Large Intestine Anatomy And Digestion

The large intestine (Figure 7.1) is a tube about 5 feet long and

Large Intestine Haustra Anatomy
Figure 7.1 The large intestine consists of the cecum, colon, rectum, and anal canal. It is last place where the body will remove nutrients before the waste is excreted.

2.5 inches in diameter. It is wider than the previous sections of the digestive tube and can store material for 12 to 24 hours until elimination. The large intestine is connected to the small intestine by the ileocecal valve.

There are four major sections of the large intestine: the cecum, colon, rectum, and anal canal. The cecum, which connects to the small intestine, is a 6-inch-long pouch-shaped portion of the tube that stores small amounts of chyme until it passes into the colon, or longest part of the large intestine.

The appendix, which is attached to the cecum, is about 3 inches long. It is open to the cecum at one end and closed at the other. The appendix has no function in digestion or absorption, but has several large aggregates of lymphoid tissue and may play a role in the immune system. The appendix can easily be twisted or blocked, causing an inflammation called appendicitis. If the appendix bursts, the bacteria that inhabit the large intestine can gain access to the abdominal cavity, causing acute infections that are difficult to treat and may lead to death.

The second, and largest, section of the large intestine is the colon. This section is divided into four regions based on the direction or shape of the tube. After chyme leaves the cecum, it goes into the ascending colon, which is on the right side of the abdomen. The part of the colon that is in front of the stomach just under the diaphragm is called the transverse colon. On the left side of the abdomen is the descending colon. The colon begins to twist and bend down toward the middle of the body at this point. This portion is called the sigmoid colon and the twisting brings the tube in line with the last two, shorter, parts of the large intestine, the rectum and anal canal.

There are several differences between the small and large intestinal walls. First, the mucosa of the large intestine has no villi. The mucosal epithelium is made up of columnar cells and goblet cells. The mucus-secreting cells increase in number throughout the large intestine, which, in turn, increases the amount of mucus secreted and assists the passage of intestinal contents that are becoming increasingly dehydrated. The muscularis contains two layers of smooth muscle, but the outer layer is made up of three bands of muscle called the teniae coli. The regular constrictions of the teniae coli result in a muscle tone that gives the appearance of a series of pouches, called haustra, along the colon. The large intestine appears as though a piece of string were loosely tied around the diameter of the tube every couple of inches. As chyme travels through the large intestine, it moves from haustra to haustra with some mixing and storage at each stop. This process is called haustral churning and results in the physical digestion that occurs in the large intestine.

As stated above, the chemical digestion that occurs in the large intestine is carried out by bacteria. These microorganisms are collectively called "normal flora" because they are normally found in the colon. E. coli, a bacterium that has received a lot of attention, is one of the organisms that normally exist in the large intestine. It does not, however, normally exist in the urinary tract, where it is the most frequent cause of urinary tract infections. The end of the intestinal tact is the anus, which is not far from the urethra, the opening for voiding urine. The organism can be carried by the person from one opening to the other and the result is a painful infection.

The intestinal bacteria use whatever carbohydrates that have not been separated and absorbed in the small intestine. The bacteria produce waste material that includes gases such as hydrogen, nitrogen, methane, carbon hydroxide, and dimethyl sulfide. About 1 pint of this gas is produced daily. The last component, dimethyl sulfide, is responsible for the strong odor of these gases. Some acids will also be produced and, in higher than normal concentrations, may cause abdominal pain and increase motility in the large intestine, resulting in diarrhea.

The bacteria also convert any amino acids that enter the large intestine into fatty acids and pass them in the feces. The bacteria convert bilirubin from the liver to a chemical called stercobilin to produce the normal brown color of feces. If there is no bile coming from the liver or gallbladder, the feces are white or gray colored, and usually high in undigested fats because bile was not present to emulsify digestive fats. This whitish coloring and increased fat content indicates gallbladder problems.

A person can live without the large intestine. In cases of cancer of the digestive tract, the large intestine can be removed and the person's intestine attached to an opening in the abdominal wall. Feces are collected in a bag attached to the outside of the abdomen. This procedure is called a colostomy. Care must be given to supply the vitamins that are usually absorbed from the large intestine.

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  • ellen
    How body wastes are removed through the large intestine\?
    8 years ago

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