The cardiovascular system is comprised of three circulatory systems (Stephenson, 2007). The systemic circulation nourishes the body, except for the lungs and heart, which have their own pulmonary and coronary circulations, respectively. Muscular arteries leave the aorta, and branch and narrow into arterioles of the microcirculation. These arterioles branch, narrow, and simplify even further into capillaries whose walls are formed by only the inner vascular lining of the endothelial cells. Here, the passage of blood is slowed and restricted to one blood cell at a time, thereby facilitating nutrient delivery and waste removal. Oxygen is exchanged for carbon dioxide, or vice versa in the lung. Moreover, there is exchange of metabolic substrate and water from metabolic waste products, such as lactic acid. The capillaries then coalesce into venules, which exit the microcirculation by fusing into elastic veins that return blood to the heart by the vena cava for the systemic circulation, or by the pulmonary vein. The high elasticity and low muscularity of veins, compared to arteries, allow them to have approximately 20-fold greater capacity for distension with increasing pressure and to store approximately two-thirds of blood volume.