Hormones are chemicals that are made in several organs of the body. They are released into the blood in tiny amounts, and travel through the body to other organs. These chemical messengers co-ordinate the different organ systems, so that they grow and develop correctly and work together properly.

Some hormones have a sudden, dramatic effect. Adrenaline, for example, is released if you are frightened or nervous. It prepares your body to run or fight, causing 'butterflies' in your stomach by sending blood to your muscles and away from your digestive system. It makes you alert and you breathe quickly. The effect doesn't last for long, though, and your body soon returns to normal after you've calmed down.

Other hormones have slower effects that last for a long time. Growth hormones and sex hormones are like this.