INVER HILLS COMMUNITY COLLEGE

BIOLOGY 2202: ANATOMY AND PHYSIOLOGY

DIGESTIVE SYSTEM

ABSORPTION

To solve the problem below, you must write a clear and convincing justification for accepting one of the possible answer choices as the best or most appropriate and for rejecting each of the other choices as less appropriate. Your reasons for rejecting choices are just as important as your reasons for choosing the best answer.

Which of the following would most likely cause the greatest decrease in the absorption of nutrients by the human digestive system?

a. Increasing the blood flow to the large and small intestines

Reject, increasing the flow of blood may enhance the absorption of nutrients to a degree. Increased flow allows some nutrients to be absorbed at a faster rate by creating steeper concentration gradients into the blood from the intestinal cells.

b. Removing half the length of the small intestine

Reject, this will reduce the absorption of nutrients into the small intestine, but the majority of absorption that takes place is due to surface area. By reducing the length of the small intestine by half does not necessarily mean you will reduce the amount of surface area by half. It does depend on which half of the small intestine was removed, but with the incredible amount of surface area available through villi and microvilli, it does not reduce the surface area to the greatest degreee (see answer d.).

c. Reducing the length of the esophagus by half

Reject, the esophagus does little to no absorption at all as a function of digestion. Its main function is to allow smooth passage of food into the stomach. No absorption can take place so early in the process of digestion.

d. Removing the villi and microvilli in the small intestine

Accept, this will greatly reduce the absorption of nutrients because this is what provides the majority of our surface area. About 92% of absorption takes place in the small intestine and most of this is due to the incredible surface area available. The walls of the intestinal lumen are folded into convoluted wrinkles which increase its surface area by a factor of 3. The epithelial layer is then folded into tiny fingerlike extensions only about 50µm wide, called villi. These increase the total epithelial surface area by another 10X, giving a surface of about 15m², about the floor area of a good-sized bedroom. The membrane of each epithelial cell is then folded again into tiny cellular extensions called microvilli. They also increase the surface area by a factor of about 20X, giving a total area of about 300m², roughly the size of a tennis court. (Moffatt, 1997)

e. Reducing the absorption of water in the large intestine

Reject, although water absorption does take place in the small intestine, very little nutrient absorption takes place here. This is evident by the obvious lack of folds (i.e., villi, microvilli). Most of the absorption of nutrients and water takes place in the small intestine, so reducing the absorption of water will have little effect on overall absorption of nutrients.