The stomach is a part of the digestive system. Food travels from the mouth to the stomach via the esophagus. This food has been masticated in the mouth, is known as bolus, and is ready for further digestion in the small intestine.
The food collects in the stomach where it resides for the time the small intestines have processed the previous batch and passed it to the large intestine. The bolus is converted into partially digested chyme in the stomach and released slowly into the duodenum, the anterior portion of the small intestine. Breaking down of protein happens in the stomach with the aid of the enzyme pepsin. Hydrochloric acid in the stomach kills harmful bacteria. The acid is secreted by the parietal cells in the stomach wall. The chemical action of the enzyme digests proteins and mechanical churning action of the stomach wall breaks down the food into still smaller particles. It is worth noting that fats, starches, and sugars pass through the stomach undigested. These are digested in the small intestine.
The stomach is made up of four parts, namely cardia, fundus, corpus, and pylorus.
Food is received in the cardia. The esophageal sphincter located at the junction of the cardia and the esophagus prevents stomach acids from traveling to the esophagus. The fundus is located at the top left of the stomach, it serves to store gases released during the chemical breakdown and churning of food.
The corpus or body is the largest part of the stomach; this is where the churning action takes place. The fourth region, pylorus, is connected to the duodenum. The four layers of the stomach wall are the mucosa, submucosa, muscularis externa, and serosa.
Although the bulk of the absorption happens in the small intestine, certain medicines like aspirin and molecules of amino acids are absorbed directly by the stomach walls. The stomach also has a very unique capability of "tasting" food items such as fats, glutamates, glucose, carbohydrates, proteins, and fats.
The transit of food through the stomach does not follow the same order in which it arrived. Very small particles of food and water can pass through almost immediately. On an average, food will stay in the stomach for up to five hours. But the total transit time of food through the digestive system will vary with the composition of food and the individual's health. The stomach's volume is around 45 ml without distension and can hold up to 3 liters of food