As material leaves the small intestine and enters the large intestine, it passes by a small saclike organ called the appendix. In some mammals, the appendix processes cellulose and other materials. The only time humans notice their appendix is when it becomes clogged and inflamed, causing appendicitis. The remedy for appendicitis is to remove the infected organ by surgery—as quickly as possible—before it can rupture or break open.

Absorption From the Large Intestine When chyme leaves the small intestine, it enters the large intestine, or colon. The large intestine is actually much shorter than the small intestine. The large intestine gets its name due to its diameter, which is much greater than the small intestine's diameter. The primary function of the large intestine is to remove water from the undigested material that is left. Water is absorbed quickly across the wall of the large intestine, leaving behind the undigested materials. Rich colonies of bacteria present in the large intestine produce compounds that the body is able to absorb and use, including vitamin K. When large doses of antibiotics are given to fight an infection, they can destroy these bacteria, and vitamin K deficiency can occur.

Elimination The concentrated waste material—the feces—that remains after most of the water has been removed passes into the rectum and is eliminated from the body through the anus. When something happens that interferes with the removal of water by the large intestine, you usually become aware of it right away. If not enough water is absorbed, a condition known as diarrhea occurs. If too much water is absorbed from the undigested materials, a condition known as constipation occurs.

FIGURE 30–17 The Large Intestine This X-ray shows the large intestine and its contents.

30.3 Assessment

Review Key Concepts

1. 1. Review Explain the function of the digestive system.

   2. Compare and Contrast What is the difference between mechanical digestion and chemical digestion?

2. 1. Review List the structures that food travels through during digestion and give the function of each.

   2. Relate Cause and Effect Some people have a disorder in which their stomach muscles cannot contract and churn food. What effect do you think this has on the length of time food stays in the stomach?

3. 1. Review Explain how nutrients are absorbed.

   2. Apply Concepts What impact do the folds and villi of the small intestine have on absorption?

Apply the Big idea

Matter and Energy

4. How would the rate of digestion be affected if the various organs and glands did not release enzymes? Hint: You may wish to refer to Chapter 2 for a review of enzyme action.

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