The walls of the gastrointestinal (GI) tract, at all levels, are muscular and capable of movement.

Movements of the GJ muscles have direct actions on ingesta in the gut lumen. GI movements have several functions: (1) to propel ingesta from one location to the next; (2) to retain ingesta at a given site for digestion, absorption, or storage; (3) to break up food material physically and mix it with digestive secretions; and (4) to circulate ingesta so that all portions come into contact with absorptive surfaces.

The dynamics of fluid movement in the gut are not as well understood as in other organ systems, particularly the cardio­vascular system. The heart and great vessels behave in a man­ner similar to that of most mechanical pumping systems: a central pump pushes fluid through a conduit of relatively fixed diameter. Because of this configuration, the cardiovascular system more or less conforms to physical laws that are well established and studied reasonably easily; sophisticated quan­titative analyses of cardiovascular function can be made clini­cally. In contrast to the situation in the heart, the fluid pump and the conduit are the same organ in the gut. This makes study of the fluid dynamics of the gut extremely complex. At this time, the mathematically defined physical laws of fluid dynamics, as applied to the gut, are of little clinical usefulness. Therefore the physiology of GI motility is usually applied clinically on a qualitative, rather than a quantitative, basis.

Movement of the gut wall is referred to as motility, and motility may be of a propulsive, retentive, or mixing nature. The time it takes material to travel from one portion of the gut to another is referred to as the transit time. An increase in pro­pulsive motility decreases the transit time, whereas an increase in retentive motility increases the transit time. Selectively increasing retentive motility and reducing propulsive motility are important aspects of diarrhea therapy.