The descending colon
- Category: Digestive system
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The descending colon is a segment of the large intestine and is the third and penultimate segment of the colon. It transports feces from the transverse colon inferiorly along the left side of the abdominal cavity to the sigmoid colon. Feces passing through the descending colon are stored until they are ready to be eliminated from the body. The walls of the descending colon absorb water as well as remaining nutrients and vitamins from the feces, depositing these valuable substances into our bloodstream.
The descending colon is a hollow tube that is part of the gastrointestinal (GI) tract. Its diameter is roughly 2.5 inches (7 cm), while its length is approximately 9 to 10 inches (25 cm). It contains many small pouches, known as haustra, along its length that increase its surface area and help to move feces through the colon. At its superior end, the descending colon connects to the transverse colon at the splenic flexure just inferior to the spleen. From the splenic flexure, the descending colon extends inferiorly toward the left hip before turning about 90 degrees to the right and forming the sigmoid colon.
Like all parts of the gastrointestinal tract, the descending colon consists of four major tissue layers surrounding a hollow lumen. The mucosa forms the innermost layer that surrounds the lumen and is in contact with fecal matter stored in the colon. Mucosa is a mucous membrane made of simple columnar epithelial tissue and a thin underlying layer of areolar connective tissue. The mucus secreted from goblet cells in the mucosa layer provides lubrication to feces moving through the lumen, protecting the delicate tissues of the colon. Epithelial cells in the mucosa absorb any remaining nutrients, vitamins, and water present in the feces. Surrounding the mucosa are the supportive tissues of the submucosa layer, where we find blood vessels, nerves and connective tissues. The muscularis layer is found deep to the submucosa and enables movement of the descending colon. Several layers of smooth muscle make up the muscularis and allow the descending colon to form the pocket-like haustra. During defecation, these muscle cells contract in waves of peristalsis to push feces down the colon toward the sigmoid colon. Finally, the outermost anterior layer of the descending colon is visceral peritoneum; on the posterior of the descending colon, the outermost layer is areolar connective tissue (known as adventitia). These tissues anchor the colon along the posterior body wall; provide blood flow to the colon; and offer protection from friction as our bodies move.
By the time feces reach the descending colon, the vast majority of nutrients, vitamins and water have been extracted by the ascending and transverse colon, leaving mostly waste products. Still, some absorption of water and vitamins produced by bacterial fermentation of feces – including vitamins K, B1, B2 and B12 – does occur in the descending colon. Its primary function, however, is the storage and accumulation of feces prior to defecation. During defecation, the descending colon helps to propel feces toward the sigmoid colon and rectum and eventually out of the body by contraction of its smooth muscle tissue.