- Veins play both a passive and active role in circulatory homeostasis. On a very basic level, veins collect blood from the circulation and return it to the heart for re-pressurization and re-circulation. However, veins are also an important functional reservoir for blood whose role in circulatory homeostasis they also regulate. Because the volume of blood in the veins is under low pressure it is known as the "Unstressed Volume".
- Capillaries progressively converge into venules and then into veins resulting in veins being the final common vessels which ultimately return blood to the heart. Importantly, veins tend offer very low resistance to blood flow and as a result can conduct large amounts of blood volume back to the heart even with the small blood pressure gradients which exist between the veins and the atria. One challenge that veins must overcome is achieving sufficient venous return given the force of gravity which tends to pool blood in veins which are lower in height than the heart. Venous return in the face of gravity is achieved by an anatomical solution which involves the existence of frequently-placed one-way valves within the veins. These valves allow blood to move in the direction of the heart but prevent backward flow. When veins are squeezed by surrounding muscles, blood can only flow in the direction of the heart; thus, muscle activity is critical for normal venous return.
- As shown in circulatory data, nearly 60% of the total volume of blood in the human body is present within the veins. The blood in veins is referred to as a "Reservoir" because it can be mobilized to boost cardiac output and in turn systemic arterial pressure when physiological demands require so. The physiological basis for this requires a deeper understanding of cardiocirculatory integration and specifically cardiovascular function integration. However, briefly, vascular compliance of the veins is under the control of the autonomic nervous system which exerts control via SNS fibers that synapse on beta1 receptors on venous vascular smooth muscle cells.
- Activation of SNS fibers results in smooth muscle cell contraction and thus venoconstriction which essentially increases the total body venous return by pushing venous reservoir blood into the heart. Increased venous return boosts cardiac preload which, courtesy of the Frank-Starling Relationship, will enhance the cardiac output. Enhanced cardiac output in the face of a constant systemic vascular resistance will enhance the systemic arterial pressure as described in systemic arterial pressure regulation