The GI tract is effectively a large coiled tube with liver and pancreas connected to it by secretory ducts. Its function is to transfer the components of food from the outside to the inside the body (Fig. 9.1). The gut is organized into different anatomical areas, each having a specific function related to digestion and absorption: the stomach and duodenum dealing with the initial process of mixing ingested food and its digestion; the jejunum continuing with the digestive processes, and beginning the process of absorption; the ileum absorbing digested foodstuffs and the large bowel being involved in the absorption of fluid and electrolytes (Table 9.1). Along the lengths of the gut, various added fluids, electrolytes, and proteins aid in the mixing, hydration, and digestion of the food. The gut does more than simply pass all digested food to the other organs. It will, for example, treat the simple monosaccharide, glucose , differently when this is received from the lumen or via the mesenteric blood supply. Glucose taken up from the lumen is transferred directly to the liver unaltered, whereas glucose received via the blood supply is metabolized to lactate prior to passage to the liver. The amino acid glutamine, which is derived from the dietary protein, is used by the enterocytes as a major energy source and does not enter the portal blood supply (see below).
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