Simple diffusive-convective model for intestinal absorption of a nonelectrolyte (urea)

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Abstract
The kinetics of absorption of urea from loops of dog small intestine were found to be reasonably consistent with a model incorporating diffusive and convective terms as independent linear contributions. The starting equation was Ju = [PHI] JW C + K(C[long dash]Cp) in which Ju and Jw are the rates of urea and water absorption, C and Cp are the urea concentrations in the lumen and plasma, is a sieving coefficient, and K is the diffusive permeability coefficient. This equation was integrated to take into account the changing luminal fluid volume and concentration of an isolated loop; and, to normalize for loop size, it was assumed that K was proportional to the filling volume of the loop: K = [alpha]VO. A variety of initial luminal fluid and plasma concentration relationships was tested and the results conformed, at least to a first approximation, to the assumed model with [PHI] = 0.8 and [alpha] = .04/min. As predicted by the model, uphill absorption of urea was observed for appropriate initial concentration relationships and rates of fluid absorption. Further analysis of the observations favored the inferences that a major portion of the diffusive flux is not associated with the convective flux, that the convective flux is by flow through channels rather than vesicular, and that the functional pore radius of the channels may be 10-13 A.