[Osmotic equilibrium between the intestinal lumen and blood of the growing rat without increase in the fluid content of portal vein blood].

In rats weighting 230-280 g, the resistance, the protein, and the electrolyte concentration of portal vein blood were measured continuously. At constant electrolyte concentration (Cel) resistance was a true measure of hematocrit. Protein concentration (Cprot) was monitored refractometrically in a continuous stream of filtered plasma. The changes brought about by intraduodenal and intragastric injections of water were studied. For comparison, equal amounts (0.5-1% of body weight) of isotonic NaCl were instilled intraduodenally. Water led in all experiments to an immediate and steep but variable drop of Cel within a minute without an accompanying drop of Cprot. The drop of Cprot was delayed by 1.4-2.6 min. The delay was even greater when a small increase of Cprot by 65 mg/dl took place. No such delay occurred when saline was given. After this initial phase, the course of Cprot and hct curves was identical for both fluids. Using these two parameters as independent indicators of volume, similar values for blood volume increase were calculated from their final change. The result suggests that in the gut, osmotic equilibration has priority over fluid transport.