Oxygen, neccessary for cellular respiration, is the answer. As oxygen becomes scarce, your cells begin to aerobically produce very little ATP, in effect fermenting and producing lactic acid.
When climbing a mountain, we can observe transitions in biological communities that are analogous to the changes in biomes at different latitudes.
The ileum contains villi that project into the lumen and increase the amount of surface area.
Villi are small finger-like structures that project into the lumen of the small intestine. Villi increase the surface area of the intestinal walls for easy and quick absorption of digested food with the addition of digestive secretions. Villi vary in length from about 0.5 to 1 mm. They are usually found in large numbers at the beginning of the small intestine and they reduce toward the end of the tract.
<span>Amylase needs an optimum pH for its activity. It is in the range of 6-7. Below or above pH will denature this enzyme. The pH level is optimum for this enzyme in the mouth and hence it catalyses the break down of sugar. The pH level of stomach is lower than its optimum level duet to the activity of gastric acid. Hence the enzyme becomes inactive in the stomach.</span>
