Capillaries are tiny blood vessels where the smallest arteries meet the smallest veins and where the blood exchanges food and oxygen for waste products. Veins, which are also blood vessels, return depleted blood to the heart.
Some capillaries are so small that they can only be seen under a microscope, and tiny blood cells can only pass through them one at a time. Because capillaries are so small, their walls are correspondingly thin. Nutrients carried by the blood are able to pass through these walls to the cells of the body. Waste materials from the cells and tissues can also pass through the walls and be carried away. In the lungs, the capillaries exchange oxygen and carbon dioxide.
Capillary-sized veins are called venules. The small veins join together to form larger ones. Eventually, the veins all flow into the superior and inferior vena cava. The superior vena cava carries blood to the heart via the head and arms. The inferior vena cava carries blood from the legs and the trunk.
The walls of veins have three layers, though they're thinner and more rigid than the walls of the arteries. Many of the larger veins have valves that make sure the blood flows toward the heart and doesn't flow backward.