Blood vessels are hollow tubes that carry blood throughout the body. They consist of arteries, veins, and capillaries. Arteries carry oxygen-rich blood away from the heart. Veins carry oxygen-poor blood back to the heart. In pulmonary circulation, this is reversed. It is the pulmonary artery that brings oxygen-poor blood into the lungs and the pulmonary vein that brings oxygen-rich blood back to the heart.
Arteries branch into smaller vessels called arterioles. Arterioles further branch into capillaries, which deliver oxygen and nutrients to cells. Most capillaries are thinner than a hair. Many are so tiny, only one blood cell can move through them at a time. Once the capillaries deliver oxygen and nutrients and pick up carbon dioxide and other waste, they move the blood back through wider vessels called venules. Venules eventually join to form veins, which deliver the blood back to your heart to pick up oxygen.