A method for continuous plasmapheresis and refractometric determination of the protein concentration of blood circulating in the vascular system of the unanesthetized rat is presented. The method is applied to demonstrate very rapid transvascular fluid exchange during physical activity of the animal, noradrenalin infusions and hemorrhage. Changes of protein concentration are compared with concomitant changes of hematocrit in the hepatic portal vein and the v. cava inferior. Though both parameters invariably change in a parallel fashion, relative variations of protein concentration fall short of hot variations by 50%.

A refractometric method was used for the continuous registration of plasma protein concentration in rats during and after 3 and 5 minutes of provoked activity. Simultaneous conductometric measurements of hematocrit (hct) showed that, although both invariably changing in the same direction, the relative change of protein concentration is always less than that of hct: plasma volume changes calculated from the former fall short of those calculated from hct by 34 +/- 23% during the hemoconcentrative period during activity and by 52 +/- 11% during the hemodilutory period after activity. The difference between these figures was significant, thus implying that fluid leaving the circulation during the filtration phase is less rich in protein than that entering it during the absorptive phase of microvascular adjustments. A kinetic analysis of the period after activity was made. The rate constants of fluid- and protein-flux were closely correlated. Both plasma volume and intravascular protein mass increased asymptotically to a new equilibrium 6% above control within 30 min after activity. It is suggested that the excess protein is mobilized from large parenchymatous organs, mainly the liver.