Human endocrine system

The endocrine system

The endocrine system uses hormones to allow the body to respond to stimuli. Stimuli are changes in the environment and can be from:

  • inside the body (internal) such as like an increase in blood glucose concentration,

or from

  • outside of the body (external) like a drop in temperature

A hormone is a chemical substance, produced by a gland and carried in the bloodstream, which alters the activity of specific target organs. Hormones bind to receptors on the target organ. The binding of the hormone onto the receptors causes the target organ to respond. For example, when insulin is released into the blood from the pancreas, insulin binds to receptors on the surface of the liver, the target organ. This causes the liver to respond and take glucose out of the blood and store it.

Once a hormone has caused the required response, it is destroyed by the liver.

Different hormones

The body produces a range of different chemical hormones that travel in the bloodstream and affect a number of different organs or cells in the body. The diagram and table below shows this in detail.

Hormones and the glands that produce them in the human body
SourceTarget organ(s)RoleEffects
ADHPituitary glandKidneysControlling the water content of the bloodIncreases reabsorption of water by the collecting ducts
AdrenalineAdrenal glandsSeveral targets including 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

Comparing the endocrine and nervous system

The endocrine system causes slower responses than the nervous system, but the responses last for longer. There are important differences between hormonal control caused by the endocrine system and nervous control.

Type of signalElectrical (chemical at synapses)Chemical
How the signal is carriedBy nerve cells (neurons)By the bloodstream
EffectorsMuscles or glandsTarget cells in particular tissues
Type of responseMuscle contraction or secretionChemical change
Speed of responseVery rapidSlower
Duration of responseShort (until nerve impulses stop)Long (until hormone is broken down)