Porth's Essentials of Pathophysiology, 4e

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Structure and Function of the Gastrointestinal System

C h a p t e r 2 8

Gastric pits

Mucosa

Mucous cell

Parietal or oxyntic cell

Chief cell

Gastric glands

FIGURE 28-7. Gastric pit from body of the stomach.

Submucosa

from vagal nerves innervating the stomach and binds to acetylcholine receptors on the parietal cells. Gastrin is secreted by G cells in the antrum of the stomach and reaches the parietal cells through the circulation. It binds to as-yet uncharacterized receptors on the parietal cells. Histamine is released from special endocrine cells in the gastric mucosa and diffuses to nearby parietal cells, where it binds to histamine-2 (H 2 ) receptors. One of the important characteristics of the gastric mucosa is resistance to the highly acid secretions that it produces. The mucosal surface is protected by prosta- glandin E 2, which inhibits acid secretion and stimulates mucus production. Hence, it is an important factor in the maintenance of the gastric mucosal barrier. Aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) decrease prostaglandin synthesis, which can impair the integrity of the mucosal surface. Ethyl alcohol, or refluxed bile salts from the intestine, can also disrupt this barrier. When this occurs, hydrogen ions move into the tissue. Hydrogen ions accumulate in the mucosal cells, intracellular pH decreases, enzymatic reactions become impaired, and cel- lular structures are disrupted. The result is local ischemia, vascular stasis, hypoxia, and tissue necrosis. Intestinal Secretions The small intestine, which is where most of the digestion and absorption of food takes place, secretes digestive juices and receives secretions from the liver and pancreas (see Chapter 30). An extensive array of mucus-produc- ing glands, called Brunner glands , is concentrated at the site where the contents from the stomach and secretions from the liver and pancreas enter the duodenum. These glands secrete large amounts of alkaline mucus that pro- tect the duodenum from the acid content in the gastric chyme and from the action of the digestive enzymes.

In addition to mucus, the intestinal mucosa produces two other types of secretions. The first is an isotonic alka- line fluid (pH 6.5 to 7.5) secreted by specialized cells in the crypts of Lieberkühn , which are tubelike glands that dip down into the mucosal surface between the intestinal villi (Fig. 28-9). Unlike the gastric pits, the intestinal crypts do not secrete enzymes, but do secrete mucus, electrolytes,

Blood

Stomach

Mucous cell

Basolateral membrane

Luminal membrane

Parietal cell

Histamine

K +

ATPase

Proton pump

Gastrin

Gastric (HCl) acid

H +

H +

Acetylcholine

Cl –

Cl –

H +

–

HCO 3

H 2 CO 3 CO 2 + H 2

O

FIGURE 28-8. Mechanism of gastric acid secretion by the parietal cells in the stomach.