Capillaries are small in order to allow red blood cells to travel through them in single file. This aids in microcirculation. They are thin in order to allow oxygen, carbon dioxide, nutrients and wastes to be exchanged through their walls.
Microcirculation is the process that allows blood to circulate from the heart to the arteries, smaller arterioles, capillaries, venules and back to the heart. Blood flow in the capillaries is controlled by structures called precapillary sphincters. These structures are located between arterioles and capillaries, and they contain muscle fibers that allow them to contract. When the structures are open, blood can flow freely to the capillary beds of the body's tissues, but when they are closed the blood is unable to flow,
Capillaries use a process called diffusion in order to exchange fluids, gasses, nutrients and waste between the blood and the body's tissues. This is because there are small pores in the capillary walls that allow certain substances to pass into and out of the blood vessel. The fluid exchange is controlled by the blood pressure inside the capillary vessel, which is known as hydrostatic pressure. Fluid exchange is also controlled by osmotic pressure of the blood inside the vessel which is produced by high concentrations of salts and plasma proteins in the blood.