An Expanded Finite Element Model of the Intestinal Mixing of Digesta

Abstract

We used a finite element model to simulate flow through the intestine induced by peristalsis and segmentation. These models explored the effects of varying the rheological properties of digesta, incorporating a slip effect at the wall and the presence of an area of lumen distension adjacent to the area of vorticeal mixing that developed during peristalsis. Reynolds numbers were consistently low (<200) throughout all simulations indicating that local conditions were insufficient to bring about turbulence. When the viscosity of the material flowing through a simple tube model was increased, the area of the vortex orad (upstream) to the zone of peristaltic coaptation was relatively reduced, whilst that aborad (downstream) to the zone of peristalsis was increased. This effect was reduced when the material in the lumen had shear thinning properties. When a region of relative distension of the lumen that travelled aborad to the zone of peristalsis was incorporated into the model, the total area of vortex formation was reduced when material of either high or low Newtonian viscosity was in the lumen. However, these reductions were small compared with those obtained when slip at the intestine wall was incorporated into the model. Incorporation of slip resulted in a marked reduction of both orad and aborad vortices, an effect that persisted when the other effects were incorporated into the same model. The authors conclude that the translocation of nutrients from the intestinal lumen to the wall by action of short-lived vortices will be significantly reduced when the apparent viscosity of digesta is high and there is significant slip at the wall, even when digesta is a shear thinning, non-Newtonian fluid. Hence, the consumption of a diet that contains a high proportion of fibre or other insoluble residue will physically impair absorption by reducing vorticeal flow and promoting creep flow.