Like other GI secretions, gastric juice consists of inorganic and organic constituents together with water. Among the important components of gastric juice are HCl, salts, • Fig.29.1The three functional regions of the stomach. The regions have different luminal secretions and patterns of smooth muscle activity indicative of their unique functions in response to food.
CHAPTER 29 The Gastric Phase of the Integrated Response to a Meal 531
Opening of gastric pit
Opening of gastric pit • Fig. 29.2 Representation of the structure of the gastric mucosa showing a section through the wall of the stomach (A) and detail of the structure of gastric glands and cell types in the mucosa (B).
pepsins, intrinsic factor, mucus, and HCO3 − . Secretion of all these components increases after a meal.
Inorganic Constituents of Gastric Juice
The ionic composition of gastric juice depends on the rate of secretion. The higher the secretory rate, the higher the concentration of H+ ions. At lower secretory rates, [H+] decreases and [Na+] increases. [K+] is always higher in gastric juice than in plasma. Consequently, prolonged vomiting may lead to hypokalemia. At all rates of secretion, Cl− is the major anion of gastric juice. Gastric HCl converts pepsinogens to active pepsins and provides the acid pH at which pepsins are active.
The rate of gastric H+ secretion varies considerably among individuals. In humans, basal (unstimulated) rates of gastric H+ production typically range from about 1 to 5 mEq/hr. During maximal stimulation, HCl production rises to 6 to 40 mEq/hr. The basal rate is greater at night and lowest in the early morning. The total number of parietal cells in the stomach of normal individuals varies greatly, and this variation is partly responsible for the wide range in basal and stimulated rates of HCl secretion.
Organic Constituents of Gastric Juice
The predominant organic constituent of gastric juice is pepsinogen, the inactive proenzyme of pepsin. Pepsins, often collectively called “pepsin,” are a group of proteases secreted by the chief cells of the gastric glands. Pepsinogens are contained in membrane-bound zymogen granules in the chief cells. Zymogen granules release their contents by exocytosis when chief cells are stimulated to secrete 29.1 Response to a Meal
Table 29.1 ). Pepsinogens are converted to active pepsins by the cleavage of acid-labile linkages. The lower the pH, the more rapid the conversion. Pepsins also act proteolytically on pepsinogens to form more pepsin. Pepsins are most proteolytically active at pH 3 and below. Pepsins may digest as much as 20% of the protein in a typical meal but are not required for digestion, because their function can be replaced by that of pancreatic proteases. When the pH of the duodenal lumen is neutralized, pepsins are inactivated by the neutral pH.
Intrinsic factor, a glycoprotein secreted by parietal cells of the stomach, is required for normal absorption of vitamin B12. Intrinsic factor is released in response to the same stimuli that elicit secretion of HCl by parietal cells.
Cellular Mechanisms of Gastric Acid Secretion
Fig. 29.3 ). Branching secretory canaliculi course through the cytoplasm • Fig. 29.3
Parietal cell ultrastructure. A, A resting parietal cell showing the tubulovesicular apparatus in the cytoplasm and the intracellular canaliculus. B, An activated parietal cell that is secreting acid. The tubulovesicles have fused with the membranes of the intracellular canaliculus, which is now open to the lumen of the gland and lined with abundant long microvilli.
• Fig. 29.4