- Understand the basic functions of the gastrointestinal system and the design features that subserve these
- Describe the functional layers of the gastrointestinal tract and the specializations that contribute to function
- Identify the segments of the gastrointestinal tract and the specialized functions attributed to each
- Understand the circulatory features of the intestine and variations that occur after meals
- Describe the basic anatomy of the neuromuscular systems of the gut
Digestion and Absorption
The gastrointestinal system primarily conveys nutrients, electrolytes, and water into the body. In unicellular organisms, metabolic requirements can be met by diffusion or transport of substances from the environment across the cell membrane. However, the greatly increased scale of multicellular organisms, along with the fact that most such organisms are terrestrial, and thus not normally swimming in a soup of nutrients, means that specialized systems have evolved to convey nutrients into and around the body. Thus, the gastrointestinal system and liver work in concert with the circulatory system to ensure that the nutritional requirements of cells distant from the exterior of the body can be met.
Most nutrients in a normal human diet are macromolecules and thus cannot readily permeate across cell membranes. Likewise, nutrients are not usually taken predominantly in the form of solutions, but rather as solid food. Thus, in addition to the physical process of food uptake, the intestines serve to physically reduce the meal into a suspension of small particles mixed with nutrients in solution. These are then chemically altered resulting in molecules capable of traversing the intestinal lining. These processes are referred to as digestion, and involve gastrointestinal motility as well as the influences of pH changes, biological detergents, and enzymes.
The final stage in the assimilation of a meal involves movement of digested nutrients out of the intestinal contents, across the intestinal lining, and into either the blood supply to the gut or the lymphatic system, for transfer to more distant sites in the body. Collectively, this directed movement of nutrients is referred to as absorption. The efficiency of absorption may vary widely for different molecules in the diet as well as those supplied via the oral route with therapeutic intent, such as drugs. The barriers to absorption encountered by a given nutrient will depend heavily on its physicochemical characteristics, and particularly on whether it is hydrophilic (such as the products of protein and carbohydrate digestion) or hydrophobic (such as dietary lipids). For the main substances vitally required by the body, the gastrointestinal tract does not rely solely on diffusion across the lining to provide for uptake, but rather has evolved active transport mechanisms that take up specific solutes with high efficiency.
There is significant excess capacity in the systems for both digestion and absorption of a meal, including an excess of enzymes and other secreted products as well as an excess in the surface area ...