The main function of the stomach is to break down and digest food in order to extract necessary nutrients from what you've eaten.
In order for this to happen, it's necessary that the stomach, the digestive glands and the intestines produce various enzymes, including pepsin, and acid. These acids and enzymes need to change the food into a semi-liquid form. Then the starch, fat and protein need to be broken down into smaller chemical units that can be absorbed into the bloodstream through the wall of the intestine.
The process of digestion
The process of digestion starts in the mouth, where food is chewed and broken into smaller pieces. Saliva is mixed with the food, which makes it easier to move around in the mouth. An enzyme called salivary amylase also starts to digest carbohydrates like sugar and starches.
Once the food has been chewed, it's pushed by the tongue to the back of the mouth. From here, muscles move it further down the oesophagus and from there past a one-way valve, which is called the oesophageal sphinctre.
The muscles of the oesophagus are strong and gravity-defying – one can swallow even if you're standing on your head or lying down.
Once the food is in your stomach, it's not supposed to move back up again, even if you're lying down, or if the stomach contracts.
The acid in your stomach
The amount of acid in your stomach varies – it obviously increases when you've eaten. There's always acid present in the stomach, but usually in small amounts. The stomach itself is made of protein, and unless it's protected, it will be consumed by the acid in the stomach.
Generally, the stomach and the duodenum are protected from the acid by a layer of mucus that stops the acid from consuming the stomach lining. Several factors, can lead to this protective layer of mucus not performing its job, and stomach and duodenal ulcers can result.
The oesophagus doesn't have this protective layer, hence the feeling of heartburn if food pushes back up past the oesophageal sphinctre.
The three main functions of the stomach
The stomach, under normal conditions, performs three main functions:
Firstly, it acts as a place of storage, so that 2 or 3 meals a day can provide all our energy needs. Without this, we would have had to eat bits and pieces all day.
Secondly, it also aids the digestive process, as it is here that food is turned into a semi-liquid substance so that the nutrients can be absorbed from it.
A third very important function of the stomach is the destruction of contaminants that the food may contain – bacteria and other micro-organisms. Very little is absorbed into the bloodstream straight through the stomach walls – aspirin and alcohol being exceptions to this rule.
When food leaves the stomach
After a few hours in the stomach, the liquidised food is pushed downward through another valve, called the pylorus, into the small intestine, so called because it is narrow and not short – it could be up to six metres long. Here chemicals are added to neutralise stomach acid.
Enzymes from the pancreas and bile from the liver are secreted to further the digestive process and to break down fats, carbohydrates and proteins.