Explain how deoxygenated blood travel from body to lung for purification . Draw well labelled diagram in support of your answer. 

Double circulation is the circulation of blood through heart twice while supplying the blood to the whole body. In double circulation, there are two loops or circuits where the blood travels to heart.

Systemic circulation: First stage of systemic circulation starts from left atrium to left ventricle. This transfers the oxygen rich blood to aorta for the supply to various parts of the body. Later, the veins and venules collect the blood from various parts of the body. This blood is rich in carbon dioxide than oxygen and is known as deoxygenated blood. Deoxygenated blood is poured into superior vena cava then to right atrium. Right atrium passes the blood to right ventricle for pulmonary circulation.

Pulmonary circulation: The pulmonary artery collects the blood from right ventricle and passes to lungs for oxygenation. Lungs collects the deoxygenated blood supplied to it and purifies. Then the oxygenated blood is pumped back to left atrium of the heart through the pulmonary vein. Left atrium passes the blood to left ventricles where they pump the oxygenated blood to aorta for systemic circulation.

The double circulation maintains a strict separation of oxygenated and deoxygenated blood. Thus it ensures the high supply of oxygen and improves the body efficiency.