| The primary roles of the small intestine (gut) are nutrient absorption and transport of digestive material (chyme). Two basic motility patterns (modes) exist to control these transport phenomena: propulsive wave-like propagations (peristalsis) for axial transport, and rhythmic contractions of short segments of the gut (segmentation) for radial transport to the surface for absorption. Once nutrients are advected to the surface, however, they must diffuse through a low-velocity fluid layer known as the "unstirred (water) layer (UL)." The thickness of this layer has been reported up to ∼1000 mum [19], which, however, is physiologically unrealistic [23]. More realistic studies have reported thicknesses of less than 50mum, suggesting that a "highly efficient stirring mechanism" may be present in vivo to produce such small UL thicknesses [21].;We hypothesize that finger-like projections (villi) on the intestinal surface, under active muscular control, generate a "micro-mixing-layer (MML)" which, when coupled with macro-scale mixing, can provide highly efficient stirring. We develop a physiologically accurate computational model to investigate the hypothesis. We use the lattice Boltzmann framework (LBM) to predict fluid and scalar motions, with moving boundary conditions, and zero-passive-scalar concentration at the epithelial surface to model rapid nutrient absorption.;Using simpler gut models of flow generated by peristaltic and segmental motility (developed to establish technology needed for the combined multi-scale model), we show that the fluid mechanics-driven absorption dynamics of the small intestine is much more complex and interesting than has been previously reported. Contrary to common wisdom, over the physiological range of occlusion ratios (Rmin/Ravg, where R min and Ravg are minimum and average radius, respectively), both peristalsis and segmentation significantly promote nutrient absorption. Segmentation has long been associated with the absorption process, but peristalsis has been generally explained as only necessary for axial transport. We show that the propulsive mechanisms generated by peristalsis for axial transport also create high nutrient gradients near the surface. These nutrient gradients lead to absorption characteristics of peristaltic motility that can be comparable to, or even exceed those of segmental motility. However, we also show that to maximize absorption while minimizing the power required by the muscle contractions necessary for gut motility, segmentation is optimal over the range of physiologically relevant occlusion ratios.;A three-dimensional, multi-scale lattice Boltzmann model was developed to study the effects of coupled macro-scale deformations and pendular villous motility on absorption. We show that active movements of the villi can enhance absorption by ∼25% beyond passive villous movements induced by macro-scale motility patterns alone. Increasing the length or the frequency of oscillation enhances the effect. Consistent with the findings of our simplified 2D and 3D villous motility models, we show that the presence of coordinated counter-oscillating groups of villi is a key mechanism in generation of the MML, and significantly promotes absorption. Azimuthally moving counter-oscillating groups of villi provide the most advantageous absorption characteristics. Because the azimuthal movements are perpendicular to the fluid patterns induced by the macro-scale motility, the interaction produces 3D fluid motions that further enhance mixing and increase the absorption in the gut. |
