In animals the blood flows through the heart once during each circuit of the body. They are said to have a single circulation. From the heart, arteries carry the blood to the gills where the blood takes up oxygen. The arteries that leave the gills carry oxygen-rich blood to various parts of the body. The blood from the body is collected by the veins. By the time the blood enters the veins, it is poor in oxygen content and the veins carry the de-oxygenated blood back to the heart. In mammals there is double circulation because of the lungs. Blood passes through the heart twice; from the main circulation of the body to the heart, then to the lungs and back to the heart again, before the blood is pumped into the main circulation. Large veins carry de-oxygenated blood from various parts of the body to the heart. From the heart the pulmonary arteries carry the blood to the lungs. Oxygenated blood is returned to the heart by the pulmonary veins. This circulation is known as pulmonary circulation. Oxygenated blood leaves the heart to be distributed to all parts of the body except lungs. Veins carry the blood from all parts of the body back to the heart. This is known as systematic circulation.
