capillaries, which are formed of a thin layer of endothelial cells covered by a basement membrane, are very small blood vessels: they measure only o.3 to 1 mm in length and have a maximum diameter of 0.01 mm. The extreme thinness of their walls encourages exchanges between the blood and the space around them. Through the capillaries, oxygen and nutritive elements are distributed to the tissues and carbon dioxide, the product of cellular metabolic activity, is carried away.