Final answer:
The four main types of secreting cells in gastric glands are parietal cells, chief cells, mucous neck cells, and enteroendocrine cells. The stomach doesn't digest itself because of the protective mucus layer. Mechanical and chemical digestion occur in the stomach through churning, mixing, and the action of hydrochloric acid.
Step-by-step explanation:
The four main types of secreting cells in gastric glands are parietal cells, chief cells, mucous neck cells, and enteroendocrine cells. Parietal cells secrete hydrochloric acid, while chief cells produce pepsinogen, which is converted into pepsin for protein digestion. Mucous neck cells secrete mucus, and enteroendocrine cells produce hormones like gastrin.
The stomach doesn't digest itself because the gastric glands produce a protective layer of mucus that covers the inner lining of the stomach. This mucus layer acts as a barrier between the stomach acid and the stomach tissues.
Mechanical digestion in the stomach involves the churning and mixing of food, breaking it down into smaller particles. Chemical digestion in the stomach occurs primarily through the action of hydrochloric acid, which denatures proteins and activates pepsinogen to pepsin.