The endocrine system uses hormones to allow the body to respond to stimuli. Stimuli are changes in the environment and can be from:
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.
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.
|ADH||Pituitary gland||Kidneys||Controlling the water content of the blood||Increases reabsorption of water by the collecting ducts|
|Adrenaline||Adrenal glands||Several targets including the respiratory and circulatory systems||Preparation for 'fight or flight'||Increases breathing rate, heart rate, flow of blood to muscles, conversion of glycogen to glucose|
|Insulin||Pancreas||Liver||Controlling blood glucose levels||Increases conversion of glucose into glycogen for storage|
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 signal||Electrical (chemical at synapses)||Chemical|
|How the signal is carried||By nerve cells (neurons)||By the bloodstream|
|Effectors||Muscles or glands||Target cells in particular tissues|
|Type of response||Muscle contraction or secretion||Chemical change|
|Speed of response||Very rapid||Slower|
|Duration of response||Short (until nerve impulses stop)||Long (until hormone is broken down)|