Gastrointestinal system secretion/absorption

Zweibaum A, M Laburthe, E Grasset, D Louvard. (1991). Use of cultured cell lines in studies of intestinal cell differentiation and function. In M Field, CA Frizzell, eds. Handbook of Physiology, Section 6, The Gastrointestinal System, Vol. IV, Intestinal Absorption and Secretion. Bethesda, MD Am Physiol Society, pp 223-255. 

Figure 1. A simple model illustrating some of the complex interactions involved in zinc absorption and metabolism. Following absorption, a fraction of the zinc goes directly into blood, another fraction passes first through the portal circulation and then into the systemic circulation. Some zinc is resecreted into the gastrointestinal tract, thereby becoming available for reabsorption. Activity in blood can exchange with tissue pools, be excreted in urine, or secreted back into the gastrointestinal tract.

A fraction of the Ingested zinc is absorbed, but the rate of absorption is not constant throughout the gastrointestinal tract(l ). Some of the absorbed zinc enters the systemic circulation directly, some enters the portal circulation first, then passes into the general circulation, and some is secreted back into the gut and is then available for reabsorption. 

The gastrointestinal (GI) tract is in a continuous contractile, absorptive, and secretory state. The control of this state is complex, with contributions by the muscle itself, local nerves (i.e., the enteric nervous system, ENS), the central nervous system (CNS), and humoral pathways. Of these, perhaps the most important regulator of physiological gut function is the ENS (Figure 37-1), which is an autonomous collection of nerves within the wall of the Gl tract, organized into two connected networks of neurons the myenteric (Auerbach s) plexus, found between the circular and longitudinal muscle layers, and the submucosal (Meissner s) plexus, found below the epithehum. The former is responsible for motor control, while the latter regulates secretion, fluid transport, and vascular flow. 

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