Pulmonary circulation refers only to the network of arteries and veins that carry blood from the heart to the lungs and back. It is not considered part of the systemic circulation.

Pulmonary circulation begins when oxygen-poor blood is pumped from the right ventricle into pulmonary arteries. This is the only instance in which arteries carry oxygen-poor blood. The blood is taken into the lungs, where it enters a series of gradually smaller arteries until it passes into pulmonary capillaries. These capillaries have the same function as ones in systemic circulation: they allow for the exchange of waste products and oxygen. In this case, the blood cells shed their waste products (including carbon dioxide) and pick up fresh oxygen.

After the blood is freshly oxygenated in the lungs, it travels through the pulmonary veins back to the left atrium. This is the only instance in which veins carry oxygen-rich blood. Once in the left atrium, the oxygen-rich blood drains into the left ventricle, where it is pumped into the systemic circulation.