Stomach

The purpose of the stomach is to break down food and liquids we have ingested into a form that the body can use. 

Most of the food and drink we ingest consists of molecules that are too large to pass into cells, so one function of the stomach is to make it into a manageable size. 

The second function is to break the food down chemically in a process we call digestion into  molecules of a type capable of being passed across the walls of the intestine.

The stomach is a highly elastic and muscular organ which can expand easily to hold a large meal or a large amount of fluids. The stomach wall is folded.  Continual movement of this muscular stomach wall churns the food mechanically and mixes it with gastric juices, helping it to break down the food mechanically.  The combination of chemical and mechanical digestion reduces the food into a uniform creamy paste called acid chyme.

The stomach wall is dotted with pits leading to gastric glands. Nerves and hormones control the secretion of gastric juices so that they are produced only when food is about to be eaten or is in the gut.  These gastric glands secrete gastric juices which consist mainly of water, along with three other types of secretion

• Mucus – which protects the stomach lining from the hydrochloric acid and pepsin

• Hydrochloric acid – which not only breaks down the food but acts a  defensive mechanism against bacteria and viruses.  Generally the stomach was thought to be too hostile for bacteria to survive, but it is clear on the discovery of bacteria like H pylori [which causes stomach ulcers] that some bacteria can survive and enter the intestine

• Pepsinogen  - which is an inactive form of enzyme which is converted by hydrochloric acid to pepsin, the active enzyme

Carbohydrates,  proteins, fats and nucleic acids are digested in the stomach by the acid in the stomach and, more importantly, pepsin. 

There is a type of opening called the pyloric sphincter that guards the opening to the small intestine.  Once the food has become acid chyme this sphincter opens and the chyme flows into the duodenum a little at a time [the duodenum is the first bit of the small intestine].  Hydrogencarbonate ions in the duodenum,  secreted by the pancreas,  neutralise the acid in the chyme.  The intestine is where all the absorption of nutrients takes place.