Hello!
I'm afraid that I can't help with the Digestive System specifically. And in general, you won't find help about specific domains or designing sims in this forum.
What I can do is describe the prerequisites for implementing a simulation using the PhET platform:
(1) You must be very familiar with Javascript. And this is not the Group for getting general Javascript help or asking Javascript questions. If you're not already a Javascript ninja, a Google search will help you locate many resources for understanding Javascript and developing your Javascript skills.
(4) Look at the example-sim repository,
https://github.com/phetsims/example-sim. This example is typical of the structure of most PhET simulations. Experiment with this example, attempt to make changes, learn how it works, use it as a starting point for your own simulation.
Apologies if that sounds daunting, but developing a simulation is an advanced software engineering task.
I hope this helps.
Chris Malley
PixelZoom, Inc.