The stomach has two smooth muscle valves, or sphincters, which keep the contents of the stomach contained. They are the esophageal sphincter that is found in the cardiac region and divides the tract above, and the Pyloric sphincter which divides the stomach from the small intestine. Food enters the stomach from the esophagus through the esophageal sphincter, which prevents food from passing back to the esophagus.
The stomach consists of five layers. Starting from the inside, the innermost layer is called the mucosa. Stomach acid and digestive juices are secreted by the mucosa layer. The next layer is called the submucosa, which is surrounded by the muscularis, a layer of muscle that moves and mixes the stomach contents. The next two layers, the subserosa and the serosa are the wrapping for the stomach. The serosa is the outermost layer of the stomach. The stomach is surrounded by parasympathetic (stimulant) and orthosympathetic (inhibitor) nervous plexuses (anterior gastric, posterior, superior and inferior, celiac and myenteric). These plexuses regulate both the secretory activity and the motor activity of the muscles.
Four major types of secretory epithelial cells cover the inner surface of the stomach and extend down into gastric pits and glands. Mucous cells secrete an alkaline mucus that protects the epithelium against shear stress and acid. Parietal cells secrete hydrochloric acid. Chief cells produce pepsin, a proteolytic enzyme. G cells secrete the hormone gastrin.