Physiology Topics   

Stomach

The epithelial cells of the stomach mucosa extend down into the connective tissue forming columns of three types of secretory cells called gastric glands. Parietal cells secrete hydrochloric acid and intrinsic factor (needed for vitamin B12 absorption). Chief cells secrete pepsinogen and gastric lipase. Pepsinogen is the zymogen of pepsin, a protein-digesting enzyme. Zymogens are enzymes secreted in an inactive form. Enteroendocrine cells called G cells secrete gastrin, a hormone that stimulates chief cells and parietal cells to secrete. Goblet cells (secrete mucus) are present both in the lumen and gastric glands.

Gastrin from G cells and acetylcholine (Ach) from the vagus nerve activate receptors in the interstitial side of parietal cells which use Ca+2 as a second messenger to increase the production of H+. Histamine also has a receptor that uses cAMP as a second messenger to increase production of H+.

Carbon dioxide enters the cell from the interstitial space by diffusion and is converted to carbonic acid by carbonic anhydratase. The acid dissociates and bicarbonate is transported back into the interstitial in antiport with Cl- by an active transport pump. H+ is transported in antiport with K+ into the lumen by another active transport pump. K+ and Cl- are also actively symported into the lumen by a third pump. As a result, the stomach pH = 1.

Stomach acid kills microbes in food, denatures proteins and converts pepsinogen into pepsin. It also has regulatory functions (negative feedback) by inhibiting the secretion of gastrin and stimulating the secretion of secretin and cholecystokinin (CCK).

Continue to "Stomach Hormones " or take a quiz: [Q1] [Q2].

Need more practice? Answer the review questions below.

1- Describe the general features of the stomach mucosa.

2- List the types of stomach secretory cells and their secretions.

3- What is intrinsic factor?

4- What is pepsinogen?

5- What is a zymogen?

6- How are parietal cells activated to increase H+ production?

7- What is the role of carbonic anhydratase in stomach acid production?

8- Which ions are transported in or out of parietal cells? How are they transported?

9- What is the pH of the stomach?

10- List the functions of stomach acid?