The Stomach
The stomach has three main sections. At the top, the fundus acts as a storage areas and can relax while eating, allowing the stomach to stretch from 50 ml to 1.5L without a significant change in pressure.
The stomach secretes 1-2 L/day from gastric pits, which cover up to 85% of the surface of the body and antrum. The body is responsible for mucus, pepsinogen, and HCl secretion. Its mucosa is very vulnerable to damage, and hemorrage is extremely common.
The antrum secretes gastrin. Mucous neck cells near the top of pits secrete mucus and bicarbonate.
Gastric mucosa is protected by high turnover, tight junctions, and mucus/bicarb secretions. Following gastric biopsy in the stomach, the cells on either side switch from columnar to squamous and fill in the hole within a few hours.
Parietal Cells
Parietal Cells, located in the body, secrete HCl in response to gastrin, histamine, or ACh. HCl is secreted via a proton pump, which goes against a H+ concentration gradient of over 1 million.
Parietal cells also secrete intrinsic factor, which binds to vitamin B12 and facilitates its uptake in the terminal ileum.
Blood CO2 and H20 produce HCO3- and H+. A lumenal proton pump exports H+ and imports K+.
Proton pump function is positively regulated by gastrin, histamine, and ACh (vagus nerve) via GPCR-mediated liberation of calcium stores. It is negatively regulated by somatostatin and prostaglandins.
Acid secretion is blocked by somatostatin.
Zantac is a H2 antagonist, while newer acid inhibitors block the proton pump.
G Cells
G Cells are located in the antrum, and produce gastrin in response to GRP or somatostatin
Gastrin induces acid secretion, antral motility, and mucosal growth. Somatostatin inhibits gastrin release and acid secretion
chief cells
secrete pepsinogen, which is activated by HCl.