Food begins its journey through the digestive system in the mouth, before being pushed by a series of involuntary muscle contractions through the esophagus, then the stomach, and then the small and large intestines. The digestive system is essentially a series of long, hollow organs lined with muscles that contract in a rhythmic pattern, known as peristalsis, to keep food moving in the correct direction.
The mouth is considered the first part of the digestive system because food is physically broken down there by chewing. Saliva also mixes with the food, starting the chemical digestion process that breaks larger molecules into small ones that the body can absorb. Once food is chewed and mixed with saliva, the tongue pushes food into the esophagus. This is the only voluntary muscle contraction in the digestive process. After swallowing, involuntary muscles begin pushing the food through the esophagus into the stomach. In the stomach, churning muscular contractions mix the food with digestive juices.
Once the food is combined with digestive juices in the stomach, it must be passed on to the small intestine. This occurs when the pyloric sphincter, a ring of muscle at the bottom of the stomach, relaxes and lets the food flow through. The partially digested food, now referred to as chyme, is then pushed through the small intestine by rhythmic, wave-like muscular contractions of the involuntary smooth muscle in the intestinal walls. After passing through the small intestine, the remaining food material is passed through the large intestine via the same process before being pushed out of the anus during a bowel movement.