Large Intestine

The large intestine absorbs water, electrolytes, and certain vitamins from the chyme it receives from the small intestine. In a process regulated by the action of sphincter muscles, the large intestine then passes waste products out of the body through the rectum and anal canal.

The large intestine, or colon, extends from the ileocecal valve to the anus, framing the small intestine on three sides. Chyme from the ileum passes into the cecum, which is a blind pouch (open only at one end) at the beginning of the large intestine. Waste material then passes in sequence through the ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal (fig. 18.17). Waste material (feces) is excreted through the anus, the external opening of the anal canal.

The mucosa of the large intestine, like that of the small intestine, contains many scattered lymphocytes and lymphatic nodules and is covered by columnar epithelial cells and mucus-secreting goblet cells. Although this epithelium does form crypts (fig. 18.18), there are no villi in the large intestine—the intestinal mucosa therefore appears flat. The outer surface of the colon bulges outward to form pouches, or haustra (fig. 18.17 and 18.19). Occasionally, the muscularis externa of the haustra may become so weakened that the wall forms a more elongated outpouching, or diverticulum (divert = turned aside). Inflammation of one or more of these structures is called diverticulitis.

^R- The appendix is a short thin outpouching of the cecum.

■J -I t It does not function in digestion, but like the tonsils, it 1 ^ contains numerous lymphatic nodules (fig. 18.18) and is subject to inflammation—a condition called appendicitis. This is commonly detected in its later stages by pain in the lower right quadrant of the abdomen. If the appendix ruptures, infectious material can spread throughout the surrounding body cavity, causing inflammation of the peritoneum—peritonitis. This dangerous event may be prevented by surgical removal of the inflamed appendix (appendectomy).

Clinical Investigation Clues

Remember that Alan had pains in particular locations only when provoked by certain food and drinks; also, he did not have an elevated white blood cell count or a fever. Is it likely that Alan has appendicitis? Why or why not?

The large intestine has little or no digestive function, but it does absorb water and electrolytes from the remaining chyme, as well as several B complex vitamins and vitamin K. Bacteria residing in the intestine, primarily the colon—collectively referred

The Digestive System

Transverse colon

Ascending — colon

Ileocecal valve

Cecum

Transverse colon

Ascending — colon

Ileocecal valve

Cecum

Ileocecal Outpouching

Haustrum Sigmoid colon

Anal canal

■ Figure 18.17 The large intestine. The different regions of the large intestine (colon) are illustrated.

Descending colon

Haustrum Sigmoid colon

Anal canal

Muscularis externa

Serosa

Crypts of Lieberkuhn

Lumen with feces

Muscularis externa

Serosa

Crypts of Lieberkuhn

Lumen with feces

Serosa Sigmoid Colon

Centers of lymphatic nodules

■ Figure 18.18 A photomicrograph of the human appendix. This cross section reveals numerous lymphatic nodules, which function in immunity.

Centers of lymphatic nodules to as the intestinal microbiota or microflora—produce significant amounts of vitamin K and folic acid (see chapter 19), which are absorbed in the large intestine.

The number of bacterial cells in the human colon is said to exceed the total number of cells in the human body! This intes tinal microbiota originates at birth and performs a number of physiologically important functions. In addition to the production of B vitamins and vitamin K, bacteria in the colon ferment (through anaerobic respiration) some indigestible molecules in the chyme and secreted mucus. They produce short-chain fatty acids

Fox: Human Physiology, I 18. The Digestive System I Text I I © The McGraw-Hill

Eighth Edition Companies, 2003

Chapter Eighteen

Osmosis Human Physiology

The handling of salt and water transport in the large intestine is made more complex by the fact that the large intestine can secrete, as well as absorb, water. The secretion of water by the mucosa of the large intestine occurs by osmosis as a result of the active transport of Na+ or Cl- out of the epithelial cells into the intestinal lumen. Secretion in this way is normally minor compared to the far greater amount of salt and water absorption, but this balance may be altered in some disease states.

Diarrhea is characterized by excessive fluid excretion in the feces. Three different mechanisms, illustrated by three different diseases, can cause diarrhea. In cholera, severe diarrhea and dehydration result from entero-toxin, a chemical produced by the infecting bacteria. Release of en-terotoxin stimulates active NaCl transport into the lumen of the intestine, followed by the osmotic movement of water. In celiac sprue, diarrhea is caused by damage to the intestinal mucosa produced in susceptible people by eating foods that contain gluten (proteins from grains such as wheat). In lactose intolerance, diarrhea is produced by the increased osmolarity of the contents of the intestinal lumen as a result of the presence of undigested lactose.

■ Figure 18.19 A radiograph of the large intestine. The large intestine is seen after a barium enema has been administered; the haustra are clearly visible.

(less than five carbons long), which are used for energy by the epithelial cells of the colon, and which aid the absorption of sodium, bicarbonate, calcium, magnesium, and iron in the large intestine.

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Responses

  • jaime
    How does the large intestine absorb water?
    8 years ago
  • Evelyn
    How the water and salt digest in big intestine?
    8 years ago
  • stefan
    What colon begins at the ileocecal junction?
    7 years ago
  • folcard roper
    Is a function of the large intestine to absorb remaining water & electroytes from chyme?
    7 years ago
  • ghenet
    What is the blind pouch located on one end of the large intestine called?
    7 years ago
  • polo
    What absorbs water and electrolytes?
    6 years ago
  • AIMONE RUSSO
    Why is it important that the large intestine reabsorbs water and not excrete water during diarrhea?
    6 years ago
  • CHRISTINA
    What vitamins are processed in the large intestine?
    6 years ago
  • shishay mebrahtu
    What vitamins are produced by large intestine?
    6 years ago
  • juliann
    How does the colon absorb water?
    2 years ago
  • ella
    Which waste product is excreted by the large intestine?
    1 year ago

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