Cardiovascular and Nervous System

Ventricular diastole is the relaxation phase of the heart muscle. Since the heart is composed of cardiac muscle tissue, the activity of this tissue governs the relaxation process.

In a healthy individual, there are two normal heart sounds called lub and dub. Lub is the first heart sound. It is associated with the closure of the tricuspid and bicuspid valves at the beginning of systole. The second heart sound dub is associated with the closure of the semilunar valves at the beginning of diastole.

Human blood passes from the inferior vena cava into the diastolic right atrium of the heart because of a pressure differential between the atrium and the vena cava. This negative pressure gradient is created by atrial diastole and pulls blood to the atrium which is then pumped to the ventricles by contraction.

Due to one complete cardiac cycle, the two atria contract during the first 0.1 seconds, followed by ventricular systole for 0.3 seconds and finally joint diastole for 0.4 seconds. 

Receptors associated with aortic arch and carotid artery can recognise a change in $CO _{2}$ and $H^+$ concentration and send necessary signals to the rhythm centre (medulla oblongata) that controls the rate of breathing.

A large twisted spiral valve divides incompletely the cavity of pylangium into a left dorsal cavum  pulmocutaneum and right ventral cavum aorticum.

The chordae tendinae are strong, fibrous strings attached to the leaflets (or cusps) of the heart on the ventricular side i.e., the lower chamber. These strings originate from small mounds of muscle tissue, the papillary muscles, which project inward from the walls of the ventricle. When the cusps close, the chordae tendinae prevent them from swinging back into the atrium cavity (the upper chamber).

Sinus venosus and conus arteriosus are not found in rabbit but they are present in the frog. In rabbits, sinus venosus is formed in the embryo, but later becomes a part of the right wall of the auricle. In rabbits, conus arteriosus forms a part of the upper wall of the right ventricle in which the pulmonary artery originates. 

In animals, rate of heart beat is connected with the size of animal. In mammals, smaller animals have higher heart beat. For example, 200 beats per minute in rabbit, 500 beats per minute in sparrow, 28 beats per minute in elephant. In frog, rate of heart beat is 64 per minute. In mammals, maximum heart beat is observed in shrew (800 per minute) and minimum in blue whale (25 per minute).

All vertebrates possess a muscular chambered heart. Fishes have a 2-chambered heart with an atrium and a ventricle. 

Sinus venosus and conus arteriosus are not found in rabbit but they are present in the frog. In the rabbit, sinus venosus is formed in the embryo but later it becomes a part of the wall of the right auricle. So the impure (deoxygenated) blood collected by the precavals and postcavals directly comes into the right auricle. The pure blood brought by the pulmonary veins from the lungs directly come into left auricle.

All vertebrates possess a muscular chambered heart. 

The frog heart has 3 chambers two atria and a single ventricle. The atrium receives deoxygenated blood from the blood vessels (veins) that drain the various organs of the body. The left atrium receives oxygenated blood from the lungs and skin (which also serves as a gas exchange organ in most amphibians).