For a nice, easy introduction to the digestive system, Khan Academy has a video that I wish I had made myself (or at least written):

The digestive system: Hank takes us through the bowels of the human digestive system and explains why it’s all about surface area.

https://www.khanacademy.org/embed_video?v=qF6TBRaE2_A

For a little deeper understanding, Marion Diamond does a great job, but over a lot longer time (three lectures)

and, finally, if you want to check out the Magic School Bus episode that Marion Diamond was consulted on and then refused to be a part of because it didn’t have the guts to finish the ride through the digestive tract, check it out here: