Effect of villosity and distension on the absorptive and secretory flux in the small intestine.

The effects of villosity and distension on the absorptive and secretory flux in the small intestine were investigated theoretically in a simplified model. In the case of low epithelial permeability, villosity increases both fluxes by surface enlargement, but in the case of high epithelial permeability, this occurred only if the intervillous spaces are very narrow. Otherwise, the flux is reduced due to the intervillous diffusion resistance, which is more effective than the enlargement of the surface area in that case. Distension increases the fluxes due to the additional surface exposed, by opening the intervillous spaces. In the case of low epithelial permeability this increase exceeds that expected from the enlargement of the smooth inner cylindrical surface area. In the case of high epithelial permeability, however, the increase of the fluxes exceeds surface enlargement only in the first phase, just after opening the intervillous spaces. Otherwise, the increase of the flux is less, since the hindrance by the intervillous diffusion resistance is more effective than the increase of the smooth inner cylindrical surface area. In the intervillous spaces the concentration gradient is non-linear with the steepest slope at the entrance due to the permeation through the lateral surfaces of the villi. The gradient approaches linearity in the center of broad intervillous spaces and becomes steeper when the width decreases and the epithelial permeability increases. In rat small intestine broad intervillous spaces are formed at the front sides of the trapezoidal villi by the predominant circular distension. The diffusion resistance in these spaces and the increase of the supravillous diffusion resistance weaken the increase of the absorptive and secretory flux by distension, especially in the case of high epithelial permeability.