24.8: Veins as Blood Reservoirs
Veins, while chiefly responsible for circulating blood back to the heart, also function as storage vessels for blood. They house approximately 64 percent of the body's total blood volume, a feat made possible by their high capacitance—the inherent ability to expand and accommodate large volumes of blood, even under low pressure. The large diameter and thin walls of veins augment their distensibility, significantly more so than arteries, due to their classification as capacitance vessels. When the body requires redirected blood flow, the medulla oblongata's vasomotor center initiates sympathetic stimulation to the venous smooth muscles, triggering a constriction or venoconstriction. Although this constriction is less severe than that seen in smaller arteries and arterioles, it can be equated to a 'rigidifying' of the vessel wall, which amplifies the internal blood pressure and accelerates its return to the heart.
It's worth mentioning that about 21 percent of venous blood is found in the venous networks within the liver, bone marrow, and skin. This blood volume is dubbed the venous reserve. Through venoconstriction, this 'reserve' volume can be rapidly transported back to the heart for redistribution to other areas of the circulatory system. In other words, venoconstriction helps to maintain circulatory homeostasis.
