Q.Schematic plan of human double circulation is given below.

Imagine a delivery van that has to make two separate trips before its round is complete — first to a fuel depot to fill up, then out to the customers. Human blood works in a similar two-trip way. In one full journey around the body the blood passes through the heart twice, and this arrangement is called double circulation.

The two trips are two separate loops. In the pulmonary circulation, the right ventricle pumps deoxygenated blood into the pulmonary artery, which carries it to the lungs. There the blood picks up oxygen, and the pulmonary veins bring this oxygenated blood back to the left atrium. This is the short loop between the heart and the lungs — the "fuel depot" trip. In the systemic circulation, the left ventricle pumps the freshly oxygenated blood into the aorta, from where arteries, arterioles and capillaries deliver it to the body tissues; the deoxygenated blood returning from the tissues is then collected by venules, veins and the vena cava and emptied into the right atrium. This is the long loop out to the whole body. …