Creating the Digestive System
The digestive system looks like this when it is finished:
Begin by creating a linear profile. Start out by using all corner points (set both Initial and Drag to Corner).
Convert all the vertex points to Smooth. Select all the vertices, right-click, and then choose Smooth.
Click the Line Refine button.
Add vertices to the long, straight part of the digestive system.
The finished line has plenty of vertices so that the digestive system can be animated to simulate peristalsis.
Apply the Lathe modifier and choose the X direction. Then choose Sub-Object.
Move the axis of the modifier downward (-Y) to make the digestive system wider. The end result should look like this: