The epithelial cells lining the stomach of vertebrates are protected from damage by HCl because

Options

A. (a) HCl is too dilute
B. (b) the epithelial cells are resistant to the action of HCl
C. (c) HCl is neutralised in the stomach
D. (d) the epithelial cells are covered by a mucus secretion.

Explanation

### Step-by-Step Solution:

1. **Understanding the Stomach's Environment**: The stomach contains gastric glands that play a crucial role in digestion. These glands secrete various substances, including hydrochloric acid (HCl), which creates an acidic environment necessary for digestion and killing pathogens.

2. **Identifying the Cells in Gastric Glands**: The gastric glands consist of three types of cells:
- **Peptic Cells**: These cells secrete pepsinogen, an inactive enzyme that is activated to pepsin in the acidic environment.
- **Auxintic Cells**: These cells secrete hydrochloric acid (HCl), which contributes to the stomach's acidity.
- **Mucous Cells**: These cells secrete mucus, which is vital for protecting the stomach lining.

3. **Role of Hydrochloric Acid**: HCl is essential for:
- Creating an acidic medium that aids in digestion.
- Activating pepsinogen to pepsin for protein digestion.
- Killing harmful bacteria present in food.

4. **Protection of Epithelial Cells**: The epithelial cells lining the stomach are at risk of damage from HCl. However, they are protected by the mucus secreted by the mucous cells. This mucus forms a protective layer over the epithelial cells.

5. **Conclusion**: The mucus covering prevents HCl from directly contacting and damaging the epithelial cells. Therefore, the epithelial cells remain unharmed due to this protective mucus layer.

6. **Final Answer**: The epithelial cells lining the stomach of vertebrates are protected from damage by HCl because they are covered by a mucous secretion (Option D).