pores of the capillary membrane. Hence, these proteins are largely responsible for the osmotic pressures (commonly referred to as colloid osmotic pressures or oncotic pressures) that are exerted across the capillary wall. The concentration of proteins in plasma (7.0 g/dL) can generate an osmotic pressure of about 28 mm Hg. A similar protein concentration and oncotic pressure is found in interstitial fluid of tissues perfused by discontinuous-type capillaries (liver, spleen). Consequently, the transcapillary oncotic pressure gradient, which is essentially zero, does not influence the rate of fluid filtration across these capillaries. However, in tissue perfused by continuous or fenestrated capillaries, the interstitial protein concentration (and oncotic pressure) is only one-third to one-half that of plasma. In these organs, a significant oncotic pressure exists across the capillary wall, with plasma oncotic pressure favoring the withdrawal of fluid from the interstitial compartment.
A number of factors can influence the oncotic pressures generated in plasma and interstitial fluid. Because plasma oncotic pressure is generated by proteins dissolved in plasma, any factor that affects plasma protein concentration can alter Sudden increases in plasma volume, as might occur in patients receiving large volumes of physiologic salt solutions, will cause nc to fall, with a resultant increase in capillary fluid filtration. A sudden increase in Pc can also alter the transcapillary oncotic pressure gradient because the accompanying accumulation of filtered fluid in the interstitium will dilute the interstitial proteins and reduce A consequence of the dilution-related fall in is a braking effect on capillary filtration because normally acts to facilitate fluid movement from blood to interstitium. If capillary permeability is increased, then plasma proteins leak out of the blood into the interstitial compartment of the affected tissue, thereby leading to a rise in and a dissipation of the normal transcapillary oncotic pressure gradient. Here again, the ultimate outcome is an accelerated rate of capillary fluid filtration. Another perturbation that will cause to rise is obstruction of lymph drainage from a tissue. The lymphatic system provides the only means for the removal of plasma proteins that escape the blood and enter the interstitium. If lymph drainage is obstructed, then the interstitial protein concentration will eventually equilibrate with the plasma protein concentration and the normal transcapillary oncotic pressure gradient will be dissipated.
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