Capillaries exchange metabolic materials such as oxygen, carbon dioxide and glucose between the blood and cells of the body.Their walls consist mostly of the endothelium layer lining the blood vessels making them extremely thin for rapid diffusion.There are numerous capillaries and they are highly branched, providing a large surface area for exchange.They have a narrow diameter and spread through tissue which means no cell is far from a capillary. Their lumen is narrow so red blood cells are squeezed flat against the side of a capillary, which reduces the diffusion distance to the cells.There are spaces between the lining cells that allow white blood cells to escape.