ABSTRACT
The gastrointestinal (GI) tract is a set of organs whose main functions are the digestion and absorption of food and the excretion of solid waste matter in the form of feces. In thinking about the GI tract, a useful analogy is an oil refinery, where crude oil is broken down into its constituent hydrocarbons for use as fuels, and in manufacturing industries such as plastics. An oil refinery uses different methods to extract these substances from the crude oil, in much the same way that the GI tract uses different chemical methods to break down food and to ensure the constituents of a meal are absorbed properly. The end-product of oil refining is bitumen that can be used in road building, but this is where the analogy falls down, as we have yet to discover such a utilitarian end-point for human feces. The important thing to realize is that the formation of all these products is done by several different processes, just like in the GI tract. Thus, different parts of the GI tract look different and have different functions.
