Physiology of digestion and absorption

The gastrointestinal (GI) tract has the sole responsibility for extracting fuel from the environment along with the physical building blocks and chemicals necessary to build and run the machinery of the body. The normal human diet comprises approximately 2 L of water admixed with 400 g of carbohydrate, 100 g of protein, and 60 to 100 g of fat. The digestive process proceeds in the stomach by mechanical, acid, and enzymatic activity resulting in chyme production. The resultant slurry is introduced to the duodenum under the control of hormones sensitive to volume, osmolality, and acidity. The role of the small intestine, then, is to absorb ingested water, electrolytes, and nutrients as well as to absorb the additional water, electrolytes, and enzymes secreted by the intestinal tract for the digestive process. This occurs sequentially along the small intestine involving intraluminal, mucosal, and intracellular processes and differs significantly for the various nutrients. Assimilation of nutrients is a complex process that we are better appreciating at the molecular level through ongoing investigation. The physicochemical steps of digestion have been well characterized along with much of the membrane events resulting in absorption. Secretion and motility are intimately related to digestion and absorption with regulation via the intrinsic and extrinsic innervation of the GI tract and interdependence of the mesenteric vasculature, lymphatics, and regulatory polypeptides. Despite the complexity, digestion and absorption are very efficient processes that result in the delivery of 1 L of effluent containing less than 5% of ingested nutrient to the large intestine. Still, many states of dysregulation are poorly understood along with overall governing mechanisms coordinating the elements of normal digestion and absorption. While some technical triumphs such as intestinal transplantation have become a reality, other processes as ubiquitous as postoperative ileus remain incompletely understood and with few treatment options. This chapter presents the current understanding of the digestion and absorption of dietary contents.