The details that you go into are all going to be dependent on the amount of information that you want to include within the model that you make. There are different aspects to the digestive system and you may want to simply show the different intestines and how the food moves around the body from entrance to exit.
You can label the model that you have, or you can simply make it as standard as possible and show the basic system that people already know and therefore you are not going to add too much detail to model that you create.
It is all going to depend on what examples you see to help you with the project that you are doing and the model that you are making. If you want to ensure that you are going to add as much detail as possible you can find the different examples on the internet which are going to have descriptions that you can follow and be aware of when you are creating the model for other people to look at and understand when they are in class.