Dependence of intestinal absorption in vivo on the unstirred layer.

The appearance rate of butanol, antipyrine, salicylic acid, and urea in the venous blood of rat jejunal loops perfused in vivo is increased up to 64%, if the intraluminal solution is mixed more efficiently by the simultaneous perfusion of air. The enhancement of the absorption can be attributed partly to the enlarged absorbing area but mainly to the reduction of the effective unstirred layer thickness by about 500 micrometers. The unstirred layer reduces the phenylalanine absorption at 0.1 mmol l(-1) but not at 100 mmol l(-1), since at high concentrations a full saturation of the transport system can be achieved in spite of the unstirred layer resistance. The interference of the unstirred layer increases with increasing absorbability of the substances.