Venous return is affected by several factors including muscle contraction, respiratory activity and gravity. Decreased venous compliance and vena cava compression also influence venous return. Other factors include valve competence, heartbeat, intrapericardial pressure, blood volume and degree of filling systematic circulation.
Venous return is the flow of blood from the periphery back to the right atrium. Under normal conditions, venous return must equal cardiac output, except for periods of a few seconds, because the cardiovascular system is primarily a closed loop. If this was not the case, then blood would accumulate in either the systemic or pulmonary circulations. Factors that essentially affect the venous side of the circulation can have a significant impact on cardiac output.
Venous return increases by the rhythmical contraction of limb muscles, as it happens during normal locomotion activities, such as walking and running. Sympathetic activation veins reduce venous compliance, raises central venous pressure and indirectly increases venous return. In addition, respiratory functions, specifically inspiration, promotes venous return because of reduction in right atrial pressure. Venous return is decreased by increase in the resistance of the vena cava, as it occurs when the thoracic vena cava compresses during late pregnancy.
Another determinant of venous return is gravity. When a person stands up, hydrostatic forces lead to decrease in pressure of the right atrial and increase in pressure in the dependent limbs. Venous return pressure increases from the dependent limbs to the right atrium, but decrease in the venous return.