Capillaries have thin walls so that oxygen, carbon dioxide, nutrients and waste can flow through the walls without difficulty, states The Franklin Institute. They consist of only one epithelial cell, which means only a single blood cell can pass through the capillary wall at a time.
Blood flows to the capillaries after passing through arteries, reports KidPort. They facilitate the delivery of essential nutrients and oxygen to surrounding tissues, such as the organs and muscles. Heat transfers from the blood in the capillaries to surrounding tissues, as well. This helps the body regulate its temperature when exposed to high temperatures or exercise.