The circulatory system requires a mechanism to manage blood volume fluctuations, ensuring enough blood is available for active circulation without overloading the heart. This need is met by a temporary holding area known as the blood reservoir, which stores a large volume of blood outside the main circuit. The systemic veins and venules are the blood vessel types that fulfill this role, acting as the body’s primary blood reservoir. Because of this specialized storage function, they are often referred to as capacitance vessels.
The Capacitance Vessels
The term “capacitance vessels” directly refers to the ability of these vessels to hold a large volume of blood without a significant rise in internal pressure. The vast network of systemic veins and smaller venules holds a substantial majority of the body’s total blood volume. Estimates suggest the systemic venous system typically contains between 60% and 70% of the entire circulating blood volume. This capacity allows the body to maintain a stable, smaller volume in the high-pressure arterial system and capillaries, where oxygen and nutrient exchange occurs. Within this reservoir, specific areas like the venous networks in the liver, skin, and bone marrow are effective storage sites, collectively holding the venous reserve.
Why Veins Are Ideal Storage Units
The structural properties of veins make them uniquely suited for their reservoir function, contrasting sharply with thick-walled arteries. Veins possess a high degree of distensibility, a property known as compliance, meaning they can stretch readily to accommodate increasing blood volumes. This allows them to store blood without causing a drastic increase in the pressure inside the vessel, which would otherwise impede blood flow. Their vessel walls are relatively thin, containing less smooth muscle and connective tissue compared to arteries. The pressure within the venous system is significantly lower than in the arterial system, which enables this high-volume storage without the risk of rupture.
Releasing the Reserve Blood
The stored blood is not static; the body can rapidly mobilize this reserve when faced with demands such as exercise, dehydration, or significant blood loss. This mobilization is primarily controlled by the sympathetic division of the nervous system, which governs the body’s “fight or flight” response. When activated, the nervous system triggers a process called venoconstriction, causing the smooth muscle within the venous walls to contract. This action effectively reduces the volume capacity of the veins, stiffening the vessel walls and increasing the pressure on the contained blood.
Mobilizing the Reserve
The “squeezed” blood is then directed more rapidly toward the heart, which increases the amount of blood the heart can pump out per minute, a measure known as cardiac output. This rapid shift of blood from the low-pressure reservoir to the active circulation is a mechanism for maintaining blood pressure and ensuring adequate blood flow to vital organs during times of physiological stress.