Porth's Essentials of Pathophysiology, 4e

690

Gastrointestinal and Hepatobiliary Function

U N I T 8

Small Intestinal Digestion and Absorption

transported through the portal vein or lymphatics into the systemic circulation.

Although some digestion of carbohydrates and proteins begins in the mouth and stomach respectively, digestion takes place mainly in the small intestine. The emulsifica- tion of fats to free fatty acids and monoglycerides takes place entirely in the small intestine. The liver, with its production of bile, and the pancreas, which supplies a number of digestive enzymes, play important roles in digestion. The distinguishing characteristic of the small intes- tine is its large surface area, which in the adult is esti- mated to be approximately 250 m 2 . Anatomic features that contribute to this enlarged surface area are the circular folds that extend into the lumen of the intes- tine and the villi, which are fingerlike projections of mucous membrane, numbering as many as 25,000, that line the entire small intestine (Fig. 28-10). Each villus is equipped with an artery, vein, and lymph vessel (i.e., lacteal), which bring blood to the surface of the intes- tine and transport the nutrients and other materials that have passed into the blood from the lumen of the intes- tine (see Fig. 28-9). Fats rely largely on the lymphatics for absorption. The villi are covered with cells called enterocytes that contribute to the absorptive and diges- tive functions of the small bowel, and goblet cells that provide mucus. The enterocytes secrete enzymes that aid in the digestion of carbohydrates and proteins. These enzymes are called brush border enzymes because they adhere to the border of the microvilli that proj- ect from the surface of the enterocytes. In this way they have access to the carbohydrate and protein molecules as they come in contact with the absorp- tive surface of the intestine. This mechanism of secre- tion places the enzymes where they are needed and eliminates the need to produce enough enzymes to mix with the entire contents filling the lumen of the small bowel. The digested molecules diffuse through the membrane or are actively transported across the mucosal surface to enter the blood or, in the case of fatty acids, the lacteal. These molecules are then

Carbohydrate Digestion and Absorption Carbohydrates must be broken down into monosaccha- rides, or single sugars, before they can be absorbed from the small intestine. The average daily intake of carbohy- drate in the American diet is approximately 350 to 400 g. Digestion of starch begins in the mouth with the action of amylase. Pancreatic secretions also con- tain an amylase. Amylase breaks down starch into several disaccharides, including maltose, isomalt- ose, and α -dextrins. The brush border enzymes con- vert the disaccharides into monosaccharides that can be absorbed (Table 28-3). Sucrose yields glucose and fructose, lactose is converted to glucose and galactose, and maltose is converted to two glucose molecules. When the disaccharides are not broken down to monosaccharides, they cannot be absorbed but remain as osmotically active particles in the contents of the digestive system, causing diarrhea. For example, per- sons with a deficiency of lactase, the enzyme that breaks down lactose, experience diarrhea when they drink milk or eat dairy products. Fructose is transported across the intestinal mucosa by facilitated diffusion, which does not require energy expenditure. Glucose and galactose move from the intes- tinal lumen into the intestinal cells by way of a sodium– glucose cotransporter (SGLT-1), against a chemical gradient. The energy for this step does not come directly from ATP, but from the sodium gradient created by the Na + /K + -ATPase pump located on the basolateral side of the membrane (Fig. 28-11). Glucose and galactose are transported from the cell into the blood across the baso- lateral membrane by facilitated diffusion using a glucose transporter-2 (GLUT-2) protein. Water absorption from the intestine is linked to absorption of osmotically active particles, such as glucose and sodium. It follows that an important consideration in facilitating the transport of water across the intestine (and decreasing diarrhea) after temporary disruption in bowel function is to include sodium and glucose in the fluids that are consumed.

Villus Crypt of Lieberkühn Mucosal muscle

Submucosa

Circular muscle

Longitudinal muscle

FIGURE 28-10. The mucous membrane of the small intestine. Note the numerous villi on a circular fold.

Serosa

Lymph node