Double Circulation

The pattern of blood flow through the human body is described as double circulation, because for one complete round trip around the body, blood must actually pass through the heart twice — once through each of two separate, fully independent circuits.

The first of these is the pulmonary circuit, which connects the heart to the lungs. Deoxygenated blood, collected from every tissue of the body, arrives at the right atrium via the great veins, passes into the right ventricle, and is pumped out through the pulmonary artery to the lungs, where it releases carbon dioxide and picks up a fresh supply of oxygen across the walls of the lung's alveoli. This now-oxygenated blood returns from the lungs via the pulmonary veins directly to the left atrium of the heart — completing one pass through the heart.

The second circuit is the systemic circuit, which connects the heart to every other tissue of the body. Oxygenated blood arriving at the left atrium passes into the left ventricle and is pumped out, under considerably higher pressure than the right ventricle generates, through the aorta, which branches repeatedly into progressively smaller arteries reaching every organ and tissue. There, across capillary walls, the blood delivers oxygen and nutrients and picks up carbon dioxide and other metabolic wastes, becoming deoxygenated again, before returning via the venous system to the right atrium — completing the second pass through the heart, and bringing the blood back to where the pulmonary circuit began. …