Intestines

The Intestines are divided into two portions, small and large. As the names indicate, the only difference is in size. The small intestine has a smaller diameter than the large intestine. The terms do not refer to length.

Small Intestine

The small intestine is about six meters long (20 feet) and 2.5 cm wide (1 inch). It extends from the stomach to the cecum of the large intestine. There are three parts to the small intestine.

The duodenum is the first 25 cm (10 inches). The common bile duct enters the duodenum at the ampulla of Vater (hepatopancreatic ampulla).
The jejunum is about 2.4 meters long (8 feet). Some digestion and absorption takes place in the first part of the jejunum.
The ileum is about 3.6 meters long (12 feet). It extends from the jejunum to the ileocecal valve, which prevents the reflux of contents from the cecum back into the ileum. The lining of the ileum contains numerous patches of lymphoid tissue called Peyer's patches, which diminish the bacterial content of the digestive system.

Functions of the small intestine:

secrete digestive enzymes (peptidases, sucrase, maltase, and lactase)
secrete buffers
absorb nutrients

Structures of the small intestine: The walls form circular folds with finger-like projections called villi. The epithelial cells of each villus form extensions called microvilli. Together they are often referred to as the brush border because of their velvety appearance. They are essential for the absorption of nutrients into the bloodstream. These nutrients, or end products, go on to the liver for processing before being distributed throughout the body. The brush borders also secrete several digestive enzymes and two important hormones (secretin and cholecystokinin).

Large Intestine

The large intestine is also known as the colon or bowel. It is about 6.3 cm in diameter (2.5 inches) and 1.5 meters (5 feet) in length, extending from the ileum of the small intestine, to the anus. The large intestine does not secrete digestive enzymes and does not have villi.

Parts of the large intestine include:

The taenia coli are three longitudinal muscle bands visible on the outer surfaces of the colon just beneath the serosa. Muscle tone within these bands produces the haustrae.
The haustrae are external pouches that permit the considerable distension and elongation required of the intestine. They also affect the mucosal lining, producing a series of internal creases.
The cecum is the expanded pouch at the start of the large intestine and where chyme first enters. Chyme is a semifluid mixture of food and digestive secretions. Attached to the cecum is the appendix.
The ileocecal valve is the muscular sphincter or junction between the ileum and the cecum.
The ascending, transverse, and descending portions of the colon encircle the small intestine
- The ascending colon begins at the ileocecal valve and ascends along the right side of the peritoneal cavity until it reaches the inferior margin of the liver. The right colic flexure or hepatic flexure marks the transition to the transverse colon.
- The transverse colon continues toward the left side, passing below the stomach, following the curve of the body wall. Near the spleen, it turns at the left colic flexure or splenic flexure.
- The descending colon continues on the left side until it reaches the iliac fossa where it curves and recurves to form the sigmoid colon, which empties into the rectum.
The rectum is about 15 cm or 6 inches long. It is the last part of the digestive tract and the last portion of the rectum is the anorectal canal.
- The anorectal canal is the last inch of the colon that surrounds the anus.

Functions of the large intestine include:

absorption of water and certain electrolytes
synthesization of certain vitamins (especially vitamin K and certain B vitamins) by the "good" intestinal bacteria
temporary storage of fecal waste
elimination of waste from the body (defecation)

This page was updated in December 2005.