CHAPTER 5 • VASCULAR FUNCTION
109
in venous pressure. Drugs that reduce venous
tone (e.g., nitrodilators) will decrease venous
pressure while increasing venous volume by
shifting the compliance curve to the left.
Mechanical Factors Affecting
Central Venous Pressure and
Venous Return
Several of the factors affecting CVP can be
classified as mechanical (or physical) factors.
These include gravitational effects, respira-
tory activity, and skeletal muscle contraction.
Gravity passively alters CVP and volume,
and respiratory activity and muscle contrac-
tion actively promote or impede the return of
blood into the central venous compartment,
thereby altering CVP and volume.
GRAVITY
Gravity exerts significant effects on CVP and
venous return. When a person is reclining
(supine
position),
systemic
blood
vessels
are positioned near the hydrostatic level of
the heart, which causes a generally uniform
distribution of the blood volume between
the head, thorax, abdomen, and legs. When
supine, CVP averages about 2 mm Hg, and
venous pressure in the legs is only a few mm
Hg above CVP. When a person changes from
supine to a standing posture, gravity acts on
the vascular volume, causing blood to accu-
mulate in the lower extremities (Fig. 5.14).
Because venous compliance is much higher
than arterial compliance, the shift in blood
volume to the legs increases their venous pres-
sure and volume. In fact, venous pressures in
the feet when a person is standing still may
reach 90 mm Hg because of the increased
hydrostatic pressure owing to the influence of
gravity. The shift in blood volume from the
thorax to the dependent limbs causes thoracic
venous volume and CVP to fall. This reduces
right ventricular filling pressure (preload)
■ FIGURE 5.14 Effects of gravity on central venous pressure
(CVP)
and venous pressure (PV) in the lower
leg. When horizontal (reclining), thoracic blood volume and CVP are relatively high, and PV is only a few
mm Hg above CVP. When standing upright, the force of gravity causes a large increase in venous pressure
in the legs, expanding the com pliant veins and increasing their volume. This translocation of blood volume
to the veins in the legs reduces thoracic volume and pressure.
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