Pulmonary circulation is the portion of the cardiovascular system which carries oxygen-depleted blood away from the heart, to the lungs, and returns oxygenated blood back to the heart. The term is contrasted with systemic circulation.
In the pulmonary circulation, deoxygenated blood exits the heart through arteries, enters the lungs and comes back through pulmonary veins.
Oxygen-depleted blood from the body leaves the systemic circulation when it enters the right heart, more specifically the right atrium. The blood is then pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.
The pulmonary arteries carry blood to the lungs, where red blood cells release carbon dioxide and pick up oxygen during respiration.
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.
Pulmonary circulation was first discovered and published by Ibn Nafis in his Commentary on Anatomy in Avicenna's Canon (1242), for which he is considered the father of circulatory physiology. It was later published by Michael Servetus in Christianismi Restitutio (1553).
Since it was a theology work condemned by most of the Christian factions of his time, the discovery remained mostly unknown until the dissections of William Harvey in 1616.
The pulmonary circulation loop is virtually bypassed in fetal circulation. The fetal lungs are collapsed, and blood passes from the right atrium directly into the left atrium through the foramen ovale, an open passage between the two atria. When the lungs expand at birth, the pulmonary pressure drops and blood is drawn from the right atrium into the right ventricle and through the pulmonary circuit. Over the course of several months, the foramen ovale closes, leaving a shallow depression known as the fossa ovalis in the adult heart.