This is a good question because in general, arteries carry oxygenated blood while veins carry de-oxygenated blood but the pulmonary artery and the pulmonary vein are exceptions and I’ll explain why.
Blood vessels are generally labeled as arteries, veins, or capillaries. The distinction between arteries and veins depends on the direction of blood flow in them. It does not depend on the type of blood (oxygenated or de-oxygenated) flowing through them.
In general, when a blood vessel transports blood away from the heart or distributes it to other structures in the body, it is regarded as an artery regardless of the nature of the blood flowing through it. Conversely, when a blood vessel transports blood from other body structures and returns it to the heart or to any network of blood vessels that returns the blood to the heart, it is regarded as a vein.
This explains why the pulmonary artery which transports de-oxygenated blood from the heart to the lungs is regarded as an artery in spite of the fact that it is carrying de-oxygenated blood. It also explains why the pulmonary vein which returns oxygenated blood to the heart from the lungs is regarded as a vein in spite of the fact that it is carrying oxygenated blood.
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