In man as in all mammals two circulations of blood takes place one is systemic circulation or body circulation and the other is pulmonary circulation or lung circulation. In systemic circulation the oxygenated blood received by the left atrium is forced into the left ventricle, when the ventricles contract the blood leaves the heart through the aorta. Through successive branchings the aorta gives rise to hundred of arteries which again branch to arterioles and capillaries. These carry blood to different parts of the body where exchange of nutrients and gases take place. From the organs similar number of venules converges to form veins which reunite to form two large veins, the superior and inferior venacava and open into the right atrium. This course of blood from the left ventricle to the right atrium is one type of circulation, which is the systemic circulation. The second type of circulation is the pulmonary circulation. In this circulation the venous blood from the right atrium is pushed into the right ventricle, which is carried by the pulmonary artery to the lungs. In the lungs the deoxygenated blood exchanges the gases, becomes oxygenated and is carried by the pulmonary vein back to the heart. This course of the blood from right ventricle to the left atrium is pulmonary circulation. Significance of double circulation is 1. Systemic circulation provides nutrients, O₂ and other essential substances to the tissues and collects CO₂ and other harmful substances for elimination. 2. The pulmonary circulation helps in carrying deoxygenated blood to the alveoli which are site for exchange of gases. It carries the oxygenated blood from the alveoli back to the heart. 3. There is no mixing of oxygenated and deoxygenated blood.