(411f) Mixing Efficiency of Longitudinal Muscle and Segmentive Contractions in the Small Intestine

It is largely recognize in literature that muscular contractions of the small intestine improve nutrient absorption by facilitating the mixing of intestinal contents during digestion and the continuous renewal of nutrients at the intestinal wall.

Two specific motilities which have been observed in the rabbit during ex-vivo experiments are longitudinal motions and segmentive contractions. Longitudinal motions act to move the organ axially with no change in the diameter of the intestine. Segmentive contractions act locally to partially occlude the intestinal core. 

The goal of this work was to numerically investigate how these two different types of contraction affect the dynamics and mixing of intestinal fluids of different viscosities. The rabbit intestine was modelled as an elastic solid (Young's modulus = 3000 Pa, Poisson's ratio = 0.35) and was simplified to be of cylindrical geometry. The motion of the intestinal wall was imposed based upon video data from ex-vivo observations of the rabbit ileum.

The governing system of mathematical equations were solved in the CFD software package Polyflow (ANSYS). The behaviour and stretching of two Newtonian fluids (10-3 Pa.s and 1 Pa.s) and one non-Newtonian fluid (of Bird-Carreau type with exponent 0.41, zero shear viscosity 0.65 Pa.s, and infinite shear viscosity 0.501 x10-8 Pa.s) were numerically analysed. The mixing efficiency was quantified using an in-built post-processor by measuring the stretching and folding of differential fluid elements.

It was found that the shear rates that develop as a result of these motions are too small to cause significant pseudoplastic behaviour of the fluid under study. Stretching rates were found to be dependent on both the properties of the fluid and the contraction type. High rates of stretching were observed along the entire wall during a longitudinal motion, but the dynamics suggest that flow is mostly axial with little mixing between the core and wall. The stretching of fluid elements was much more localised during a segmentive contraction, but more movement between the core and the wall was observed. Viscosity was shown to be an important factor with much higher stretching rates in the low viscous fluid. The non-Newtonian fluid behaved similarly to a viscous-Newtonian fluid, due to the low shear experienced.

 Simulations also showed that blocking one end of the intestine increased the rate of element stretching during a segmentive contraction, with the rate of stretching increasing the closer to the blockage the contraction occurred. 

Results from this study illustrated the effect of different intestinal motility patterns of the wall on the dynamics and mixing efficiency of intestinal contents during digestion