Pulmonary circulation is the portion of the cardiovascular system that carries oxygen-depleted blood away from the heart to the lungs, and returns oxygenated (oxygen-rich) blood back to the heart. The term "pulmonary circulation" is readily paired and contrasted with systemic circulation. A separate system known as the bronchial circulation supplies blood to the tissue of the larger airways of the lung.
Oxygen-depleted blood from the body leaves the systemic circulation when it enters the right atrium through the superior (upper) vena cava and inferior (lower) vena cava. The blood is then pumped through the tricuspid valve (or right atrioventricular valve) into the right ventricle. Blood is then pumped through the semilunar valve and into the pulmonary artery.
From the right ventricle, blood is pumped through the pulmonary semilunar valve into the left and right pulmonary arteries (one for each lung) and travels through the lungs.
The pulmonary arteries carry deoxygenated blood to the lungs, where it releases carbon dioxide and pick up oxygen during respiration. Arteries are further divided in to very fine branches called the capillaries. In structure the capillaries are very thin walled, allowing for efficient gas exchange. Their function is to carry blood to all cells of the body.
The oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. This blood then enters the left atrium, which pumps it through the bicuspid valve, also called the mitral or left atrioventricular valve, into the left ventricle. The blood is then distributed to the body through the systemic circulation before returning again to the pulmonary circulation.