The large intestine is a large tube that extends upward along the right side of the body, across the right side to the left side in the middle of the body, and downward along the left side of the body.  The part of the large intestine that extends from the junction of the small intestine and the large intestine to the anal canal is called the colon. The large intestine extends upward on the right side of the body. This portion is called the ascending colon. The large intestine then makes a ninety-degree turn near the right lobe of the liver and extends across the body from right to left. The part that extends across the body from right to left is called the transverse colon. The large intestine makes another ninety-degree turn near the spleen and extends downward on the left side of the body. This part is called the descending colon. When the large intestine reaches the lower part of the body, it curves into an “S” shape. This “S” shape is called the sigmoid colon. The sigmoid colon empties into the rectum and the anal canal. The anal canal empties into the anus, and the digestive tract ends.

Along the length on the right and left sides of the colon are circular segmented pockets called haustra. These haustra are present in the ascending, transverse, and descending colons but are absent in the rectum and anal canal.  A thin strip of muscle called the teniae coli runs along the length of the large intestine in between the haustra.

Overall, the cells of the epithelium in the mucosa layer of the small intestine are shaped like columns. The one exception are the cells of the epithelium in the lower half of the anal canal, which are shaped like reptile or fish scales. This shape makes the cells better at withstanding the scratching of feces against the epithelium when we have a bowel movement. Unlike the small intestine, the large intestine does not have villi or circular folds. However, it does have crypts that extend deeper than those in the small intestine.  Secreting mucus is the chief function of those crypts.

The large intestine also has several species of bacteria that are native to it.  They form B vitamins and vitamin K, which the colon then absorbs. By forming these vitamins, and in particular vitamin K, these bacteria help the body to absorb adequate amounts of vitamins so that it will function correctly. These bacteria also ferment cellulose, a carbohydrate that is found in the cell walls of plants; other carbohydrates that have not been digested by the body; and fats. Sometimes, undigested nutrients pass through the large intestine. Because they are not digested, they provide an abnormal substance for the bacteria to ferment.   The action of the bacteria can lead to cramping and can be a factor in causing gas to form. Gas consists of hydrogen, nitrogen, carbon dioxide, methane, and hydrogen sulfide. The gas may also contain indole and skatole, two organic compounds that are derived from ammonia. If the gas contains indole, skatole, and hydrogen sulfide, it will smell like feces. Otherwise, it will not have an odor.

Last Updated: Saturday, July 16, 2011