Porth's Essentials of Pathophysiology, 4e

164

Integrative Body Functions

U N I T 2

Capillary Fluid

U N D E R S T A N D I N G

Movement of fluid between the vascular compartment and the interstitial fluid compartment surrounding the body cells occurs at the capillary level.The direction and amount of fluid that flows across the capillary wall are determined by: (1) the hydrostatic pressure of the two compartments, (2) the colloidal osmotic pressures of the two compartments, and (3) the removal of excess fluid and osmotically active particles from the interstitial spaces by the lymphatic system.

Capillary with fluid leaving and entering through capillary pores

Hydrostatic Pressure. The hydrostatic pressure is the pushing force exerted by a fluid. Inside the capillaries, the hydrostatic pressure is the same as the capillary filtra- tion pressure, about 30 mm Hg at the arterial end and 10 mm Hg at the venous end. The interstitial fluid pressure is the force of fluid in the interstitial spaces pushing against the outside of the capillary wall. Evidence suggests that the inter- stitial pressure is slightly negative (−3 mm Hg), contributing to the outward movement of fluid from the capillary. 1 Colloidal Osmotic Pressure. The colloidal osmotic pressure is the pulling force created by the presence of evenly dispersed particles, such as the plasma proteins, that can- not pass through the pores of the capillary membrane. The capillary colloidal osmotic pressure is nor- mally about 28 mm Hg throughout the length of the capillary bed. The interstitial colloidal osmotic pres- sure (about 8 mm Hg) represents the pulling pressure exerted by the small amounts of plasma proteins that leak through the pores of the capil- lary wall into the interstitial spaces. The capillary colloidal osmotic pres- sure, which is greater than both the hydrostatic pressure at the venous end of the capillary and the inter- stitial colloidal osmotic pressure, is largely responsible for the move- ment of fluid back into the capillary. 2

Endothelial cell

Capillary filtration pressure

Venous end

Arterial end

30 mm Hg

10 mm Hg

Interstitial fluids

Interstitial fluid pressure –3 mm Hg

Plasma proteins

Capillary colloidal osmotic pressure 28 mm Hg

Arterial end

Venous end

Interstitial fluids

Interstitial colloidal osmotic pressure 8 mm Hg