Is movement of food through the gastrointestinal tract aided mostly by pressure from more food, by gravity, or by smooth muscles?
The large, hollow organs of the GI tract contain a layer of muscle that enables their walls to move. The movement of organ walls—called peristalsis—propels food and liquid through the GI tract and mixes the contents within each organ. Peristalsis looks like an ocean wave traveling through the muscle as it contracts and relaxes. (http://www.niddk.nih.gov/health-information/health-topics/Anatomy/your-digestive-system/Pages/anatomy.aspx)