I use this webquest with my 11th/12th grade A&P students. I use it in the middle of my unit; you can easily use it as an intro. Students do not need background information bout the digestive system to complete it.
This webquest walks students through a McGraw-Hill animation (https://goo.gl/fM90vs) which has two parts: a video animation and a "click on each organ and learn about it" part. I walk students through both parts--they actually have to read and think to complete it!--and have them complete the quiz at the bottom of the page.
Following the MGH animation, students transition to this page (https://goo.gl/XsSGj3 ) which has cool facts about the digestive system. Again, students actually have to THINK to be able to answer the questions.
NOTE: This product DOES have the key. I DO sell a version of it without the key at a discount; seek out that product in my store if you are interested in that version.