Why are the pulmonary vein and artery not like the rest of circulatory system? I'm learning anatomy. What I learnt is that we have arteries that have oxygenated blood which appears red in color, and branch blood to arterioles to deliver blood to cells via capillaries from where de-oxygeneated blood is collected via venules into veins to go back for oxygenation. So: > artery = oxygenated > > vein = de-oxygenated However, in defining the pulmonary vein and pulmonary artery, I see the reverse of these definitions. The pulmonary artery is for de-oxygenated blood. Why is it so?

It is true that nearly all arteries carry oxygenated blood and nearly all veins carry de-oxygenated blood, but that is not what defines them. Arteries are blood vessels that carry blood _away_ from the heart, and veins carry blood _towards_ the heart. If you look at the situation in that light, the naming makes sense: the pulmonary artery is carrying de-oxygenated blood _away_ from the heart to the lungs, and the pulmonary vein is carrying the _re_ -oxygenated blood back to the heart, to be pumped through the rest of the body.