Food digestion is the process by which our bodies break down foods and liquids into smaller molecules so they can be absorbed and used for energy and nourishment. The digestive tract is a series of organs joined in a long tube from the mouth to the anus. The tract's lining contains tiny glands that synthesize the digestive juices as well as a layer of smooth muscle that helps propel the food in a wave-like movement called peristalsis.
Mouth
Food digestion begins in the mouth. Your teeth chew the food, mixing it with saliva. Saliva contains enzymes that initiate carbohydrate digestion. Once food is chewed, it gets shaped into a ball called a bolus and swallowed. The food travels down the esophagus, a straight and narrow tube, to the stomach.
Stomach
Besides being a receptacle for swallowed foods, the stomach produces digestive juices and an enzyme to break down proteins. It mixes up the foods, liquids and digestive juices to produce chyme, or a partially digested food mixture. The stomach empties the chyme slowly into the small intestine through the pyloric sphincter.
Small Intestine
Most of food digestion occurs in the beginning of the small intestine. As the food enters, so do other digestive juices from the pancreas, liver and intestine to further break down the food particles. The contents are mixed and pushed forward, and the digested nutrients are absorbed through the intestinal walls. The undigested materials, fiber and dead cells are pushed into the large intestine, remaining there until a bowel movement.
Breakdown
Carbohydrates are converted from complex forms, or polysaccharides, to monosaccharides including glucose, galactose and fructose, by salivary, pancreatic and intestinal enzymes. Proteins are broken down from polypeptides to single amino acids as well as di- and tri-peptides. Bile from the liver mixes with fat in food, dissolving it and allowing it to be digested into its simplest forms, including free fatty acids, monoglycerides, glycerol, phospholipids and cholesterol.
Regulation
Hormonal and nerve regulation control digestion. Gastrin, secretin and cholecystokinin stimulate digestive juices and cause organ movement. Ghrelin and peptide YY, which are produced in the intestinal tract, regulate appetite. Intrinsic nerves that are embedded in the intestinal walls release substances that either speed up or slow down digestion. Extrinsic, or outside, nerves come from the brain or spinal cord and release acetylcholine to increase peristalsis and production of digestive juices and adrenaline to produce the opposite effect.
References
- National Digestive Diseases Information Clearinghouse: Your Digestive System and How It Works
- "Nutrition Therapy and Pathophysiology"; Marcia Nelms, Kathryn Sucher, and Sara Long; 2007.
- The Ohio State University Medical Center: The Digestive System
