Gastric acid secretion in the stomach is one of the important functions of the digestive system. It plays a crucial role in converting the ingested food into a semiliquid mass called chyme. Gastric acid is produced by the gastric glands present in the stomach lining. These glands contain different types of cells that secrete various components of gastric juice.

Parietal Cells
The main type of cell that secretes gastric acid is the parietal cell. These cells are found in the gastric glands of the stomach lining and are responsible for producing hydrochloric acid (HCl). Parietal cells also secrete intrinsic factor, a glycoprotein that is essential for the absorption of vitamin B12 in the small intestine.

Stimulation of Parietal Cells
Parietal cells are stimulated to secrete gastric acid by various mechanisms, such as:

- Histamine: Histamine is released from enterochromaffin-like (ECL) cells in response to gastrin or acetylcholine. It binds to H2 receptors on parietal cells and stimulates the production of cyclic AMP, which in turn activates proton pump to secrete HCl.
- Acetylcholine: Acetylcholine is released from nerve endings in response to the presence of food in the stomach. It binds to muscarinic receptors on parietal cells and stimulates the production of cyclic AMP, which in turn activates proton pump to secrete HCl.
- Gastrin: Gastrin is a hormone secreted by G cells in the stomach lining. It binds to CCK2 receptors on parietal cells and stimulates the production of cyclic AMP, which in turn activates proton pump to secrete HCl.

Other Types of Cells in Gastric Glands
Apart from parietal cells, gastric glands also contain other types of cells that secrete various components of gastric juice. These include:

- Chief cells: Chief cells secrete pepsinogen, an inactive precursor of pepsin, a proteolytic enzyme that breaks down proteins in the stomach.
- Mucous cells: Mucous cells secrete mucus, which forms a protective layer on the stomach lining and prevents it from being damaged by gastric acid.
- Enterochromaffin-like (ECL) cells: ECL cells secrete histamine, which stimulates parietal cells to secrete gastric acid.
- D cells: D cells secrete somatostatin, a hormone that inhibits the secretion of gastric acid and other digestive enzymes.

Conclusion
In conclusion, gastric acid is primarily secreted by parietal cells in the stomach lining. These cells are stimulated to secrete acid by various mechanisms, such as histamine, acetylcholine, and gastrin. Apart from parietal cells, gastric glands also contain chief cells, mucous cells, ECL cells, and D cells that secrete other components of gastric juice.

Gastric acid is mainly composed of HCl and that is secreted by parietal cells/oxyntic cells of Copro fundic gland of stomach.