You start in the Esophagus, falling down with some grass that was eaten with you, which you must avoid and beat to the exit. If you continue to fall, you proceed to the next the level in the stomach. If you decide to use the thrusters and go up, you make it to a secret level and can then try to escape back out through the mouth.
The mouth level, you need to time it perfectly to fit between the teeth to escape.
Stomach Chamber 1 has some worms that home in on you. You need to outrun them to the exit or they will cause you to crash.
Stomach Chamber 2 has a lot of motion. The larger capillaries move at varying speeds as well as the right side wall.
Stomach Chamber 3 has platelets. They can’t hurt you, but they can knock you about and they all move randomly.
Stomach Chamber 4 has a collapsing roof, trying to squeeze you and stop you hitting the exit.
The first stage of the intestines is a test of precision flying.
The final intestine stage, the alien ship starts upside down. So you need to act quickly to upright yourself.