Intestine, Small Health Article

Definition

The small intestine is a long coiled tube located in the abdominal cavity. It is the major site of chemical digestion and absorption of nutrients by the body and consists of three sections: the duodenum, the jejunum, and the ileum.

Description

The small intestine is the longest section of the digestive system. This important body system digests food by breaking it down into nutrient molecules small enough to pass into the bloodstream. Nutrient molecules are absorbed in the small intestine and sent into the blood circulatory system. The digestive system also eliminates solid waste, recycles water, and absorbs vitamins from nutrients in the large intestines. Even though some starch breakdown takes place in the mouth and some breakdown of protein is done in the stomach, most of the digestion occurs in the duodenum.

The small intestine is a coiled, tube-like organ held in place by two membrane sheets attached to the walls of the abdominal cavity and referred to as the mesentery. Nerves, blood and lymph vessels to and from the small

KEY TERMS

Abdomen—A part of the body that lies between the thorax and the pelvis. It contains a cavity (abdominal cavity) that holds organs such as the pancreas, stomach, intestines, liver, and gallbladder. It is enclosed by the abdominal muscles and the vertebral column (spine).

Abdominal cavity—Hollow part of the body located between the diaphragm, which is the thin muscle below the lungs and heart, and the pelvis, which is the basin-shaped cavity that contains the reproductive organs, the bladder, and the rectum. The abdominal cavity contains the abdominal organs.

Acid—Refers to a compound that is acid or sour. When dissolved in water, acids yield hydrogen ions.

Amino acids—Organic compounds containing mostly the elements carbon, nitrogen, and oxygen that combine to form peptides and proteins.

Base—Refers to a compound that is caustic (soda, lime). Bases dissolve in alcohol and in water, and can combine with fats.

Bile—A greenish yellow fluid produced by the liver and stored in the gallbladder that is secreted in the small intestine to assist in the absorption of fats.

Bile ducts—Passages external to the liver for the transport of bile.

Digestion—The conversion of food in the stomach and in the intestines into substances capable of being absorbed by the blood.

Digestive system—Organs and paths responsible for processing food in the body. These are the mouth, the esophagus, the stomach, the liver, the gallbladder, the pancreas, the small intestine, the colon (large intestine), and the rectum.

Duodenum—The first section of the small intestine, extending from the stomach to the jejunum, the next section of the small intestine.

Epithelial cells—Cells covering the surface of the body and the lining of its cavities. In the small intestine, the epithelial cells cover the villi and contain microvilli.

Epithelium—The covering of the internal and external surfaces of the body and of the lining of blood vessels and small cavities. It consists of cells joined by cementing substances.

Gastric juice—An acidic secretion of the stomach that breaks down the proteins contained in the ingested food, prior to digestion.

Gland—An organ that produces and releases substances for use in the body, such as fluids or hormones.

Ileum—The last section of the small intestine located between the jejunum and the large intestine.

Jejunum—The section of the small intestine located between the duodenum and the ileum.

Lymph—Colorless liquid that bathes body tissues and circulates in the lymph vessels.

Mesentery—The membrane that connects the jejunum and the ileum to the abdominal cavity walls.

Mucosal folds—Mucosal folds are circular folds found on the inner surface of the small intestine. They increase surface area and help mix the partially broken down foods by acting as baffles.

Mucous membrane—The lubricated lining of several body organs that contains mucus-secreting glands.

Mucus—Slimy secretion of glands found on mucous membranes composed of various proteins, salts, and white blood cells.

Pancreas—The pancreas is a flat, glandular organ lying below the stomach. It secretes the hormones insulin and glucagon that control blood sugar levels and also secretes pancreatic enzymes in the small intestine for the breakdown of fats and proteins.

Peritoneum—Thin membrane that lines the abdominal and pelvic cavities and covers most abdominal organs.

Pylorus—The opening from the stomach into the small intestine.

Small intestine—The part of the digestive tract located between the stomach and the large intestine.

Villi intestinales—Microscopic hair-like structures covered with epithelial cells measuring 1–1.5 mm that line the mucous inner membrane of the small intestine. The epithelial cells of the villi contain microvilli and are responsible for the absorption of nutrients. Being so small and numerous, they effectively increase the absorptive surface area of the small intestine.

intestine lie between the two sheets of the mesentery. In the adult, the small intestine measures on average about 22 ft (6.7 m) with a diameter of 1–2 in (2.5–5 cm). It consists of three segments: the short duodenum, the jejunum, which represents 40% of the small intestine, and the terminal ileum, which accounts for the remaining 60% and which empties into the large intestine. The small intestine thus forms a passage going from the pylorus (opening from the stomach) to the large intestine.

The inner mucous membrane of the small intestine is not flat and uniform but folded to such a great extent that its inner lining is referred to as the mucosal folds. The mucosal folds are covered with approximately 20,000 tiny hair-like projections called villi that are lined with epithelial cells studded with microvilli. One villus contains about 500 microvilli and in one square inch of small intestine, there are some ten billion microvilli.