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more thoroughly with the gastric secretions. These contractions also push partially digested food from the antrum through the py-loric sphincter and into the first part of the small intestine.

The inner surface of the stomach is thrown into long folds called rugae, which can be seen with the unaided eye. Microscopic examination of the gastric mucosa shows that it is likewise folded. The openings of these folds into the stomach lumen are called gastric pits. The cells that line the folds deeper in the

Cardia

Pyloric antrum

Pyloric sphincter

Duodenum

Esophagus -Adventitia. Longitudinal muscle

Fundus

Esophagus -Adventitia. Longitudinal muscle

Body

Circular muscle

Oblique muscle

Mucosa

Body

Circular muscle

Oblique muscle

Mucosa

■ Figure 18.5 Primary regions and structures of the stomach. Notice that the pyloric region of the stomach includes the pyloric antrum (the wider portion of the pylorus) as well as the pyloric sphincter.

■ Figure 18.6 A radiograph of the stomach. Note the rugae, which are foldings of the inner wall of the stomach (including the submucosa and mucosa).

mucosa secrete various products into the stomach; these cells form the exocrine gastric glands (fig. 18.7).

Gastric glands contain several types of cells that secrete different products:

1. goblet cells, which secrete mucus;

2. parietal cells, which secrete hydrochloric acid (HCl);

3. chief (or zymogenic) cells, which secrete pepsinogen, an inactive form of the protein-digesting enzyme pepsin;

4. enterochromaffin-like (ECL) cells, found in the stomach and intestine, which secrete histamine and 5-hydroxytryptamine (also called serotonin) as paracrine regulators of the GI tract;

5. G cells, which secrete the hormone gastrin into the blood; and

6. D cells, which secrete the hormone somatostatin.

In addition to these products, the gastric mucosa (probably the parietal cells) secretes a polypeptide called intrinsic factor, which is required for the intestinal absorption of vitamin B12. Vitamin B12 is necessary for the production of red blood cells in the bone marrow (see the next boxed clinical application). Also, the stomach has recently been shown to secrete a hormone named ghrelin. Secretion of this newly discovered hormone rises before meals and falls after meals. This may serve as a signal from the stomach to the brain that helps regulate hunger, as described in chapter 19.

The exocrine secretions of the gastric cells, together with a large amount of water (2 to 4 L/day), form a highly acidic solution known as gastric juice.

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