An Amphibian

The right auricle receives deoxygenated blood from the body via sinus venosus, and the left auricle receives oxygenated blood from the lungs. Both the samples of blood pass into the ventricle, where some mixing of the two occurs. The conus arteriosus with the help of special valves in it, supplies deoxygenated, mixed and oxygenated blood to the lungs, the general body and the head respectively. The amphibians have a double circulation as the blood passes through the heart twice in each complete circuit around the body. The course of blood from the heart to the lungs and back to the heart is called pulmonary circulation. The flow of blood from the heart to the body and head and back to the heart is called systematic circulation. An advantage of double circulation is that the blood supplied to the body is also pumped by the heart. This raises the blood pressure and accelerates the supply of oxygen and food to the body cells. Most amphibians spend a good deal of time under water where air breathing is not possible. Their undivided ventricle can route the blood from the body past the lungs and can send it to the skin for further gas exchange with the water.

The double circulation of amphibians is partial as the ventricle is undivided and some mixing of blood occurs in it.

Reptiles. Reptilian heart also receives both deoxygenated and oxygenated blood, and is, thus, arteriovenous heart. It is incompletely four chambered, having two auricles and a single partly divided ventricle. Only one accessory chamber, namely, sinus venosus is present. The conus arteriosus is partly merged with the ventricle and partly contributes to three large arteries, the aortic arches that arise from the ventricle.

Fig.3. Double circulation of blood in reptiles

The right auricle receives deoxygenated blood from the body via sinus venosus and the left auricle receives oxygenated blood from the lungs. Both the samples of blood pass into the ventricle, which sends deoxygenated blood to the lungs via pulmonary arch and oxygenated blood to the rest of the body via a pair of systemic arches. Little mixing of blood occurs in the ventricle because of valves in it. Thus, the ventricle is functionally divided, though not structurally. Reptiles also have a partial double circulation. In crocodiles the ventricle is completely divided.

Birds and mammals. Mammalian heart is four-chambered, having right and left auricles and right and left ventricles. The right auricle receives the deoxygenated blood from the body and sends it into the right ventricle that pumps it to the lungs via a pulmonary arch. The left auricle receives oxygenated blood from the lungs and sends it into the left ventricle, which pumps it to the body through aortic arch. Thus, the deoxygenated and oxygenated blood remains fully separate, and there is a complete double circulation. This is the main advancement in the avian and mammalian heart over the amphibian and reptilian heart. There are no accessory chambers. The sinus venosus is merged with the right auricle, and the conus arteriosus is partly merged with the ventricles and partly contributes to the aortic arches.

Blood leaves the ventricle for supply to the body by two aortic arches loses pressure faster than when if flows through one aortic arch. Higher blood pressure in birds and mammals means faster circulation, and quicker supply of oxygen and food to the tissues and rapid removal of waste from them. Complete separation of pulmonary and systemic circulations allows the two circulations to have different blood pressures according to the need of the organs the supply.

Fig.4. Heart and blood flow in a bird and mammal.