Mixing, digestion, and emptying in human-stomach: a computational fluid dynamics study

Abstract

The stomach is essential for food digestion, however, its operation is highly complicated while its mechanisms are still not fully understood. Understanding gastric digestion mechanisms is important for the design of functional foods. Muscular movements of the stomach wall play critical roles in the human digestive process. They enhance the mixing of ingested foods with the gastric juice secreted from the stomach wall. In this thesis, a computational fluid dynamics (CFD) model is developed to better understand the flow dynamics in the process of mixing liquid foods with the gastric juice. The gastric motility is modeled with a dynamic mesh. The gastric juice secretion is modelled with sources of mass and hydrogen ions continually added into the stomach lumen. Gastric emptying rates of different foods are determined according to their calorie content. The numerical model is first used to investigate flow dynamics in the stomach. The numerical results show that the terminal antral contractions (TACs) can considerably increase the kinetic energy in the stomach. The TAC creates retropulsive “jets” and reduces the pH of liquid gastric contents. The mixing efficiency of liquid gastric contents and gastric juice is low, and it takes about 40-50 min to approach an acidic environment with a pH of 1.6 inside the stomach lumen. The simulations in this thesis show that the density difference in food/stomach-juice has significant effects on the dynamic spatial distribution of gastric pH. With the developed numerical model, the process of mixing and emptying of gastric contents is further investigated. The numerical results confirm that a fast pathway is located close to the lesser curvature (relatively short concave border on the right) of the stomach when water is emptied. However, this fast pathway doesn’t exist when the gastric contents are composed of water and food boluses with different properties. The muscle contractions enhance the mixing of light food boluses and water, while they have limited effects on heavy food boluses. As a result, food is distributed in layers; heavy food boluses are located in the bottom layer. Besides the gastric motility and high viscosity of foods, the food matrix made of heavy food particles is also important to the formation of the Magenstrasse (stomach road). The food matrix and the zone of wrinkles behave like a porous medium which has higher flow resistance to the light food particles than to the water, leading to faster emptying of water. The water is emptied along the stomach wall since the flow resistance in the stomach wrinkles is smaller than the one in the food matrix. This mechanism is supported by the numerical results, and it may explain the phenomena observed in the experiments. The CFD model is further developed to investigate the digestion of large food particles in an acidic and dynamic environment. The concept of food matrix introduced in this thesis is used to model the large food-particles. A reaction-diffusion-convection model is used to calculate the rate of disintegration. The numerical results suggest that the digestion and emptying become faster when the meat is treated at a higher temperature. The digestion rate is reduced considerably when the gastric motility or the secretion of hydrogen ions is weakened due to a stomach disorder. TACs stimulate strong retropulsive backflows which enhance the transport of enzymes and hydrogen ions, thereby accelerating the digestion process. Due to the flow resistance by the food matrix made of large food particles, liquid gastric contents are emptied in a pathway close to the stomach inner-surface. Large food-particles are mainly disintegrated in the region next to the stomach inner-surface. Therefore, the characteristic length scale of mass transfer (for enzymes or hydrogen ions) should be the size of the food matrix, instead of the size of large food-particles.