On close examination, our digestive system – which goes from the mouth to the rectum via the stomach – is a single continuous duct that internalises the outside world. With a length of eight metres and a total surface area of over 200 m², it stands guard over the body’s insides, monitoring any new substances that are taken in. As well as this, it has the all-important function of taking foods sourced externally – i.e. foreign substances – and processing them in such a way that they can be accepted and utilised by the body. Once the stomach’s gastric acid has broken down the food and killed the bacteria, the digestive juices of the small intestine separate the digested material into its individual components – amino acids, carbohydrates and fats – with the aid of bile secretion and the pancreas. This process turns the formerly foreign substances into utilisable components, which is the most important prerequisite for being able to accept them in the body afterwards and use them to produce new complex substances. In the small intestine – which is located right below – and in the colon, all liquids and vital substances such as salts are taken out of the digested material, which in turn causes the intestinal content to thicken. In the colon, this process is aided by some 400 different types of bacteria, some of which even produce vital substances for us, such as Vitamin K. Finally, the remaining digested material that can no longer be utilised is expelled at the end of the colon.