Large Intestine

After absorption in the small intestine is complete, peristalsis moves the remaining material on to the large intestine. The large intestine, or colon, is the final organ of digestion. Study Figure 48-14 to identify the four major parts of the colon: ascending colon, transverse colon, descending colon, and sigmoid colon. The sigmoid colon leads into the very short, final portions of the large intestine called the rectum and the anal canal.

Most of the absorption of nutrients and water is completed in the small intestine. About 9 L (9.5 qt) of water enter the small intestine daily, but only 0.5 L (0.53 qt) of water is present in the material that enters the large intestine. In the large intestine, only nutrients produced by bacteria that live in the colon, as well as most of the remainder of the water, are absorbed. Slow contractions move material in the colon toward the rectum. Distension of the colon initiates contractions that move the material out of the body. As this matter moves through the colon, the absorption of water solidifies the mass. The solidified material is called feces.

As the fecal matter solidifies, cells lining the large intestine secrete mucus to lubricate the intestinal wall. This lubrication makes the passing of the feces less abrasive. Mucus also binds together the fecal matter, which is then eliminated through the anus.

figure 48-14

This X ray shows the large intestine, or colon. The ascending colon is on the left. The transverse colon crosses the abdominal cavity. The descending colon can be seen on the right. The sigmoid colon is the small section that leads to the anal canal.

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