What is the small intestine?

Quick Answer
The section of the alimentary canal where the digestion of nutrients is completed and nutrients, vitamins, minerals, and fluids are absorbed. It is connected proximally to the stomach and distally to the large intestine and divided into the duodenum, jejunum, and ileum.
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Structure and Functions

In the average adult man, the small intestine is about 6 to 7 meters long, with a diameter of 2.5 to 3.0 centimeters. Its wall contains layers of smooth muscle and nervous tissue, which allow contraction movements and their control; its inner layer consists of absorptive mucosa, rich in blood and lymph vessels that drain into the portal vein. The mucosa is lined with absorptive epithelium.

Chyme (partly digested food) from the stomach is mixed in the duodenum with bile and pancreatic enzymes, which neutralize its acidity and break nutrients down into simpler molecules. Proteins are degraded to smaller peptides, fats to fatty acids and glycerol, and carbohydrates to oligosaccharides. Enzymes linked to mucosal cells complete the breakdown, enabling nutrients to cross the epithelium and reach the bloodstream (absorption).

The mucosa is lined with millions of fingerlike projections (villi), 0.5 to 1.5 millimeters long, surrounded by moatlike invaginations (crypts), and each absorptive cell has microvilli protruding into the lumen (the space inside the digestive tract). This yields an enormous absorptive area: about 250 meters2 (the size of a tennis court). The mucosa includes absorptive, secretory, endocrine, and antimicrobial cells and is constantly renewed. Absorptive cells originate in the crypts and migrate up the villus, where they become mature and fully functional. Here they live for about three days, then are shed into the lumen. Nutrients enter the absorptive cells either via specialized cellular transport mechanisms (simple sugars and amino acids) or by simple diffusion (lipids), exit the cell through the basolateral membrane, and diffuse into the villus capillary vessels. Chyme is then moved down the intestinal lumen.

Three layers of smooth muscle allow segmentation contractions that mix the chyme, and peristaltic movements propel it distally. Between meals, periodic contractions called “migrating motor complex” propagate caudally along the small intestine and remove residual debris and bacteria. Intestinal movements are controlled locally by the enteric nervous system (a network of nerve fibers in the intestinal wall) and are modulated by the central nervous system. Hormones secreted by the small intestine regulate gastrointestinal secretion and motility and interact with the nervous system in modulating hunger and satiety.

Disorders and Diseases

Absorption-related disorders are linked to defects of mucosal enzymes or transporters and are usually genetic: In lactose intolerance (not to be confused with milk allergy), the enzyme lactase is lacking or insufficient; in glucose-galactose malabsorption, the mucosal transporter for glucose and galactose is defective. Unabsorbed sugars in the lumen cause gastrointestinal symptoms, which can be reversed by avoiding the offending sugars.

Celiac disease is caused by an immunological response to gluten (a protein present in wheat, barley, and rye) and causes chronic intestinal inflammation with flattening of the villi. Mucosal surface and function are restored by a gluten-free diet.

Tropical sprue is a disorder of unknown cause (possibly an infection) in tropical areas, which causes abnormalities in the lining of the small intestine and anemia. It is treated with tetracycline.

In intestinal lymphangiectasia (idiopathic hypoproteinemia) the mucosal lymph vessels are enlarged and obstructed, so that fat and proteins cannot be absorbed. A low-fat, high-protein diet with supplements can help manage the resulting diarrhea. Short bowel syndrome occurs after surgical removal of a large portion of the small intestine. It causes diarrhea and malabsorption and often requires long-term total parenteral nutrition (intravenous feeding).

Whipple’s disease (intestinal lipodystrophy) is a rare bacterial infection that damages the mucosa. It is successfully treated with antibiotics but can recur. Bacterial overgrowth syndrome is the result of slow peristalsis, which allows intestinal bacteria to grow excessively, causing diarrhea and malabsorption. It is treated with antibiotics.

Cancerous tumors in the small intestine are rare. Adenocarcinoma develops in the glandular cells of the mucosa. It requires surgical removal. Noncancerous (benign) tumors can affect different kinds of intestinal cells. Small noncancerous growths may be destroyed by endoscopic surgery.

Parasitic infestations include giardiasis (caused by a unicellular organism, mainly treated with metronidazole) and ascariasis (treated with mebendazole) and tapeworm infections (treatments vary widely), both caused by worms.

Perspective and Prospects

Intestinal ailments and remedies are documented in ancient civilizations. Medieval anatomists maintained the gut could influence the humoral balance of the body. In the seventeenth century, intestines were described as being “made up of tunics, and these from fibers, flesh, parenchyma, veins, arterie, mesenterics, mucous crust, and fat.” It was not until the mid-twentieth century that the complexity and importance of the small intestine became apparent. The glucose-transport mechanism was discovered in the 1960s, and molecular mechanisms of absorption have been studied extensively since then. Recent techniques now allow investigation of their regulation through transporter gene expression. Also, the discovery of a host of intestinal hormones has opened the exploration of the endocrine and neural pathways that regulate nutrient intake, particularly in connection with the problem of obesity control.

Bibliography

Badman, Michael K., and Jeffrey S. Flier. “The Gut and Energy Balance: Visceral Allies in the Obesity Wars.” Science 307 (March 25, 2005): 1909–1914.

Levin, Roy J. “Digestion and Absorption of Carbohydrates: From Molecules and Membranes to Humans.” American Journal of Clinical Nutrition 59, 3d suppl. (March, 1994): 690S–698S.

Sherwood, Lauralee. “The Digestive System.” In Human Physiology: From Cells to Systems. 8th ed. Belmont, Calif.: Brooks/Cole, 2013.

"Small Intestine Disorders." MedlinePlus, June 27, 2013.

"Your Digestive System and How It Works." National Digestive Diseases Information Clearinghouse, April 23, 2012.