As we've seen already, most arteries carry oxygenated blood away from the heart, and most veins carry deoxygenated blood back to the heart.

The only exceptions are:

• the pulmonary artery, which carries dexoxygenated blood away from the heart to the lungs
• the pulmonary vein, which carries oxgenated blood from the lungs back to the heart, and
• the portal vein, which carries blood to the liver.