Gastric Secretions

The mucous membrane that forms the inner lining of the stomach is thick, its surface studded with many small openings. These openings, called gastric pits, are located at the ends of tubular gastric glands (oxyntic glands) (fig. 17.19). Although their structure and the composition of their secretions vary in different parts of the stomach, gastric glands generally contain three types of secretory cells. One type, the mucous cell (goblet cell), occurs in the necks of the glands near the openings of the gastric pits. The other types, chief cells (peptic cells) and parietal cells (oxyntic cells), are found in the deeper parts of the glands (figs. 17.19 and 17.20). The chief cells secrete digestive enzymes, and the parietal cells release a solution containing hydrochloric acid. The products of the mucous cells, chief cells, and parietal cells together form gastric juice.

Of the several digestive enzymes in gastric juice, pepsin is by far the most important. The chief cells secrete it as an inactive, nonerosive enzyme precursor called pepsinogen. When pepsinogen contacts the hydrochloric acid from the parietal cells, however, it changes rapidly into pepsin. Pepsin, in turn, can also convert pepsinogen into more pepsin.

Pepsin begins the digestion of nearly all types of dietary protein. This enzyme is most active in an acid environment, which the hydrochloric acid in gastric juice provides.

Shier-Butler-Lewis: V. Absorption and 17. Digestive System © The McGraw-Hill

Human Anatomy and Excretion Companies, 2001

Physiology, Ninth Edition

Esophagus

Esophagus

Circular fibers Longitudinal fibers

Circular fibers Longitudinal fibers

Duodenum

Pyloric canal

Figure 17.17 Pyloric region

(a) Some parts of the stomach have of stomach three layers of muscle fibers. (b) Major regions of the stomach. (b)

Lower esophageal sphincter

Lower esophageal sphincter

Esophagus

Fundic region of stomach

Esophagus

Cardiac region of stomach of stomach

Duodenum

Alexis Martin Stomach

Rugae

Body of stomach

Rugae

Much of what we know about the stomach's functioning comes from a French-Canadian explorer, Alexis St. Martin, who in 1822 accidentally shot himself in the abdomen. His extensive injuries eventually healed, but a hole, called a fistula, was left, allowing observers to look at his stomach in action. A U.S. Army surgeon, William Beaumont, spent eight years watching food digesting in the stomach, noting how the stomach lining changed in response to stress.

In 1984, another chapter unfolded in our knowledge of digestive function when medical resident Barry Marshall at Royal Perth Hospital in western Australia performed a daring experiment. His mentor, J. Robin Warren, had hypothesized that a bacterial infection causes gastritis

(inflammation of the stomach lining) and peptic ulcers (sores in the lining of the esophagus, stomach, or small intestine). At the time, the prevailing view was that a poor diet and stressful lifestyle caused these conditions. Marshall concocted what he called "swamp water" — and drank billions of bacteria. He developed gastritis, which, fortunately, cleared up. A colleague who repeated the experiment developed an ulcer and required antibiotics. After a decade of debate, the medical community concurs that the bacterium Helicobacter pylori, which thrives under acidic conditions, indeed causes gastritis and peptic ulcers. A short course of antibiotics and acid-lowering drugs has replaced lifelong treatments.

Shier-Butler-Lewis: V. Absorption and 17. Digestive System © The McGraw-Hill

Human Anatomy and Excretion Companies, 2001

Physiology, Ninth Edition

Acid Digestion Vessels

Figure 17.19

Gastric pits

Mucous cell Mucous cell mmM

Chief Blood Vessels

Chief cell -

Figure 17.19

Gastric gland

Lamina propria Parietal cell

Chief cell -

Gastric pits

Mucous cell Mucous cell mmM

Gastric Secretions

Submucosa

Submucosa

(a) Gastric glands include mucous cells, parietal cells, and chief cells. (b) The mucosa of the stomach is studded with gastric pits that are the openings of the gastric glands.

Shier-Butler-Lewis: I V. Absorption and I 17. Digestive System I I © The McGraw-Hill

Human Anatomy and Excretion Companies, 2001

Physiology, Ninth Edition

Gastric juice also contains small quantities of a fat-splitting enzyme, gastric lipase. However, its action is weak due in part to the low pH of gastric juice. Gastric li-pase acts mainly on butterfat.

The mucous cells of the gastric glands secrete copious thin mucus. In addition, the cells of the mucous membrane, associated with the inner lining of the stomach and between the gastric glands, release a more viscous and alkaline secretion, which coats the inside of the stomach wall. This coating is especially important because pepsin can digest the proteins of stomach tissues, as well as those in foods. Thus, the coating normally prevents the stomach from digesting itself.

Still another component of gastric juice is intrinsic factor (in-trin'sik fak'tor). The parietal cells of the gastric glands secrete intrinsic factor, which is required for vitamin B12 absorption from the small intestine. Table 17.5 summarizes the components of gastric juice.

Mucous cell Gastric pit

Parietal cell

Gastric gland

Chief cell

Figure

Mucous cell Gastric pit

Parietal cell

Gastric gland

Chief cell

Figure

Enlarged Photo Blood Vessels

A light micrograph of cells associated with the gastric glands (50x micrograph enlarged to 100x).

U Where is the stomach located?

^9 What are the secretions of the chief cells and parietal cells?

^9 Which is the most important digestive enzyme in gastric juice?

□ Why doesn't the stomach digest itself?

Essentials of Human Physiology

Essentials of Human Physiology

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