Gastric juice, like other gastrointestinal secretions, is composed of inorganic and organic components along with water. Key constituents of gastric juice include HCl, salts, pepsins, intrinsic factor, mucus, and HCO3-. The secretion of these components increases after a meal.

The composition of gastric juice in terms of ions depends on the rate of secretion. Higher secretory rates result in higher concentrations of H+ ions, while lower secretory rates lead to a decrease in [H+] and an increase in [Na+]. The concentration of K+ in gastric juice is always higher than in plasma. Prolonged vomiting can cause hypokalemia. Chloride (Cl-) is the primary anion in gastric juice. Gastric HCl plays a role in converting pepsinogens to active pepsins and provides the acidic pH necessary for pepsin activity.

The rate of gastric H+ secretion varies among individuals. Basal rates of gastric H+ production in humans typically range from 1 to 5 mEq/hr, but during maximal stimulation, HCl production can rise to 6 to 40 mEq/hr. Basal rates are higher at night and lower in the early morning. The total number of parietal cells in the stomach varies greatly among individuals, contributing to the wide range of basal and stimulated rates of HCl secretion.

Pepsinogen is the main organic constituent of gastric juice. It is the inactive proenzyme form of pepsin. Pepsins, a group of proteases secreted by the chief cells of the gastric glands, are derived from pepsinogens. Pepsinogens are stored in membrane-bound zymogen granules in chief cells and are released through exocytosis upon stimulation. Pepsinogens are converted to active pepsins through the cleavage of acid-labile linkages, with a faster conversion rate at lower pH. Pepsins can also proteolytically act on pepsinogens to form more pepsin. Although pepsins can digest up to 20% of the protein in a meal, they are not essential for digestion as pancreatic proteases can perform their function. Pepsins are inactivated by neutral pH in the duodenal lumen.

Intrinsic factor, a glycoprotein secreted by parietal cells, is necessary for the absorption of vitamin B12. It is released in response to the same stimuli that trigger the secretion of HCl by parietal cells.

The cellular mechanisms of gastric acid secretion involve parietal cells. Secretory canaliculi extend through the cytoplasm of parietal cells. In resting parietal cells, the tubulovesicular apparatus and intracellular canaliculus can be observed. When the parietal cells are activated and secrete acid, the tubulovesicles fuse with the membranes of the intracellular canaliculus, which then opens into the lumen of the gland and is lined with abundant long microvilli.
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