The digestive system When you eat, the food's journey through the body is governed by the digestive system.

In this article

What is digestion? In the mouth

The oesophagus The stomach

The small intestine The large intestine

Digestion is the process of breaking down food so that it's small enough to be absorbed and used by the body for energy or in other bodily functions.

Digestion involves a number of different stages. The first phase is known as the cephalic (head) phase. It starts before food has even entered your mouth. The sight, smell, taste or even the thought of food will activate saliva in the mouth as well as digestive juices, which contain enzymes to break down food.

Prebiotics

These are mainly indigestible carbohydrates called oligosaccharides. On reaching the large intestine, they selectively stimulate the growth and/or activity of beneficial microorganisms already in the colon, such as bifidobacteria and lactobacilli.

Once food is in the mouth, the tastebuds begin determining the chemicals within the food via their nerve endings, in order to give you the taste sensations of salt, sweet, sour or bitter. As your teeth chew and grind the food, breaking it down, it's mixed with saliva. This comprises many enzymes including salivary amylase, which begins to break down the long chains of starch found in foods such as bread, cereals, potatoes and pasta. Saliva also contains mucin, which moistens the food so it can pass easily through the digestive (gastrointestinal) tract.

After the food has been swallowed, it's carried down the oesophagus (a muscular tube) towards the stomach. The oesophagus can contract and relax in order to propel the food onwards, and each mouthful of food takes about six seconds to reach the stomach once swallowed.

Probiotics

Probiotics are live bacteria similar to the bacterial micro-organisms that live in the large intestine. They are often referred to as 'friendly bacteria', and come from food sources or dietary supplements. The mix of these 'friendly' bacteria and other gut microorganisms is important for good health, and many factors can alter this delicate balance, such as infection or use of antibiotics. Friendly bacteria are vital for proper development of the immune system, to protect against micro-organisms that could cause disease, and to aid the digestion and absorption of food and nutrients.

The stomach is a sack made of muscle and, when it's empty, it has a volume of only 50ml but this can expand to hold up to 1.5 litres or more after a meal. The walls of the stomach are made of three different layers of muscle that allow it to churn food around and make sure it's mixed with the stomach's acidic digestive juices. The presence of hydrochloric acid in the stomach prevents the action of salivary amylase and helps to kill bacteria that might be present. The stomach also produces the enzyme pepsin, which breaks down proteins (mostly found in meat, fish, eggs and dairy products).

The hormone ghrelin is produced by cells lining the stomach. Ghrelin stimulates hunger and tends to increase before a meal and decrease after eating. This hormone forms part of the communication system between the gut and the part of the brain that controls hunger and satiety (how full you feel).

Food can stay in the stomach for a few minutes or several hours in the gastric phase where numerous acids and enzymes are released, including the hormone gastrin. When the food has been churned into a creamy mixture known as chyme, the pyloric sphincter (an opening controlled by muscle) opens and chyme passes gradually into the small intestine.