Health Library

Digestion

Food passes from the mouth through the esophagus to the stomach. The stomach churns the food and breaks it down further with hydrochloric acid and an enzyme called pepsin. The process of breaking food down in the stomach takes a few hours. From there, it goes to the duodenum, which the first part of the small intestine. Within the duodenum, digestive bile produced by the liver and stored in the gallbladder along with enzymes from the pancreas break it down more.

Enzymes are chemicals that speed up the digestion of specific types of food. For example, the enzyme trypsin breaks down the protein in steak, and lipase helps to break down fat. Humans don’t have enzymes to break down certain plant fibers, which is why they can’t be fully digested. The enzyme called lactase breaks down the sugar in milk. Sometimes, lactase is not produced by the body at all, or in insufficient amounts, making a person lactose intolerant. So, when a person who is lactose intolerant eats ice cream or yogurt, the digestive system gets bloated and expels gas.

Once everything is broken down, the small intestine absorbs the nutrients the body needs. From there the nutrients go into the bloodstream and to the liver, where poisons are removed. Undigested food and water continue through the small intestine and go into the large intestine, where water is reabsorbed. Then, at the end of the line, feces are eliminated through the rectum and anus.

Review Date: 10/20/2022  

Reviewed By: Linda J. Vorvick, MD, Clinical Professor, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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