Hormone that stimulates parietal cells to secrete HCl and chief cells to secrete pepsinogen

The hormone that stimulates parietal cells to secrete Hydrochloric Acid (HCl) and chief cells to secrete pepsinogen is Gastrin.

Here are the steps of how it works:

1. When food enters the stomach, it triggers the G cells in the stomach lining to release Gastrin.

2. Gastrin then enters the bloodstream and reaches the parietal cells in the stomach lining.

3. Gastrin stimulates the parietal cells to secrete Hydrochloric Acid (HCl). The HCl helps to create the acidic environment necessary for the digestion and breakdown of food.

4. Gastrin also stimulates the chief cells in the stomach lining to secrete pepsinogen.

5. Pepsinogen is an inactive enzyme that is activated by the acidic environment in the stomach created by the HCl. Once activated, pepsinogen becomes pepsin, which is an enzyme that helps to break down proteins in the food.

6. The secretion of HCl and pepsinogen continues until the food has been sufficiently broken down and the acidity of the stomach triggers the G cells to stop releasing Gastrin.