Why do pulmonary veins carry oxygen-rich blood?

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Silhouette diagram of the human body highlighting the heart and major blood vessels, with labeled arteries in red and veins in blue to show the structure of the cardiovascular system. — Circulatory system Human circulatory system.

Pulmonary veins carry oxygen-rich blood because they are responsible for transporting blood that has just undergone gas exchange within the lungs.1

During pulmonary circulation, oxygen-poor blood is pumped from the heart into the lungs, where it reaches a dense network of capillaries surrounding the alveoli. In these tiny air sacs, carbon dioxide diffuses out of the blood to be exhaled, while oxygen from inhaled air diffuses into the bloodstream.12

Once the blood is replenished with oxygen, it is collected into venules and then into the pulmonary veins, which return this oxygenated blood to the left atrium of the heart to be distributed throughout the body via systemic circulation.21