The Small Intestine

Food leaves the stomach as a semifluid mass called chyme. Chyme enters the small intestine, a tube about 7 m (23.1 ft.) long and 3 cm (1.2 in.) in diameter.

The small intestine has three sections. The uppermost section, the duodenum is about 25 cm (1.2 ft.) long. The next section, the jejunum,is about 4m (13.1 ft.) long. The last 2.5 m (8.2 ft.) form the ileum. The majority of chemical digestion and absorption takes place in the small intestine.

The Pancreas and the Liver

The pancreas and the liver secrete digestive juices into the small intestine and so play important roles in digestion. The pancreas, located behind the stomach, has many small lobes that secrete enzymes and sodium bicarbonate. The sodium bicarbonate neutralizes the acidity of the chyme leaving the stomach. The chief enzymes secreted by the pancreas are pancreatic amylase, pancreatic lipase, trypsin, and chymotrypsin. Pancreatic amylase continues the chemical digestion of starch that began in the mouth. It converts starches into maltose. Pancreatic lipase breaks down fats into their component molecules, fatty acids and glycerol. Trypsin and chymotrypsin break down the proteins by splitting them into smaller chains of amino acids called peptides.

The liver is the largest internal organ, weighing about 1.5 kg. It produces bile, which is a salt solution, not an enzyme. Bile emulsifies fat – that is, it breaks down large fat globules into tiny droplets. This process greatly increases the surface area of fat particles. Lipase can than act on the fat more effectively. This process is referred to as the detergent effect of bile, because detergent does the same thing to fat. Bile generally enters the duodenum from the gall bladder, a small sac where bile is stored.

Digestion in the Small Intestine

Most chemical digestion occurs in the duodenum. A heavy layer of mucus protects the first few centimeters of the duodenum from the acidic chyme released by the stomach. If the mucus protection is not sufficient, the high acid level can cause duodenal ulcers, which are even more common than stomach ulcers.

Enzymes produced by the small intestine include peptidases, maltase, lactase, sucrase, and intestinal lipase. Various peptidases break down peptides into amino acids. Maltase, lactase, and sucrase convert disaccharides into monosaccharides. Intestinal lipase, like pancreatic lipase splits fats into fatty acids and glycerol.

Absorption in the Small Intestine

Amino acids, monosacharides, fatty acids, glycerol, water, and minerals are all absorbed in the small intestine. Absorption occurs quickly in the small intestine because of its lining. The mucuous lining consists of folds covered with millions of tiny projections called villi. Each villus has a brush border composed of approximately 600 microvilli, which are extensions of the epithelial tissue covering the villi. Intestinal enzymes are not released into the cavity of the small intestine. Instead, the enzymes remain in the brush border where they act upon the food molecules. The molecules are then absorbed into the bloodstream through the microvilli walls. The lining folds, the villi, and the microvilli together increase the surface are of the small intestine 600 times. The result is an absorptive are equal in size to the area of a tennis court.

Absorption occurs through the processes of diffusion and active transport. Each villus contains tiny blood vessels called capillaries, through which monosaccharide and amino acid molecules enter the bloodstream. The blood carries these nutrient to the body’s tissues. The liver converts excess glucose into glycogen, a form of starch, and stores it as a future energy source. The cells of the villi resynthesize fatty acids and glycerol into the fats. The villi contain tiny vessels called lacteals, which absorb the fats. These fats eventually pass from the lacteals into the bloodstream.