The endocrine system is a control system of the human body much like the nervous system. It produces chemical messages in the form of hormones; whereas the nervous system produces electrical messages.

It is made up of lots of specialised endocrine glands that secrete hormones to the bloodstream. The hormones that are secreted by the endocrine glands are chemical messengers which are carried by the bloodstream to other tissues or organs in the body.

The messages they deliver tell these tissues or organs to either increase or decrease their activity. Hormones act only on target tissues or organs that have the appropriate receptor sites for that given hormone.

In this way hormonal messages are delivered to, and act on, only the areas of the body they are intended for. The endocrine system is linked to the nervous system by effects of the hypothalamus on the pituitary gland, as seen in the adjacent image.

The pituitary gland is known as the 'master gland' because its secretions control the activity of other endocrine glands. The activity of the pituitary gland is however controlled by the hypothalamus which as well as being an endocrine gland,.