Page 49 - Libro 2

3 — Venous Physiology
29
exercise, the pressure in the veins falls lower than 20 mm Hg. This has the result of increasing the pres- sure gradient (to about 177 mm Hg) across the capil- laries. This increased pressure gradient will increase blood flow needed during exercise. Remember from the preceding chapter, blood will move down an en- ergy (pressure) gradient; and the bigger the gradient, the more blood flow.
In an uplifted arm, the hydrostatic pressure is negative. The hydrostatic pressure at the wrist would be approximately 􏰂50 mm Hg. Combining the hydrostatic pressure with a dynamic pressure of 15 mm Hg would yield a total intravascular pres- sure of 􏰂35 mm Hg. However, pressure within the veins cannot fall lower than the tissue pressure of 5 mm Hg or the veins would collapse and no blood flow would occur. The pressure gradient across the capillary in this uplifted arm does decrease to 40 mm Hg as compared with 80 mm Hg in the supine position.
PRESSURE–VOLUME RELATIONSHIPS
Because veins are collapsible tubes, their shape is determined by transmural pressure. Transmural pressure equals the difference between the pres- sure within the vein and the tissue pressure. At low transmural pressure, a vein will assume a dumbbell shape. As the pressure within a vein increases, the vein will become elliptical. At high transmural pres- sures, the vein will become circular (Fig. 3-4).
Changes in vein shape are associated with large increases in venous volume (Fig. 3-5). As such, veins can accommodate large changes in volume with very little changes in pressure. The walls of a vein are rather elastic, but a very large change in pressure is needed to change the volume of the vein when it is circular as compared with partially col- lapsed and elliptical. When supine, the transmural pressure is low. However, upon standing, the pres- sure increases and the walls stiffen such that the
High transmural pressure
Low transmural pressure
Figure 3-4 The effects of transmural pressure on the shape of a vein.
5 4 3 2 1
0
0 10 20 30 40 50 60 70 80 90
TRANSMURAL VENOUS PRESSURE mmHg
Figure 3-5 Venous pressure–volume curve.
venous volume will change little even with large changes in pressure.
Elastic compression stockings are available in varying degrees of pressure with some patients wear- ing stockings with 15 mm Hg pressure. This pres- sure is exerted on the limb producing an increase in the tissue pressure. Thus, with 15 mm Hg stockings, the tissue pressure increased from about 5 mm Hg to 15 mm Hg. This results in a net decrease in the transmural pressure of 10 mm Hg. When supine, this 10 mm Hg difference will greatly reduce venous vol- ume. However, when standing, these low-pressure compression stockings will have little effect because of the increased pressure within the vein caused by the hydrostatic pressure. This is one reason why higher pressure compression stockings are often em- ployed to aid individuals while sitting or standing.
EDEMA
Edema is a consistent sign of increased venous pres- sure. The Starling equilibrium describes the move- ment of fluid across the capillary (Fig. 3-6). Forces that act to move fluid out of the capillary are the intracapillary pressure and the interstitial osmotic pressure. Forces that tend to favor the reabsorption of fluid from the interstitium are the interstitial pres- sure and the capillary osmotic pressure. Osmotic pressure is the pressure exerted by fluid when there is a difference in the concentrations of solutes across a semipermeable membrane, in this case, the capil- lary endothelium. Normally, the forces are fairly bal- anced so that there is little overall fluid loss. What little fluid normally moves out into the interstitial space is picked up by the lymphatics. While stand- ing, the increased capillary pressure is no longer balanced by the reabsorptive forces and fluid loss
INCREASE IN VENOUS VOLUME ml per 100 ml of calf