Capillary permeability to protein as a factor in the control of plasma volume.

1. Measurements of plasma volume, blood volume, extracellular fluid volume and capillary permeability to albumin were made in rats before and after plasmapheresis extending over 2 days.2. In spite of a mean reduction of albumin concentration of nearly 40%, plasma volume showed a significant increase of 12%. There was a slight increase in blood volume and a slight decrease in extracellular fluid volume but these changes were not statistically significant.3. The mean capillary permeability to albumin was significantly lower after plasmapheresis than before and changes in permeability showed a strong negative correlation with changes in plasma volume. The possibility of a causal relationship is discussed.