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Why do pulmonary arteries carry oxygen-poor 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.

The pulmonary arteries carry oxygen-poor blood because they are part of the pulmonary circulation, a specific circuit designed to transport blood from the heart to the lungs for gas exchange.12

In the cardiovascular system, the term "artery" is defined by its function of carrying blood away from the heart, rather than by the oxygen content of the blood it transports. The right ventricle pumps deoxygenated blood into the pulmonary artery specifically so it can reach the lungs. Once in the lungs, this blood travels through capillaries surrounding the alveoli, where it releases carbon dioxide and picks up oxygen. Only after this process is complete does the blood return to the heart via the pulmonary veins as oxygen-rich blood.12