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 branch off from the metarterioles which connect |arterioles| with |venules|. The |capillaries| have thin walls, only one cell thick, 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~.
