Capillaries are composed of endothelium, which is a form of simple squamous epithelial tissue. Capillary walls are thin and only 5-10 microns wide, so red blood cells can only flow through them in single file.
Small pores in capillaries enable the exchange of fluids, gases, nutrients and wastes through a process called diffusion. Blood pressure and osmotic pressure of the blood within the capillary vessels controls the fluid exchange. High concentrations of salts and plasma proteins in the blood control osmotic pressure. Capillary walls allow water and small solutes to pass through but block proteins.
Metabolically active body parts, such as muscles and kidneys, have many capillaries. Metabolically inactive body parts, such as connective tissues, have few.
Blood flow between arterioles and capillaries is controlled by muscled structures called precapillary sphincters that open and close. When open, blood flows freely to capillary beds. When the sphincters are closed, blood cannot flow through capillary beds and must flow directly from the arterioles to venules through the thoroughfare channel.
Capillaries are an important part of microcirculation. In microcirculation, blood circulates from the heart to arteries, to smaller arterioles, to capillaries, to venules, to veins and back to the heart.
Blood is always supplied to all body parts, but all capillary beds do not always contain blood. Blood goes where the body needs it the most, such as to the digestive tract after a meal to aid in digestion and nutrient absorption.