The stomach is a muscular bag and is part of the digestive system, sometimes called the gastrointestinal tract. The upper part of the stomach is joined to the gullet (oesophagus) and the lower part of the stomach is joined to the beginning of the small bowel (duodenum).

Once food has been swallowed it passes down the gullet and into the stomach where it is mixed with gastric juices. The semi-solid food then passes into the small bowel where it is broken down further and nutrients are absorbed. The gastric juices in the stomach help the bowel to absorb some important substances from our food such as Vitamin B12, iron and calcium.

The wall of the stomach has four layers:

• The mucosa – this is the name for the lining of the stomach. It contains glands which produce chemicals (enzymes and acid) that make gastric juices.
• The submucosa – a layer underneath the mucosa.
• The muscularis – a layer of muscle beneath the submucosa.
• The serosa – this is a strong membrane that forms the outer  layer of the stomach.
  

As well as being part of the digestive system the stomach is also connected to the lymphatic system. The lymphatic system has two main roles. It helps to protect the body from infection and it drains fluid from the tissues.

All the tissue layers of the stomach are bathed in fluid called lymph. This fluid drains through tiny tubes (lymph vessels) in the stomach layers, which are connected to small bean-like structures (lymph nodes). The lymph nodes filter debris (such as old cells or bacteria) from the fluid before returning it through larger lymph vessels to the main blood circulation.