Some ten billion capillaries lace all body tissues, bringing blood within reach of every cell. They are the smallest blood vessels, microscopic in size, and contain less than five percent of the total circulating blood volume at any one time. Capilaries connect arterioles with venules and have thin walls, across which oxygen and metabolic exchanges take place. As blood flows through the capillaries in the lungs, it changes from venous blood to arterial blood by unloading carbon dioxide and picking up oxygen. Its color changes in the process from a deep crimson to a bright scarlet. As blood flows through tissue capillaries, it changes back from arterial blood to venous blood. The oxygen leaves the blood to enter cells, and the carbon dioxide leaves the cells and enters the blood.
