The stomach is a temporary reservoir between 15 and 25 cm in length, in which food is transformed in to semifluid mass known as chyme. The digestion of proteins starts here, and the stomach performs the vital function of producing intrinsic factor, a substance necessary for the absorption of vitamin B12, which is required by the body to produce red blood cells. The stomach is approximately 50 ml in volume when empty, and can expand to up to 4 litres following a particularly heavy meal. It is divided into four parts: the cardia, which is the area in which the food enters the stomach from the oesophagus, the fundus, the body and the pylorus, which in turn is connected to the small intestine. Having reached the stomach, food does not stay there for long, as it starts to drain again just a few minutes after a meal and is completely empty within less than four hours. What is more, the speed at which the stomach is emptied can be accelerated if the meal is particularly heavy and liquid, while it can be slowed down by the contents of the small intestine.
The form and position of the stomach varies from one person to another, and it is particularly elongated and in a more vertical position in individuals who are tall and thin.