Arteries are muscular blood vessels that carry oxygenated blood away from the heart (except the pulmonary and umbilical arteries). Arteries carry oxygen-rich blood at high pressure, so they are tough on the outside and smooth on the inside. The arteries have three layers and the smoothness of the inner layer enables blood to flow easily with no obstacles.

Veins are blood vessel channels that carry waste-rich blood back to the lungs and heart. Veins carry blood at a lower pressure than the arteries, so they are not as tough as the arteries. They have three layers, which are thinner and contain less tissue than those in the arteries. The veins have valves inside them, which help ensure the blood flows in one direction through the veins and that the blood flows against the force of gravity.

Capillaries are very thin, fragile blood vessels that receive oxygen-rich blood from arteries, exchange oxygen and carbon dioxide and then deliver the waste-rich blood to the veins. Capillaries are only one epithelial cell thick and blood can only flow through them in a single file. The red blood cells inside the capillary release their oxygen, which passes through the wall and into the surrounding tissue. The tissue releases its waste products, e.g. carbon dioxide, which pass through the wall and into the red blood cells. The exchange occurs and the waste blood is carried back to the heart and lungs through the veins.

Arteries and veins run parallel throughout the body, with a web-like network of capillaries, embedded in tissue, connecting them.