Some of the earliest investigations of biochemical and physiological events involved the study of the digestive fluids and processes. In 1822, William Beaumont, an American physician, studied gastric juice obtained from a gastric fistula that remained in a patient who had recovered from a gunshot wound. This led to the discovery that hydrochloric acid was secreted into the stomach. In 1836, the German anatomist and physiologist Theodor Schwann described the ability of gastric juice to break down albumin. This was the first recognition of the enzymatic breakdown of food, and Schwann coined the word "pepsin" from the Greek pepsis (digestion) to describe this new factor in gastric juice. In 1899-1902, in the laboratory of the Russian physiologist Ivan Pavlov, the observation that an intestinal factor was required for activation of pancreatic proteases was first made; Pavlov named this factor enterokinase. These processes are understood in much more detail today and research is continuing to provide new insights into digestive and particularly absorptive processes.
The average American diet contains approximately 15% protein, 60% carbohydrate, and 25% fat by weight. Because fat has a higher caloric density than does protein or carbohydrate, typical diets provide about 11%, 46%, and 43% of total energy needs in the form of protein, carbohydrate, and fat, respectively with the average diet containing relatively more fat and less complex carbohydrate than is generally recommended by nutritionists. In addition to handling the digestion and
/ Electron micrograph of the brush border membrane of an enterocyte; from Poley, J. R. (1988) Loss of the glyeocalyx of enterocytes in small intestine. J Pediatr Gastroenterol Nutr 7:388.
absorption of nutrients provided by the diet, the digestive system must also process endogenous proteins (—70 g'day) and endogenous lipids (—25g/day, mainly from bile), which are secreted into the lumen of the digestive tract in the salivary, gastric, pancreatic, biliary, and intestinal secretions or are components of the epithelial cells cast off into the lumen of the gastrointestinal tract.
Digestion largely involves the processes of enzymatic breakdown of these complex macronutrients to their smaller units (e.g., digestion of polysaccharides such as starch to monosaccharides such as glucose). Absorption of these smaller molecules across the epithelial cell layer of the intestinal mucosa into the interstitial fluid allows them to enter the blood or lymph for circulation to the rest of the body. Release and absorption of vitamins and minerals are also essential, and these processes are described in Units V and VI.
The gastrointestinal tract can be considered as a tubular structure extending from the mouth to the anus, and the contents in the lumen of the gastrointestinal tract can be considered as "outside" the cellular tissues of the body. Uptake of nutrients into the circulatory systems (blood and lymph), which supply the cellular tissues of the body with nutrients, depends upon the efficiency with which complex nutrients are broken down to smaller components that can be transported across the epithelial cells; upon the transporters located in the brush border (luminal) membrane of the mucosal epithelial cells (enterocytes), which allow their uptake from the luminal contents; upon further hydrolysis (e.g., peptide hydrolysis to amino acids) or processing (e.g., triacylglycerol synthesis and formation of chylomicrons) within the enterocytes; and upon transport out of the enterocyte across the contra-luminal or basolateral membrane into the interstitial fluid. From the interstitial fluid, products of digestion and absorption enter either the capillaries (and hence the portal blood) or the Iacteals (and hence the lymph and ultimately the blood). The processes involved in the entrance of nutrients into the body circulation are discussed in this unit, and the subsequent utilization of these absorbed nutrients by body tissues is discussed in Unit III.
Certain dietary components, especially complex carbohydrates from plant cell walls, are not hydrolyzed by enzymes of the human digestive system and pass into the large intestine undigested. Some of these undigestible residues may be fermented by colonic bacteria to form short-chain fatty acids and gases, whereas others add directly to the mass of the stool or are used by the colonic bacteria for their own growth. The short-chain fatty acids may cross the colonic epithelium by diffusion, enter the portal blood, and be used as a fuel by tissues. The undigestible components of plant cells are called dietary fiber; fiber is not an essential nutrient, but it is considered to have physiological and health benefits and to be an important component of healthy diets.
Martha H. Stipanuk
Was this article helpful?