(a) Humans have four chambered heart which consists of two atria and two ventricles. In a four chambered heart, the left side and right side of the heart are completely separated to prevent the oxygenated blood from mixing with deoxygenated blood. Such a separation allows a highly efficient supply of oxygen to the body cells which is necessary for producing a lot of energy. This energy is useful for warm blooded animals (like humans) which have high energy needs to maintain their body temperature.
(b) All the animals which have four chambered heart have double circulation in which the blood passes through the heart 'twice' in one complete cycle of the body. This ensures the separation of oxygenated blood from deoxygenated blood.
Explanation :
Double circulation. The blood travels twice through the heart in one complete cycle of the body and is called double circulation. It involves two circulations :
(i) Pulmonary circulation: The pathway of the blood from the heart to the lungs and back to the heart is called pulmonary circulation. It is small circulation. Deoxygenated blood in the right ventricle flows into the vascular system of the lungs, becomes oxygenated and returns to the heart left atrium through pulmonary veins.
(ii) Systemic circulation : The pathway of the blood from the heart to the rest of the body and back to the heart is called systemic circulation. It is at large circulation. Left ventricle sends the blood into the aorta. Aorta divides into arteries, arterioles and capillaries and supplies oxygenated blood to various parts of the body. From there the deoxygenated blood is collected by venules, which join to form veins and finally vena cava pours blood back into right atrium.