Large Intestine

LARGE INTESTINE

The large intestine is the end of the alimentary canal. Its main function is to complete the

absorption of nutrients, absorption of water, vitamin synthesis, and the elimination of feces. It

is a structure that starts at the appendix and ends at the anus. It is only about 3-feet long with

a diameter of three inches. There are four regions to the large intestine: the cecum, colon,

rectum, and anus. The start of the large intestine is the ileocecal valve, which controls the flow

of chyme into the large intestine.

The cecum is about six centimeters or 2.4 inches in length and contains the appendix. The

appendix contains lymphoid tissue but is not considered a functional organ. The colon is divided

into the ascending colon, transverse colon, descending colon, and sigmoid colon. There is right

and left splenic flexure that act as “bends” in the upper right (hepatic side) and upper left

(splenic side) of the colon. The rectum is about 8 inches in length and curves around the

sacrum. There are three folds called the “rectal valves” that prevent the simultaneous passage

of gas and feces. The anal canal has two sphincters, the smooth muscle internal anal sphincter

(which is involuntary), and the skeletal muscle external anal sphincter (which is voluntary).

There are no circular folds and no villi in the large intestine and there are few enzymes

secreted. It consists of simple columnar epithelium (other than the anal canal), which is made

from absorptive cells (called enterocytes) and interspersed goblet cells. Goblet cells secrete

mucus that eases the passage of feces and protects the intestine from the effects of the acidic

substances and gases made by the large intestinal bacteria.

The major unique features of the large intestine are the bands of longitudinal muscle in the

muscularis, divided into three parts called the teniae coli. When these muscles contract, they

form the haustra, which are pouches that give the wrinkly appearance of the large intestine.

Attached to the teniae coli are fat-filled sacs made by visceral peritoneum called epiploic

appendages, which have no known function. These structures are seen in the entire colon

except for the rectum and anal canal.

The anal canal is made from stratified squamous epithelium that connects to the outside skin.

This mucosa is stratified to counteract the abrasive forces of feces. The anus has mucosal folds

called anal columns, that are highly vascularized. Between these folds are the anal sinuses that

secrete mucus. Below the sinuses is the “pectinate line”—a horizontal line that is the

connection point of the gut to the external skin.

One of the unique things about the large intestine is the presence of trillions of bacteria in the

lumen, called intestinal bacterial flora. There are hundreds of different kinds of pathogenic

organisms living in the gut lumen. They facilitate chemical digestion and absorption and

participate in vitamin synthesis. The main vitamins made are vitamin K, pantothenic acid, and

biotin. Dendritic cells will continually evaluate the pathogenicity of these bacteria to ensure

that the bacteria are not invasive or pathogenic. If pathogenic, an IgA-mediated immune

response is generated so as to kill off any dangerous bacterial organisms.

The large intestine receives the chyme, which contains a large amount of water and few

nutrients. It takes 12-24 hours for the chyme to travel through the large intestine. Because it

doesn’t have much in the way of nutrients, very little digestion occurs in this area. There is

mostly mechanical digestion, caused by haustral contractions that is involved in segmentation.

These are slow-moving contractions that aid in the absorption of water by the large intestine. In

addition, peristalsis occurs in the large intestine as a “mass movement,” which quickly forces

the distal contents of the colon through to the rectum. This occurs because of the gastrocolic

reflex.

There are no digestive enzymes secreted by the large intestine. The chemical digestion that

does occur happens because of intestinal bacteria that ferment the remaining carbohydrates,

giving off gases that create the flatus or gas in the large intestine. About 1500 milliliters of this

gas is produced in the colon each day, with more produced when soluble dietary fiber and

indigestible sugars are eaten by the individual. This is why beans cause excess flatus.

The small intestine actually absorbs 90 percent of ingested water, with only 10 percent

absorbed by the large intestine in the form of feces. Feces is made of unabsorbed nutrients,

indigestible food particles, bacteria, water, old epithelial cells, and inorganic salts. Rectal