Hormones and glands

A hormone is a chemical messenger.

Hormones are produced by endocrine glands and carried in the bloodstream. They alter the activity of specific target organs.

One example of this is the hormone adrenaline, which is released by the adrenal gland. One of its target organs is the heart, where it increases the heart rate.

Once a hormone has been used, and its action complete, it is destroyed by the liver.

The effects of hormones on the body are almost always much slower than the nervous system, but they last for longer.

Contraceptive pill packaging
Contraceptive pills contain hormones to reduce the chances of becoming pregnant

There are important differences between nervous and hormonal control.

Type of signalElectrical (chemical at synapses)Chemical
Transmission of signalBy nerve cells (neurones)By the bloodstream
EffectorsMuscles or glandsTarget cells in particular tissues
Speed of responseVery rapidSlower
Duration of responseShort (until nerve impulses stop)Long (until hormone is broken down)

Master gland

The pituitary gland, at the base of the brain, is known as a 'master gland'. It secretes several hormones that can act on other glands to stimulate the release of other types of hormone.

The endocrine system

The endocrine system produces a range of different hormones that travel in the bloodstream and affect a number of different organs or cells in the body. The diagram shows the location of some of these glands.

Hormones and the glands that produce them in the human body