Food is propelled down the esophagus. At the base of the esophagus there is a circular muscle (the lower esophageal sphincter) that relaxes and allows the food go into the actual stomach. The stomach grinds the food and mixes it with acid. When one observes, smells or eats food, the vagal nerve stimulates the stomach to produce acid. The roles of acid are to help digest protein and to kill bacteria that are swallowed. The vagal nerve also stimulates the activity of the small intestine and the large intestine (colon).
Fat consumption is a powerful stimulus for the body to release a digestive hormone called cholecystokinin. This hormone causes gallbladder contractions which releases bile into the duodenum (the first part of the small intestine) to help digest fat. This hormone also stimulates the muscular activity of the intestines.
Now that the food is partially digested and broken up into tiny particles, the small intestine can absorb the nutrients and water. Carbohydrates are absorbed by the small intestine and some require that an enzyme is present in the intestinal lining, which will break it down into small absorbable components. The best example of this is lactase, which breaks down the dairy sugar lactose into two small sugar particles that can be absorbed.
When food arrives in the stomach, the pancreas also starts secreting hormones and enzymes. Insulin in secreted into the blood stream, which enables the body's cells to incorporate glucose (or sugar). The pancreatic digestive enzymes are secreted into the duodenum where they help breakdown proteins and fat into absorbable particles. The liver is responsible for manufacturing the bile and for processing food byproducts that travel in the bloodstream.
The remaining liquid and semisolid waste matter in the small intestine travels to the colon where it is stored temporarily in the right side of the colon where most of the water is absorbed. The waste matter then turns into formed stool. The colon is able to absorb 9 times the amount of water that the small intestine can. With progressive contractions the stool passes along the route of the colon and becomes ready for evacuation. The rectum can expand so that stool can be temporarily stored until a bathroom is convenient.