Which of the following factors contributes to reabsorption of tissue fluid in dependent regions of the body?
a. Hydrostatic pressure of 100 mm Hg
b. Low capillary permeability
c. Low interstitial osmotic pressure
d. Pumping action of skeletal muscles
ANS: D
Because of hydrostatic effects, capillary pressure in the feet can be as high as 100 mm Hg when an individual is standing. Reabsorption of tissue fluid can be accomplished although hydrostatic pressure greatly exceeds colloidal osmotic pressure. Three factors favor reabsorption under these circumstances. First, high intravascular hydrostatic pressure is somewhat balanced by a propor-tionally greater interstitial pressure. Second, the "pumping" action of the skeletal muscles sur-rounding leg veins lowers venous pressures. Third, lymph flow back to the thorax is enhanced by a similar mechanism. This facilitates clearance of excess interstitial fluid.
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