Blood returning to the left or right side of the heart per minute is called the venous return. The cardiac output (CO) supplies systemic tissues with the oxygen and nutrients required for those tissues to perform their functions. The CO is called venous return (VR) when it is making its way back to the right side of the heart from the systemic venous circulation (or back to the left side of the heart from the pulmonary venous circulation). Therefore, the CO is equal to VR at steady state and increases in CO are matched by mechanisms to increase VR appropriately.
One of the major characteristics of the systemic venous circulation is that the veins are very compliant—approximately 24 times more compliant than the arteries. Compliance is the ratio of a change in volume divided by a change in pressure (Compliance = Δ volume/Δ pressure or ΔV/ΔP). Any chamber or structure that undergoes changes in volume will undergo changes in the pressure within that chamber. For a more compliant structure, there will be a smaller change in pressure with the change in volume than for a less compliant structure. For example, the volume in the different chambers of the heart changes during systole and diastole. During diastole, the ventricle triples its volume but pressure changes only by 2 to 5 mm Hg because the ventricle is very compliant normally although many cardiac diseases can decrease ventricular compliance. This affects diastolic filling, coronary perfusion, and contractility (as discussed in Chapter 15). Veins are also very compliant and hold approximately 60% of the blood volume in the resting state. For a 70-kg person with a blood volume of approximately 5.6 l that means approximately 3.3 l of blood is in the systemic venous circulation making its way back to the heart. Pressures, however, are quite low in the systemic veins ranging from 25 to 30 mm Hg (ignoring the effects of gravity) just distal to the capillaries down to 0 mm Hg at the vena cavae entry into the right atrium in normal individuals.
VR, like all blood flow, is determined by a pressure gradient. Since in a steady state, CO and VR are the same, the pressure gradient for VR is mean arterial blood pressure minus right atrial pressure:
where VR = venous return; MABP = mean arterial blood pressure; RAP = right atrial pressure; SVR = systemic vascular resistance.
In analyzing blood flow from the beginning of the venous system, just distal to the capillaries, to the right atrium, one can look at just the venous system and analyze the effective pressure gradient for VR, which is venous pressure minus right atrial pressure:
where PV = venous pressure; VVR = venous vascular resistance.