Effect of plasma and red blood cells on water permeability in cat hindlimb.

Capillary filtration coefficient (CFC) was measured to investigate the effect of blood in the perfusate of an isolated, maximally vasodilated cat hindlimb. CFC was measured during perfusion with several different perfusates: a blood-albumin mixture, albumin alone, a plasma-albumin mixture, and a plasma-albumin mixture containing varying volumes of red blood cells. In some experiments the plasma was ultrafiltered to remove low-molecular-weight components. It was found that perfusion with albumin alone usually raised CFC above the level found with the blood-albumin mixture, plasma could reduce the CFC when CFC was elevated above that value obtained with the blood-albumin mixture, ultrafiltered plasma failed to reduce CFC when CFC was elevated, and the red cell content of the perfusate had no influence on CFC over the hematocrit range of 0-20% when the plasma solute concentrations were kept constant. It was concluded that the reduction of CFC by blood was caused by a small molecule (less than 10,000 mol wt) present in plasma.