Why are the pulmonary veins called veins if they carry oxygenated blood? Why are pulmonary arteries called arteries if they carry deoxygenated blood?
Veins transport blood towards the heart, while arteries transport blood away from the heart.
All veins in the body transport deoxygenated blood to the heart except for the pulmonary veins. Recall that in internal respiration, oxygen diffuses from the alveoli to the deoxygenated blood. When this happens, the blood then becomes oxygenated.
The function of the pulmonary veins is to transport that oxygenated blood from the lungs to the heart. They are still called veins because they transport blood to the heart, regardless of whether or not the blood is deoxygenated or oxygenated.
Similarly, all arteries in the body transport oxygenated blood away from the heart except for the pulmonary arteries. Recall that in systemic circulation, when deoxygenated blood is transported back to the heart, the blood must be reoxygenated.
The function of the pulmonary arteries is to transport the deoxygenated blood to the lungs where it can be reoxygenated through diffusion. They are still called arteries because they transport blood away from the heart, regardless of whether or not the blood is deoxygenated or oxygenated.
A pulmonary "artery" is classified as an artery because it has more anatomical and physiological features in common with other arteries than it does with veins.
A comparison between arteries and veins:
......... ARTERIES ................................................... VEINS ...................
Most carry oxygenated blood . . . . . . . . . Most carry deoxygenated blood
Leave the heart . . . . . . . . . . . . . . . . . . . . . Usually go toward the heart
Anatomically they are arteries . . . . . . . . Anatomically they are veins
It's true that pulmonary arteries are not like other arteries in one way -- they carry deoxygenated blood.
However, they are like any other artery in another way -- they carry blood away from the heart.
But the best reason to classify pulmonary arteries as arteries is because of their anatomy.
Arteries have thick muscular walls to withstand the pressure of the blood as it is pumped by the heart. In comparison, the walls of veins are thin and delicate.
The lumen of an artery is relatively smaller than the lumen of a vein of the same diameter.
The lumen of an artery stays open even when blood is no longer filling it, but the lumen of a vein collapses.
When blood is no longer filling a pulmonary artery, it stays round; a pulmonary vein, like all other veins, forms an irregular shape as it collapses when empty.
So a pulmonary artery is classified as an "artery" because it leads away from the heart, and because its anatomy is that of an artery, enabling it to withstand the high pressure.
The same principle applies to the classification of a pulmonary vein.
Although it carries oxygenated blood, anatomically it's a vein.