An integrated view of the cardiovascular system requires an understanding of the relationships among cardiac output, venous return, and central blood volume and how these relationships are influenced by interactions among various neural, hormonal, and other control mechanisms. Consideration of the responses to standing erect provides an opportunity to explore these elements in detail. Figure 18.7 compares venous pressures for the recumbent and standing positions. When a person is recumbent, pressure in the veins of the legs is only a few mm Hg above the pressure in the right atrium. The pressure distending the veins—transmural pressure—is equal to the pressure within the veins of the legs because the pressure outside the veins is atmospheric pressure (the zero-reference pressure).
When a person stands, the column of blood above the lower extremities raises venous pressure to about 50 mm Hg at the femoral level and 90 mm Hg at the foot. This is
Venous pressures in the recumbent and standing positions. In this example, standing places a hydrostatic pressure of approximately 80 mm Hg on the feet. Right atrial pressure is lowered because of the reduction in central blood volume. The negative pressures above the heart with standing do not actually occur because once intravascular pressure drops below atmospheric pressure, the veins collapse. These are the pressures that would exist if the veins remained open.
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