Fluid dynamics and mass transport driven by out-of-phase villi movements in the small intestine

The inner wall of the human small intestine is lined with columnar protrusions called villi. The movement of these villi generates micro-scale fluid motion, enhancing the transport of nutrients from the lumen to the epithelial cells on the villi surfaces for absorption. A high-fidelity numerical study using the lattice Boltzmann method has been conducted to investigate the fluid motion and mass transport induced by villous movement. The study found that grouping the villi arrays and making adjacent groups perform out-of-phase movements creates a micro-scale mixing layer. This mixing layer contains a series of eddies that not only facilitate the transport of nutrients from the upper bulk fluid to the villi surfaces through advection but also transport substances released from the villi surface back to the upper bulk fluid. Additionally, these eddies, interacting with the upper flow, travel downstream along the villi arrays, carrying fluid and dissolved substances downstream. The transport speed is influenced by the width of the villi groups and the frequency of their movements. This study provides a mechanistic foundation for how the human body can control the mixing and transport processes in the small intestine by regulating villi movement.