Factors influencing mixing and mass transfer in the small intestine : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Digestive Biomechanics (Physical Process of Digestion) at Massey University, Turitea, New Zealand

Abstract

This work sought to determine the factors influencing mixing and mass transfer in the small intestine. Specifically, the work was focussed on the gut periphery (i.e. perivillous region) of the terminal ileum in the brushtail possum (Trichusurus vulpecula). The salient questions to answer were; 1. What are the microrheological properties and disposition of mucus in the perivillous space? 2. What are the disposition and movements of the mucosa and the associated villi during postprandial gut motility patterns of pendular contractions? 3. Are villi rigid structures during physiological levels of lumen flow? The following three main experimental works of this thesis were all conducted using live gut wall samples maintained ex vivo. In addition, computational models were developed incorporating the novel findings detailed in this thesis to assist in visualizing mixing and mass transfer in the perivillous space. 1. The properties of the perivillous fluid environment were assessed by multiple-particle-tracking of the Brownian motion of fluorescent microbeads on gut samples. 2. The movements and disposition of the mucosal surface and associated villi during pendular contractions were observed for whole lengths of everted gut samples. 3. Flow velocities in the perivillous space of gut samples were determined by microparticle-image-velocimetery of microbeads. The movement of villi in response to physiological levels of lumen flow were quantified by image analysis. The following are the main findings and implications of the work. 1. The perivillous fluid environment consisted of discrete viscoelastic bodies dispersed within a watery Newtonian phase. Such characteristics of the fluid environment were thought to be conducive for mixing and mass transfer, and likened to the processes of gel filtration. 2. Gut pendular contractions generated transient mucosal microfolds, which resulted in the formation of periodic congregation and separation of villous tips. Such a mechanism was predicted (using computational simulations) to augment mixing and mass transfer of nutrients at the gut periphery. 3. Villi were rigid structures, which were more prone to pivot than to bend, while intervillous fluid was predicted to be quasi-static during physiological levels of lumen flow. Such a feature of villi supports a perivillous mixing and mass transfer mechanism driven by mucosal microfolding In conclusion, mixing and mass transfer in the perivillous space are governed by more complex dynamics than previously assumed and by factors previously unknown.