatory system. Contraction of heart is called systole, while its relaxation is termed as diastole. Different chambers of heart contract in a regular sequence and the values present prevent the backflow of blood.

 When sinus venosus contracts, its non-oxygenated venous blood is forced into the right auricle through sinu-auricular aperture. Meanwhile the oxygenated blood from lungs is poured into left auricle through common pulmonary vein. The two auricles now contract almost simultaneously forcing their blood into ventricle through the single auriculo- ventricular aperture.

Old View:

According to the conventional older view, the ventricle contained only deoxygenated blood in its right side received from right auricle and only oxygenated blood in its left side received from left auricle with some mixed blood in the middle region. The two kinds of blood could not mix to any great extent because of their viscous nature and the spongy nature of ventricle due to the presence of the network of columnae corneae.

When ventricles starts contraction, first the deoxygenated blood from the right side being nearer flows into the truncus arteriosus and directed by the spiral valve into the common opening of pulmo-cutaneous arches and carried to lungs and skin for oxygenation. Spiral valve now closes the opening of pulmo-cutaneous arches. Next follows the mixed blood which is pushed through cavum aorticum into the systemic arches and sent to the body and limbs. Finally enters the oxygenated blood of left -side and directed through corotid arches to the head. Thus, spiral value in truncus plays an important role in directing blood into different arches.


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