These observations suggest that the hormone gastrin, secreted by the pyloric mucosa, may exert stimulatory, or trophic, effects on the gastric mucosa. The structure of the gastric mucosa, in other words, is dependent upon the effects of gastrin.

In the same way, the structure of the acinar (exocrine) cells of the pancreas is dependent upon the trophic effects of CCK. Perhaps this explains why the pancreas, as well as the GI tract, atrophies during starvation. Since neural reflexes appear to be capable of regulating digestion, perhaps the primary function of the GI hormones is trophic—that is, maintenance of the structure of their target organs.

Test Yourself Before You Continue

1. Describe the positive and negative feedback mechanisms that operate during the gastric phase of HCl and pepsinogen secretion.

2. Describe the mechanisms involved in the intestinal phase of gastric regulation and explain why a fatty meal takes longer to leave the stomach than a meal low in fat.

3. Explain the hormonal mechanisms involved in the production and release of pancreatic juice and bile.

4. Describe the enteric nervous system and identify some of the short reflexes that regulate intestinal function.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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