Anatomical and physiological considerations
Digestion of food begins in the mouth where through the mechanical action of chewing and the chemical action of enzymes found in saliva, a bolus of food is formed and then passes through the esophagus. Contraction of the muscles in the esophagus moves the food bolus into the stomach by a process called peristalsis.
Once in the stomach, the food is broken down further by strong contractions which expose it to gastric juices including hydrochloric acid and digestive enzymes which are secreted by glands in the lining of the stomach. The food is eventually converted into a liquid material called chyme which is then passed into the first part of the small intestine, the duodenum.
The intestine is where food is eventually absorbed. In the intestine, the presence of chyme stimulates the production and release of a variety of enzymes from the pancreas and small intestinal glands.
Each enzyme has a specific role in digestion. There are special enzymes that digest protein into amino acids, starch into glucose and fat into fatty acids. The liver produces a substance called bile which helps to digest fat. Bile is formed in the liver, stored in the gallbladder and released into the small intestine as needed.
The small intestine is about 23 feet long in the adult and has three segments. The duodenum is the first part of the small intestine and is about 10 inches long. The jejunum is the middle segment of the small intestine and is about 8 feet long. The last portion of the small intestine, the ileum, is about 12 feet long.
The absorptive surface of the small intestine is greatly enhanced by the numerous folds and finger-like projections called villi and microvilli. It has been estimated that given all the folds, villi and microvilli, the total absorptive surface of the small intestine is about the size of a half of a basketball court!
Absorption of nutrients takes place all along the intestine, but each segment of the intestine absorbs only certain nutrients. Carbohydrates are ingested primarily in the form of starch or carbohydrates, sucrose (table sugar), fructose (fruit sugar) and lactose (milk sugar). The suffix "ose" refers to a form of carbohydrate.
Salivary and pancreatic amylase (digestive enzymes) break up the starch into long chains of sugars called oligosaccharides and shorter chains called disaccharides and trisaccharides. Most starch hydrolysis (breaking up a molecule into its smaller components) occurs in the duodenum and absorption takes place in the duodenum and jejunum.
Specific enzymes that hydrolyze disaccharides (disaccharidases) into their simplest forms or monosaccharides are located along the small intestine microvilli. Once the sugar is in the form of a monosaccharide, it can then be absorbed into the blood.
Carbohydrate malabsorption occurs in pancreatic disease, in selective deficiency of disaccharidases such as lactase (digests lactose) or sucrase (digests sucrose), in disorders of small intestinal cell function such as sprue or regional enteritis, and in loss of intestinal mucosal surface which occurs after resection of bowel. Abdominal distention, bloating and gas may be signs of carbohydrate malabsorption.
Proteins are broken down into long chains of amino acids by pancreatic enzymes. Small intestinal enzymes activate the pancreatic enzymes so that digestion and absorption of protein can take place. Absorption of amino acids and peptides occurs in the duodenum and jejunum.
Dietary fat is normally absorbed in the duodenum and jejunum. Before fat can be absorbed, however, it must first be made into a water-soluble form. Broken down dietary fats combine with bile salts and phospholipids (substances present in bile from the liver) to form a packet called a micelle. The micelle is water-soluble and is easily absorbed in the duodenum and jejunum.
Large amounts of water are involved in digestion and must be recycled in order to prevent dehydration. Water is reabsorbed in the large intestine. From the large intestine, water goes back into the bloodstream and the waste passes into the rectum and out the anus.