In addition to the nervous system, the body has another communication system called the endocrine system. This system uses chemical messengers called hormones to coordinate responses. Hormones travel in the blood and control slower, longer-term processes.

For your iGCSE exam, you need to know the precise definition of a hormone. A hormone is a chemical substance, produced by a gland, carried in the blood, and it alters the activity of specific target organs.

Let's look at some key glands. The adrenal glands are located on top of your kidneys. They produce the hormone adrenaline. It's a common mistake to confuse the gland with the hormone, so remember: the adrenal gland produces adrenaline.