22.12: Elimination removes unusable materials from your body.

You are much better at conservation and recycling than you imagine. Especially when it comes to the fluids and solids you consume. The last phase in the breakdown of food, elimination, takes place as what’s left of the chyme—mostly indigestible materials—leaves the small intestine and enters the large intestine, also called the colon. Much larger in diameter than the small intestine (about 3 inches vs. 1 inch for the small intestine) but only about 3–6 feet long, the large intestine serves to absorb water, salts, and some vitamins (FIGURE 22-23). The last part of the large intestine is the rectum and serves as a storage compartment for the remaining parts of consumed food, the feces, which can then be defecated.

In the bite of hamburger described earlier, some of the material, including molecules in the lettuce, cannot be digested or absorbed. This material is called fiber and includes gums and cellulose. For this reason, when fiber is consumed, it increases fecal mass. With additional mass, more water is attracted, speeding the movement of chyme through the colon, softening the feces and making it easier to eliminate (that’s why too much fiber can lead to diarrhea). Fiber also binds to bile, causing some of it to be eliminated and thereby reducing the body’s ability to absorb cholesterol from food. It is important to achieve just the right balance of water absorption in the large intestine. If too much is absorbed, the remaining indigestible material becomes too solid and can’t move easily through the last part of the digestive system, causing constipation.

Alternatively, if too little water absorption occurs, the body loses more water through diarrhea.

Huge colonies of bacteria live in the colon. Before we are born, our guts are sterile and free of bacteria. During birth, we acquire some of our mother’s vaginal and fecal bacteria. By the age of two, all of the numerous bacterial species present in adults have been acquired from food and the environment. At any given time, we generally have more bacterial cells than our own body cells (see Figure 13-5). The bacteria are usually harmless and live off the undigested materials that end up in the colon. Bacteria also release important metabolic by-products, such as vitamin K and one of the B vitamins, biotin. About half of the feces that we excrete each day is made up of dead bacterial cells, and the rest is mostly indigestible materials such as cellulose from the cell walls of plant matter and other types of fiber.

Antibiotics frequently (and unintentionally) kill a large proportion of the colon bacteria, in addition to whatever illness-causing microbe they were prescribed to kill. This can have multiple negative side effects. First, the transit of undigested materials through the colon may not be slowed down as much as usual, in which case less water is removed and diarrhea results. Also, a reduction in the production of vitamin K and biotin can lead to deficiencies.

In the colon, water is removed from feces through osmosis. First, salts are pumped out of the colon into surrounding cells, and then water moves out by simple diffusion (to an area where there are more ions). This process can be manipulated as a treatment for constipation. Laxatives contain magnesium salts, which are so slowly absorbed that they are still largely intact when they reach the colon. Laxatives work by increasing the salt concentration in the colon, so that more water remains. This additional water makes it easier to excrete feces (but can sometimes cause diarrhea).