By Donald Miller Updated Mar 24, 2022

In mammals, including humans, blood circulates through the cardiovascular system, powered by a four‑chambered heart. After delivering nutrients and oxygen to tissues, blood returns to the heart depleted of oxygen. The lungs continuously extract oxygen from the air to replenish the blood, but this requires a dedicated route to transport deoxygenated blood to the lungs and oxygenated blood back to the body. The heart, arteries, and veins coordinate this process.

Generally, arteries carry oxygenated blood while veins transport deoxygenated blood. Two vessels break this pattern: the pulmonary artery, which carries deoxygenated blood, and the pulmonary vein, which carries oxygenated blood.

Each of the heart’s four chambers—two atria and two ventricles—has a major vessel either entering or leaving it, ensuring continuous flow.

The pulmonary artery originates from the right ventricle. When the right ventricle contracts, it ejects deoxygenated blood into the pulmonary artery, directing it toward the lungs.

Within the pulmonary capillary network, blood releases carbon dioxide and absorbs oxygen. These vessels gradually coalesce into larger arteries, eventually forming the pulmonary vein, which returns oxygen‑rich blood to the left atrium.

From the left atrium, oxygenated blood moves to the left ventricle. Contraction of the left ventricle pumps the blood into the aorta, the body's main artery. Branching from the aorta, a hierarchy of arteries supplies oxygenated blood to every organ and tissue.

Capillaries, the smallest blood vessels, connect the arterial and venous systems, completing the circulatory loop. Red blood cells (erythrocytes) contain hemoglobin—a complex iron‑based protein that binds oxygen and carbon dioxide, facilitating gas exchange in the lungs.