The structural and functional efficiency of mammalian heart over the Amphibian heart

Position:

The heart in rabbit is situated in the anterior part of the thoracic cavity between the lungs, slightly shifted to the left. It is protected by double fold of pericardium. The space between the two layers is filled with pericardial fluid, which protects the heart against external shocks.

Structure:

External Structure of heart:

The heart is a highly muscular triangular structure, formed of special cardiac muscles. Its anterior part is broad, while the posterior is somewhat conical or pointed. IT is divided into four chambers: two auricles and two ventricles.

1. Auricles –

These form the anterior part of the heart and are comparatively darker in colour. The two auricles are completely separated from one another and are known as right and left auricles.

Each is a thin-walled bag filled with blood.

The sinus venosus is absent in rabbit and is supposed to be absorbed in the right auricle early in the embryonic stage. The vena cavae, therefore, directly lead in the right auricle. These bring impure or deoxygenated blood from the entire body. The left auricle receives only the oxygenated blood from lungs by a pair of pulmonary veins.

2. Ventricles –

The two ventricles situated behind the auricles constitute the posterior or ventricular part of heart. It is formed of very thick and muscular wall which is somewhat lighter in colour. Its right and left chambers are known as right ventricle is marked externally by an oblique groove which runs obliquely backwards towards the right. The right ventricle is, therefore, smaller in size and possesses less muscular walls than the left ventricle. It does not extend upto the apex of heart.

From the right ventricle originates pulmonary aorta at its left anterior angle which carries impure blood to the lungs. The left ventricle gives rise to caroticosystemic antero-dorsally.

Internal Structure of heart:

The longitudinal section of heart reveals that the auricles are formed of thin wall and are separated. It carries a rounded depression, the fossa ovalis. The openings of anterior and posterior vena cavae in the right auricle are protected by Eustachian valves, but the pulmonary opening into the left auricle is devoid of any valve.

The two ventricles are also separated by a thick and muscular longitudinal spectrum, the interventricular septum. The septum is placed obliquely in such a way that the right ventricle is smaller than the left and does not reach the apex of the heart. The right ventricle possesses comparatively thin walls than those of left.

The walls of the ventricles are produced into muscular processes or ridges which project into ventricular cavity. These are known as musculi papillaris or columnae carineae. The right auriculo-ventricular aperture is guarded by a tricuspid valve. It consists of three membranous flaps or cusps attached to the columnae carnae or muscular processes of ventricle wall by means of tendirous threads chordeae tendineae. Similarly, the left auriculo-ventricular aperture is protected by a bicuspid valve. The function of these valves is to check the flow of blood in opposite direction.

The entrance of pulmonary aorta to the right ventricle is guarded by three semilunar valves, which prevent the backward flow of blood from pulmonary aorta to the ventricle. The origin of carotico-systemic trunk from the left ventricle is also guarded by three semilunar valves.

Features of Advance over that of Frog’s Heart

1. In frog, the heart is three chambered consisting of two auricles and one ventricle, whereas in rabbit the heart is four-chambered as the ventricle has also divided into two chambers.

2. As the ventricle in frog is single, the pure and impure blood from the left and right auricles get mixed in the ventricle. In rabbit, however, the blood from auricles enters separately in the two ventricles and hence there is no mixing.

3. In rabbit, the sinus venosus is absent; as such the three vena cavae bringing blood from the body open separately into the right auricle but in frog the blood is received in sinus venosus.

4. There is no truncus I rabbit and hence the aortic arches pulmonary and systemic circulations are separate. In frog, the truncus arteriosus divides into two aortic arches.

5. In rabbit, only the left aortic arch is present which is known as carotico-systemic aorta. It emerges out of the left ventricle. In frog however, the carotid and systemic aorta are paired vessels arising from the truncus.

6. The openings between aortae and heart are guarded by valves in rabbit, but not in frog.

7. Separate contractile centres are present in the auricular and ventricular parts in rabbit, but only one in frog.