2.3.4. Small Intestine

The small intestine (or small bowel) is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. It divides in three parts

A) The DUODENUM is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. The duodenum precedes the jejunum and ileum and is the shortest part of the small intestine, where most chemical digestion takes place. The name duodenum is from the Latin duodenum digitorum, or “twelve fingers’ breadth”.

In humans, the duodenum is a hollow jointed tube about 25-38cm (10-15 inches) long connecting the stomach to the jejunum. It begins with the duodenal bulb and ends at the ligament of Treitz.

The duodenum is largely responsible for the breakdown of food in the small intestine, using enzymes. Brunner’s glands, which secrete mucus, are found in the duodenum. The duodenum wall is composed of a very thin layer of cells that form the muscularis mucosae. The duodenum is almost entirely retroperitoneal.

The duodenum also regulates the rate of emptying of the stomach via hormonal pathways. Secretin and cholecystokinin are released from cells in the duodenal epithelium in response to acidic and fatty stimuli present there when the pylorus opens and releases gastric chyme into the duodenum for further digestion. These cause the liver and gall bladder to release bile, and the pancreas to release bicarbonate and digestive enzymes such as trypsin, lipase and amylase into the duodenum as they are needed.

B) JEJUNUM is the middle section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. The jejunum lies between the duodenum and the ileum. The change from the duodenum to the jejunum is usually defined as the Duodenojejunal flexure and is attached, and thus “hung up”, to the ventricle (see stomach) by the ligament of Treitz. In adult humans, the small intestine is usually between 5.5 and 6m long, 2.5m of which is the jejunum. The pH in the jejunum is usually between 7 and 9 (neutral or slightly alkaline).

The jejunum is the second portion of the small intestine, and it has a lining which is specialized in the absorption of monosaccharides (fully digested carbohydrates) and amino acids (fully digested proteins). The proteins have been broken down in the stomach by acid and an enzyme called pepsin into amino acids. The carbohydrates are broken down in the duodenum by enzymes from the pancreas and liver into sugars. Fats are broken down in the duodenum by “lipase” from the pancreas into fatty acids. Amino acid, sugar, fatty acid particles, vitamins, minerals, electrolytes and water are small enough to soak into the villi of the jejunum and drop into the blood stream. The blood takes all these nutrients to all the other parts of the body to provide fuel to do their jobs.

C) The ILLEUM is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. The ileum follows the duodenum and jejunum and is separated from the cecum by the ileocecal valve (ICV). In humans, the ileum is about 2–4 m long, and the pH is usually between 7 and 8 (neutral or slightly alkaline).

The function of the ileum is mainly to absorb vitamin B12 and bile salts and whatever products of digestion that were not absorbed by the jejunum. The wall itself is made up of folds, each of which has many tiny finger-like projections known as villi on its surface. In turn, the epithelial cells that line these villi possess even larger numbers of microvilli. Therefore the ileum has an extremely large surface area both for the adsorption (attachment) of enzyme molecules and for the absorption of products of digestion. The DNES (diffuse neuroendocrine system) cells of the ileum secrete various hormones (gastrin, secretin, cholecystokinin) into the blood. Cells in the lining of the ileum secrete the protease and carbohydrase enzymes responsible for the final stages of protein and carbohydrate digestion into the lumen of the intestine. These enzymes are present in the cytoplasm of the epithelial cells.

The villi contain large numbers of capillaries that take the amino acids and glucose produced by digestion to the hepatic portal vein and the liver. Lacteals are small lymph vessels, and are present in villi. They absorb fatty acid and glycerol, the products of fat digestion. Layers of circular and longitudinal smooth muscle enable the digested food to be pushed along the ileum by waves of muscle contractions called peristalsis. The undigested food (waste and water) are sent to the colon.

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