ABSTRACT

The primary function of the gastrointestinal system (Fig. 6.1) is to supply the body with nutrients and water. The ingested food is moved along the alimentary canal at an appropriate rate for digestion, absorption, storage, and expulsion. To fulfill the various requirements of the system, each organ has adapted one or more functions. The esophagus acts as a conduit for the passage of food into the stomach for trituration and mixing. The ingested food is then emptied into the small intestine, which plays a major role in the digestion and absorption processes. The chyme is mixed thoroughly with secretions and it is propelled distally (1) to allow further gastric emptying, (2) to allow for uniform exposure to the absorptive mucosal surface of the small intestine, and (3) to empty into the colon. The vigor of mixing and the rate of propulsion depend on the required contact time of chyme with enzymes and the mucosal surface for efficient performance of digestion and absorption. The colon absorbs water and electrolytes from the chyme, concentrating and collecting waste products that are expelled from the system at appropriate times. All of these motor functions are performed by contractions of the muscle layers in the gastrointestinal wall.