The smallest arteries are arterioles. These lead to capillary beds. Capillaries are composed of a single layer of squamous epithelium. Constriction and dilation of arterioles control the distribution of blood in the numerous capillaries of the body. At the origin of each capillary is a circular band of muscle called a precapillary sphincter. When the sphincters are open, the capillaries are well perfused with blood and they engage in exchanges with the tissue fluid. When the sphincters are closed, blood bypasses the capillaries, flows through a thoroughfare channel to a venule and does not engage in fluid exchange.