The stomach is part of the digestive system, which is sometimes called the gastrointestinal tract. It is a muscular, bag-like organ which lies between the lower end of the gullet (oesophagus) and the beginning of the small bowel (small intestine). Once food has been swallowed it passes down the gullet and into the stomach.

The wall of the stomach has four layers:

• The inner lining (the mucosa) contains glands. The glands produce chemicals (enzymes and acid) which are released into the stomach and help to break down food so that when it leaves the stomach it is in a semi-solid form.
• Underneath the mucosa is a layer called the submucosa.
• Beneath that is a layer of muscle called the muscularis.
• The outer layer of the stomach is a strong membrane called the serosa.

The stomach lining also produces a substance which helps to absorb vitamin B12. This is important for the development of red blood cells.

Close to the stomach are a number of lymph nodes.