Info

Major Components of Gastric Juice

Component

Source

Function

Pepsinogen

Chief cells of the gastric glands

Inactive form of pepsin

Pepsin

Formed from pepsinogen in the presence of hydrochloric acid

A protein-splitting enzyme that digests nearly all types of dietary protein

Hydrochloric acid

Parietal cells of the gastric glands

Provides the acid environment needed for the conversion of pepsinogen into pepsin and for the action of pepsin

Mucus

Goblet cells and mucous glands

Provides a viscous, alkaline protective layer on the stomach wall

Intrinsic factor

Parietal cells of the gastric glands

Required for absorption of vitamin B12

1. Parasympathetic preganglionic nerve fiber (in vagus nerve)

gastrin is inhibited. When the pH reaches 1.5, gastrin secretion ceases.

4. Gastrin stimulates glands to release more gastric juice

1. Parasympathetic preganglionic nerve fiber (in vagus nerve)

4. Gastrin stimulates glands to release more gastric juice

2. Parasympathetic ' 3. Impulses stimulate postganglionic the release of gastrin impulses stimulate the release of gastric juice from gastric glands

Gastrin stimulates cell growth in the mucosa of the stomach and intestines, except where gastrin is produced. This effect helps replace mucosal cells damaged by normal stomach function, disease, or medical treatments.

2. Parasympathetic ' 3. Impulses stimulate postganglionic the release of gastrin impulses stimulate the release of gastric juice from gastric glands

Figure 17.21

The secretion of gastric juice is regulated in part by parasympathetic nerve impulses that stimulate the release of gastric juice and gastrin.

Histamine is very effective in promoting secretion of gastric acid. Consequently, drugs that block the histamine receptors of gastric mucosal cells (H2-block-ers) are often used to inhibit excess gastric acid secretion.

Three stages of gastric secretion are recognized— the cephalic, gastric, and intestinal phases. The cephalic phase of gastric secretion begins before any food reaches the stomach and possibly even before eating. In this stage, parasympathetic reflexes operating through the vagus nerves stimulate gastric secretion whenever a person tastes, smells, sees, or even thinks about food. Furthermore, the hungrier the person is, the greater the gastric secretion. The cephalic phase of secretion is responsible for 30%-50% of the secretory response to a meal.

The gastric phase of gastric secretion, which accounts for 40%-50% of the secretory activity, starts when food enters the stomach. The presence of food and the distension of the stomach wall trigger the stomach to release gastrin, which stimulates production of still more gastric juice.

As food enters the stomach and mixes with gastric juice, the pH of the contents rises, which enhances gas-trin secretion. Consequently, the pH of the stomach contents drops. As the pH approaches 3.0, secretion of

For the stomach to secrete hydrochloric acid, hydrogen ions are removed from the blood, and an equivalent number of alkaline bicarbonate ions are released into the blood. Consequently, following a meal, the blood concentration of bicarbonate ions increases, and the urine excretes excess bicarbonate ions. This phenomenon is called the alkaline tide.

The intestinal phase of gastric secretion, which accounts for about 5% of the total secretory response to a meal, begins when food leaves the stomach and enters the small intestine. When food first contacts the intestinal wall, it stimulates intestinal cells to release a hormone that again enhances gastric gland secretion. Many investigators believe this hormone is identical to gastrin, and they call it intestinal gastrin.

As more food moves into the small intestine, secretion of gastric juice from the stomach wall is inhibited due to a sympathetic reflex triggered by acid in the upper part of the small intestine. Also, the presence of proteins and fats in this region of the intestine causes the release of the peptide hormone cholecystokinin from the intestinal wall, which decreases gastric motility. Similarly, fats in the small intestine stimulate intestinal cells to release intestinal somatostatin, which inhibits release of gastric juice. Overall, these actions decrease gastric secretion and motility as the small intestine fills with food. Table 17.6 summarizes the phases of gastric secretion.

O What controls gastric juice secretion?

Distinguish among the cephalic, gastric, and intestinal phases of gastric secretion.

What is the function of cholecystokinin?

Diet Tweak System

Diet Tweak System

Trying To Lose Weight Can Be Tough. But... Not Losing Weight and Gaining What You Lost Back, Sucks. If you've ever felt that no matter what you do to lose weight nothing seems to work. If you've ever felt that there has got to be some kind of a system or way to lose weight...but just have not found it yet.

Get My Free Ebook


Post a comment