Capillaries have the simplest structure to all the components of the cardiovascular system. They are just a single cell layer thick and approximately 1 mm long, and are little more than tubes. The whole body is permeated with a fine mesh of these capillaries.
Actually, their combined length is on the order of 60,ooo miles. Therefor, extensive is their branching, that no cell in the body is more than a fraction of a millimeter away from any capillary. What’s more, a total surface area of 1,000 square miles is offered by these tiny vessels for exchanges between blood and interstitial fluid. Capillaries are composed of endothelial cells only, and they are the basic functional units of the circulatory system.
There is only nearly 250 ml of blood is contained by the capillaries at any time, out of a total blood volume of almost 4,000 ml (most is included within veins), in spite of their large number. The action of the precapillary sphincter muscles partly decides the amount of blood flowing through a capillary bed. The degree of resistance to blood flow offered by the small arteries and arterioles and the contraction of these smooth muscle sphincters regulate the blood flow to an organ.