Physical digestion

The food we eat consists of large lumps of material. We must bite off small pieces and chew them up into even smaller parts, before swallowing them. When the food gets into our stomach it is broken down further as it is squeezed and moved around by the stomach’s muscular walls. This is physical digestion.

Food is moved through the digestive system by the contractions of two sets of muscles in the walls of the gut. One set runs along the gut, while the other set circles it. Their wave-like contractions create a squeezing action, moving down the gut.

Bile is an alkaline substance produced by the liver and stored in the gall bladder. It is secreted into the small intestine, where it emulsifies fats. This is important, because it provides a larger surface area in which the lipases can work.

Drug testing

New medical drugs have to be tested to ensure they work and are safe before they can be prescribed. There are three main stages:

• The drugs are tested using computer models and human cells grown in the laboratory. Many substances fail this test because they damage cells or do not seem to work.
• Drugs that pass the first stage are tested on animals. In the UK, new medicines have to be. But it is illegal to test cosmetics and tobacco products on them. A typical test involves giving a known amount of the substance to the animals, then monitoring them carefully for any side-effects.
• Drugs that have passed animal tests are used in clinical trials. They are tested on healthy…