Colorectal Cancer Chapter 3
Colon Anatomy & Function
The Digestive System
The digestive system is like a long tube that runs from the mouth to the anus and includes organs that help the body digest and absorb food and nutrients. The organs that make up the digestive system include the mouth, esophagus, stomach, small intestine, large intestine (or colon), rectum, and anus. Inside these organs is a lining called the mucosa. The digestive tract also contains a layer of smooth muscle. The muscle moves food through the tract in a wavelike movement called peristalsis, helping to mix it and break it down.
The liver and the pancreas also belong to the digestive system. They produce digestive juices that empty into the small intestine through ducts. The gallbladder stores the liver's digestive juices.
Digestion begins in the mouth. Chewed food is swallowed and moves into the pharynx, and then is pushed down the esophagus into the stomach. There, food is churned by smooth muscle contractions and mixed with digestive juices produced by the stomach. The stomach slowly empties its contents into the small intestine, where food dissolves into juices secreted from the pancreas, liver, and intestine. As the digested food moves through the small intestine, its nutrients are absorbed into the intestinal wall and into the bloodstream, where it is transported throughout the body for distribution. The colon removes water from the digested matter and converts it into a mostly solid mass of waste material, consisting largely of indigestible elements of the food, called fiber, and older cells that have been shed from the mucosa. Waste material is then expelled as feces.
The small intestine joins the large intestine, or colon, in the right lower abdomen. The colon is a muscular tube about 5 feet long. It's made up of four sections: the ascending colon, which extends upward on the right side of the abdomen; the transverse colon, which goes across the body from the right to the left side in the upper abdomen; the descending colon, which continues downward on the left side; and the sigmoid colon (called this because of its "S" or "sigmoid" shape). The sigmoid colon attaches to the rectum, which ends in the anus.
The colon is composed of a number of different layers of tissue:
- The mucosa, the innermost layer, includes a single layer of epithelial cells, a layer of connective tissue, and a thin muscle layer. It is lined with goblet cells, glands that secrete mucous to help the passage of material through the colon.
- The submucosa is a layer of connective tissue beneath the mucosa.
- The circular muscle is a band of muscle that wraps around the entire colon and helps move waste material through it.
- The longitudinal muscle runs lengthwise along the colon. It works in conjunction with the circular muscle to create the wavelike motion of peristalsis.
- The serosa is the outermost layer of the colon.
The major function of the colon is to extract water, salts, and nutrients from partially digested food, and to propel the residue to the rectum and anus for expulsion. When the contents of the small intestine reach the colon, they're liquid. By the time they are expelled as feces, they're solid. About 2 pints of liquid matter enter the colon each day; stool volume is about a third of a pint. The difference between these two amounts is what the colon has absorbed in the course of digestion. If the digested matter lacks enough water, the colon can also add water to it to soften the stool. Maintaining this balance is one of the colon's most important functions.
By the time the partially digested food enters the colon, most of its nutrients have been absorbed by the small intestine. However, the process of digestion isn't over. The colon contains large numbers of bacteria. The most important of these are the lactobacilli and bifidus bacteria, which live in a symbiotic (mutually beneficial) relationship with the colon. They ferment the soluble fiber in food, forming valuable short-chain fatty acids that nourish intestinal cells, help regulate production of cholesterol, and are thought to help prevent a variety of diseases (including cancer). Beneficial bacteria also help to keep harmful bacteria in check. In addition, intestinal bacteria synthesize vitamin K, important in blood clot formation.
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