capacitance

Compliance/capacitance is inversely related to elastic recoil; thin-walled veins distend easily, while stiff arteries do not. The venous system contains the majority of blood volume and acts as a reservoir that can be mobilized by sympathetic-mediated venoconstriction (alpha-1 receptors) to increase venous return and preload. Aging and atherosclerosis decrease arterial compliance, widening pulse pressure (isolated systolic hypertension). Venodilators like nitroglycerin act primarily on capacitance vessels, decreasing preload to relieve angina. In septic shock, loss of venous tone (distributive shock) pools blood peripherally and reduces effective circulating volume. The relationship MAP = CO x TPR is complemented by the capacitance system, which sets the mean systemic filling pressure that drives venous return per the Guyton curves.