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Chapter: Medical Physiology: Physiology of Gastrointestinal Disorders

Disorders of the Small Intestine

A serious cause of abnormal digestion is failure of the pancreas to secrete pancreatic juice into the small intestine.

Disorders of the Small Intestine

Abnormal Digestion of Food in the Small Intestine—Pancreatic Failure

A serious cause of abnormal digestion is failure of the pancreas to secrete pancreatic juice into the small intestine. Lack of pancreatic secretion frequently occurs (1) in pancreatitis(which is discussed later), (2) when the pancreatic duct is blocked by a gallstone at the papillaof Vater, or (3) after the head of the pancreas has beenremoved because of malignancy.

Loss of pancreatic juice means loss of trypsin, chy-motrypsin, carboxypolypeptidase, pancreatic amylase, pancreatic lipase, and still a few other digestive enzymes. Without these enzymes, as much as 60 per cent of the fat entering the small intestine may be unab-sorbed, as well as one third to one half of the proteins and carbohydrates. As a result, large portions of the ingested food cannot be used for nutrition, and copious, fatty feces are excreted.

Pancreatitis. Pancreatitis means inflammation of thepancreas, and this can occur in the form of either acutepancreatitis or chronic pancreatitis.

The most common cause of pancreatitis is drinkingexcess alcohol, and the second most common cause is blockage of the papilla of Vater by a gallstone; the twotogether account for more than 90 per cent of all cases. When a gallstone blocks the papilla of Vater, this blocks the main secretory duct from the pancreas as well as the common bile duct. The pancreatic enzymes are then dammed up in the ducts and acini of the pancreas. Even-tually, so much trypsinogen accumulates that it over-comes the trypsin inhibitor in the secretions, and a smallquantity of trypsinogen becomes activated to form trypsin. Once this happens, the trypsin activates still more trypsinogen as well as chymotrypsinogen and car-boxypolypeptidase, resulting in a vicious circle until most of the proteolytic enzymes in the pancreatic ducts and acini become activated. These enzymes rapidly digest large portions of the pancreas itself, sometimes completely and permanently destroying the ability of the pancreas to secrete digestive enzymes.

Malabsorption by the Small Intestinal Mucosa—Sprue

Occasionally, nutrients are not adequately absorbed from the small intestine even though the food has become well digested. Several diseases can cause decreased absorption by the mucosa; they are often classified together under the general term “sprue.” Mal-absorption also can occur when large portions of the small intestine have been removed.

Nontropical Sprue. One type of sprue, called variouslyidiopathic sprue, celiac disease (in children), or gluten enteropathy, results from the toxic effects of gluten present in certain types of grains, especially wheat and rye. Only some people are susceptible to this effect, but in those who are susceptible, gluten has a direct destruc-tive effect on intestinal enterocytes. In milder forms of the disease, only the microvilli of the absorbing entero-cytes on the villi are destroyed, thus decreasing the absorptive surface area as much as twofold. In the more severe forms, the villi themselves become blunted or disappear altogether, thus still further reducing the absorptive area of the gut. Removal of wheat and rye flour from the diet frequently results in cure within weeks, especially in children with this disease.

Tropical Sprue. A different type of sprue calledtropicalsprue frequently occurs in the tropics and can often betreated with antibacterial agents. Even though no spe-cific bacterium has been implicated as the cause, it is believed that this variety of sprue is usually caused by inflammation of the intestinal mucosa resulting from unidentified infectious agents.

Malabsorption in Sprue. In the early stages of sprue, intes-tinal absorption of fat is more impaired than absorption of other digestive products. The fat that appears in the stools is almost entirely in the form of salts of fatty acids rather than undigested fat, demonstrating that the problem is one of absorption, not of digestion. In fact, the condition is frequently calledsteatorrhea, which means simply excess fats in the stools.

In very severe cases of sprue, in addition to malab-sorption of fats there is also impaired absorption of pro-teins, carbohydrates, calcium, vitamin K, folic acid, and vitamin B12. As a result, the person suffers (1) severe nutritional deficiency, often developing wasting of the body; (2) osteomalacia (demineralization of the bones because of lack of calcium); (3) inadequate blood coag-ulation caused by lack of vitamin K; and (4) macrocytic anemia of the pernicious anemia type, owing to dimin-ished vitamin B12 and folic acid absorption.

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