Your stomach s a "crescent-shaped hollow organ" and is about the size of a large melon. The average stomach of an adult holds about three quarts of fluid.
The stomach is made up of a variety of layers, including the outer layer that acts as a covering for the other layers, which is called the serosa. There are two muscle layers - they are the middle layers that propel food into the small intestine. The inner layer is called the mucosa and is made up of specialized cells, which are parietal cells, g-cells and epithelial cells.
Parietal cells produce hydrochloric acid which helps to break down food. It is so concentrated that one drop would eat right through a piece of wood. The g-cells produce gastrin, which is a hormone that helps the the parietal cells to produce hydrochloric acid.
It is the epithelial cells that protect the stomach by producing and secreting a bicarbonate-rich solution to coat the mucosa. Bicarbonate is alkaline and neutralizes the acid secreted by the parietal cells; this produces water in the process. The continuous supply of bicarbonate is the primary way for your stomach to protect itself from the acidic environment ? and keeps the stomach from digesting itself.
This defense system does not work as well as it should in some people due to problems with the blood supply to the stomach, or when overproduction of acid occurs. This can cause gastric ulcers. Sometimes specific bacteria, called Helicobacter pylori, may cause interfere with the stomach's defenses and can also result in ulcers.