The Digestion Process

Breaking down digestion

The digestion process is a combination of chemical and mechanical digestion. Chemical digestion breaks down food via a chemical change, in which the digestive juices and enzymes break down food into components small enough to enter the GI tract, the blood cells or the lymph cells. Mechanical digestion breaks down food via chewing or grinding in the mouth as well as via the muscular activity in the stomach and intestines. Both types of digestion begin in the mouth.

The mouth and stomach perform both chemical and mechanical digestion.

Nutrition is the process of taking in organic substances and minerals for the seven life processes. E.g. energy for growth and tissue repair. A person is described as malnourished if

Increased contractions of the stomach push the food through the sphincter and into the small intestine as the stomach empties over a 1 to 2-hour period. High fat diets significantly increase this time period.

The small intestine is the major site for digestion and absorption of nutrients. The upper part, the duodenum, is the most active in digestion. Secretions from the liver and pancreas are used for digestion in the duodenum. Epithelial cells of the duodenum secrete a watery mucus. The pancreas secretes digestive enzymes and stomach acid neutralizing bicarbonate. The liver produces bile, which is stored in the gall bladder before entering the bile duct into the duodenum.

Digestion of carbohydrates, proteins, and fats continues in the small intestine. Starch and glycogen are broken down into maltose. Proteases (enzymes secreted from the pancreas) continue the breakdown of protein into small peptide fragments and some amino acids.

Bile emulsifies fats, facilitating their breakdown into progressively smaller fat globules until they can be acted upon by lipases. Bile contains cholesterol, phospholipids, bilirubin, and a mix of salts. Fats are completely digested in the small intestine, unlike carbohydrates and

Each villus has a capillary network supplied by a small arteriole. Absorbed substances pass through the brush border into the capillary, usually by passive transport.

Maltose, sucrose, and lactose are the main carbohydrates present in the small intestine; they are absorbed by the microvilli. Starch is broken down into two-glucose units (maltose) elsewhere. Enzymes in the cells convert these disaccharides into monosaccharides that then leave the cell and enter the capillary. Lactose intolerance results from the genetic lack of the enzyme lactase produced by the intestinal cells.

Digested fats are not very soluble. Bile salts surround fats to form micelles that can pass into the epithelial cells. The bile salts return to the lumen to repeat the process. Fat digestion is usually completed by the time the food reaches the ileum (lower third) of the small intestine. Bile salts are in turn absorbed in the ileum and are recycled by the liver and gall bladder. Fats pass from the epithelial cells to the small lymph vessel that also runs through the