Human endocrine system
Hormones and nerves
A Chemical messenger produced in glands and carried by the blood to specific organs in the body. is a chemical substance, produced by a An organ or tissue that makes a substance for release, such as a hormone. and carried in the bloodstream, which alters the activity of specific The organ with receptor molecules on its cell surfaces which recognise a specific hormone.. 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.
There are important differences between nervous and hormonal control.
| Nervous | Hormonal | |
| Type of signal | Electrical (chemical at synapses) | Chemical |
| Transmission of signal | By nerve cells (neurones) | By the bloodstream |
| Effectors | Muscles or glands | Target cells in particular organs |
| 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) |
| Type of signal | |
|---|---|
| Nervous | Electrical (chemical at synapses) |
| Hormonal | Chemical |
| Transmission of signal | |
|---|---|
| Nervous | By nerve cells (neurones) |
| Hormonal | By the bloodstream |
| Effectors | |
|---|---|
| Nervous | Muscles or glands |
| Hormonal | Target cells in particular organs |
| Type of response | |
|---|---|
| Nervous | Muscle contraction or secretion |
| Hormonal | Chemical change |
| Speed of response | |
|---|---|
| Nervous | Very rapid |
| Hormonal | Slower |
| Duration of response | |
|---|---|
| Nervous | Short (until nerve impulses stop) |
| Hormonal | 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.
Important hormones released into the bloodstream include ADH (anti-diuretic hormone), adrenaline and insulin.
| Hormone | Source | Target organ(s) | Role | Effects |
| 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 organs in 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 |
| Hormone | ADH |
|---|---|
| Source | Pituitary gland |
| Target organ(s) | Kidneys |
| Role | Controlling the water content of the blood |
| Effects | Increases reabsorption of water by the collecting ducts |
| Hormone | Adrenaline |
|---|---|
| Source | Adrenal glands |
| Target organ(s) | Several targets including organs in the respiratory and circulatory systems |
| Role | Preparation for 'fight or flight' |
| Effects | Increases breathing rate, heart rate, flow of blood to muscles, conversion of glycogen to glucose |
| Hormone | Insulin |
|---|---|
| Source | Pancreas |
| Target organ(s) | Liver |
| Role | Controlling blood glucose levels |
| Effects | Increases conversion of glucose into glycogen for storage |
Master gland
The An endocrine gland that is located just below the centre of the brain. It releases a number of important hormones. 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 Part of the brain which controls water balance, temperature and secretion of hormones by the pituitary gland. 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.