Human endocrine system

Hormones and nerves

A hormone is a chemical substance, produced by a gland and carried in the bloodstream, which alters the activity of specific target organs. An example of this is the release of the hormone adrenaline, which is released by the adrenal glands. One of its target organs is the heart, where it increases the heart rate.

Once a hormone has been used, it is destroyed by the liver.

Like the nervous system, hormones can control the body. The effects are much slower than the nervous system, but they last for longer.

The foil packaging for the contraceptive pill
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 organs
Type of responseMuscle contraction or secretionChemical change
Speed of responseVery rapidSlower
Duration of responseShort (until nerve impulses stop)Long (until hormone is broken down)

Different hormones

The glands in the body produce a range of different chemical hormones that travel in the bloodstream and affect a number of different organs in the body. The diagram below shows this in detail.

Hormones and the glands that produce them in the human body

Important hormones released into the bloodstream include ADH (anti-diuretic hormone), adrenaline and insulin.

HormoneSourceTarget organ(s)RoleEffects
ADHPituitary glandKidneysControlling the water content of the bloodIncreases reabsorption of water by the collecting ducts
AdrenalineAdrenal glandsSeveral targets including organs in the respiratory and circulatory systemsPreparation for 'fight or flight'Increases breathing rate, heart rate, flow of blood to muscles, conversion of glycogen to glucose
InsulinPancreasLiverControlling blood glucose levelsIncreases conversion of glucose into glycogen for storage

Master gland

The pituitary gland in the brain is known as a 'master gland'. It secretes several hormones into the blood in response to the body's condition, such as blood water levels. The hypothalamus detects changes in hormone levels and will release hormones which control the pituitary gland or other organs. The hormones from the hypothalamus and pituitary can also act on other glands to stimulate the release of different types of hormones and bring about effects.