  Over the past few decades, in silico modeling of organ systems has
significantly furthered our understanding of their physiology and biomechanical
function. In this work, we present a detailed numerical model of the upper
gastrointestinal (GI) tract that not only accounts for the fiber architecture
of the muscle walls, but also the multiphasic components they help transport
during normal digestive function. Construction details for 3D models of
representative stomach geometry are presented along with a simple strategy for
assigning circular and longitudinal muscle fiber orientations for each layer.
Based on our previous work that created a fully resolved model of esophageal
peristalsis, we extend the same principles to simulate gastric peristalsis by
systematically activating muscle fibers embedded in the stomach. Following
this, for the first time, we simulate gravity driven bolus emptying into the
stomach due to density differences between ingested contents and fluid contents
of the stomach. This detailed computational model of the upper gastrointestinal
tract provides a foundation on which future models can be based that seek to
investigate the biomechanics of acid reflux and probe various strategies for
gastric bypass surgeries to address the growing problem of adult obesity.
