At rest, the average person pumps five liters of blood from his heart each minute. The cardiovascular system is a closed network, which means five liters of blood must also be returned to the heart each minute for this rate of circulation to continue. During exercise, cardiac output can increase, as much as eight-fold in world-class endurance athletes. If exercise is performed in the upright position, returning blood to the heart at this rate is challenging because the pull of gravity provides opposition.
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Your cardiovascular system circulates blood to deliver oxygen and fuel to tissues and remove metabolic waste. After leaving the heart, blood flows through the arterial system to capillaries where exchange with tissues takes place. Capillaries feed deoxygenated blood into the venous system, which has a smaller cross-sectional area that helps to increase the velocity of flow. The venous system’s blood vessels, or veins, also have valves spaced at short intervals that ensure blood flow in one direction only. These two features are important because venous blood pressure is low and without them, blood would pool in your extremities.
At rest, venous blood vessels typically contain 65 percent of total blood volume. Venous valves ensure unidirectional flow, but there is still a challenge of returning this blood to the heart. This is difficult because by adopting bipedal locomotion, man's corresponding move from horizontal to erect posture means there is a considerable gravitational pressure gradient that our circulatory systems must overcome. During upright exercise, when blood flow to the extremities is markedly increased, this is even more challenging. There are three principal mechanisms that ensure adequate venous return during upright exercise. They are venoconstriction, the muscle pump and the respiratory pump.
Venoconstriction involves reflex signals that cause your veins to constrict. These signals are sent by your sympathetic nervous system during exercise. Venoconstriction decreases the volume of blood that the venous system can contain, which facilitates venous return.
The muscle pump occurs due to the mechanical action of rhythmic skeletal muscle contractions during exercise. These contractions compress your veins and push blood back to your heart. In between contractions, blood draining from your capillaries refills your veins and is held in place by their valves until the next contraction takes place. The considerable influence of the muscle pump provides the rationale for performing an active cool down after exercise because stopping abruptly would remove this mechanism and risk considerable blood pooling.
The rhythmic pattern of breathing during exercise also provides a pumping action within your vasculature system. When you inhale, the pressure within your chest cavity decreases and the pressure within your abdominal region increases. This pushes blood back to your heart. This mechanism, which is called the respiratory pump, has been shown to be the predominant factor that returns blood to the heart during exercise in the upright position.