Digestion is the process by which the gastrointestinal system retrieves important nutrients for the body and chemically changes the unused food into waste.

Mastication, or the chewing of food in the mouth, is the first step of digestion. Saliva initiates digestion and changes the chewed food into a soft mass, or bolus. Saliva makes the bolus slippery, making it easier to be swallowed and slide down the back of the throat and esophagus. The bolus passes through the esophageal sphincter before it enters the stomach. Inside the stomach, hydrochloric acid is released, breaking down large food molecules into smaller ones and liquefying the bolus. The liquefied bolus, now called chyme, then passes through the pyloric sphincter and enters the duodenum, the first section of the small intestine. It is here that enzymes released from the pancreas, liver, and gallbladder further breakdown chyme into elements that can be easily absorbed and used by the body.

The small intestine is lined with a heavily folded inner mucosa and small fingerlike projections called villi. The villi enable digested food to enter the bloodstream. It is here, in the small intestine, where all nutrients and vitamins are absorbed.

Chyme can travel through up to 20 feet of small intestine before it passes through the ileocecal valve to enter the large intestine. Very little digestion occurs in the large intestine. Undigested chyme that enters the large intestine is considered waste. The waste becomes more and more solid as it passes through the large intestine because water is continuously being reabsorbed from the waste. Waste collects in the rectum, or end of the large intestine, until the brain signals for it to be expelled from the body.