Analysis of intestinal water absorption and changes in circulating blood volume in rats

To analyze the change in blood volume after intestinal absorption, circulating blood volume was continuously monitored in rats after infusion of various solutions into the small intestine. Arterial and venous catheters were connected to a system for continuous monitoring of blood volume by the dilution method using 51Cr-labeled erythrocytes. Test solutions (tap water, 0.45, 0.9, 1.8% NaCl, 0.45% NaCl with 2% glucose, and 5% glucose) were infused at a rate of 1 ml/100 g body wt. for 10 min through a duodenal catheter. After the infusion, blood volume increased except in the 1.8% NaCl group, which showed a transient decrease in blood volume by about 10%. The rate of blood volume increase was highest in the 0.45% NaCl with 2% glucose group and lowest in the 1.8% NaCl group. The retention ratio of infused solution in the vascular space was almost identical among the groups and was about 22%. These results indicate that the rate of increase in blood volume after intestinal administration of fluid is modified by the osmolality of the fluid and Na-glucose co-transport, whereas the retention ratio of the infused fluid in blood is constant.