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Hormones are chemicals that are carried in the blood and released by glands. They are used to facilitate changes in certain areas of the body to carry out a specific function. There are two different types of glands that can produce hormones: exocrine and endocrine. Exocrine glands secrete hormones through a tube or duct. A good example of this is the salivary glands which secrete saliva down the salivary ducts. In contrast, endocrine glands are ductless and instead secrete hormones into blood vessels that pass through the gland.
The difference between hormones and nerves
Both the nervous and hormonal systems work to alter the body in specific areas and adapt it to change. However, they work in quite different ways. The nervous system, as we have explored already in this topic, sends electrical signals through neurones to make changes. However, hormones work to release a chemical messenger into the blood to target cells. This process takes longer to have an effect than the nervous system but has a longer effect. With the hormonal system, responses can often be permanent and will have a lasting effect. With the nervous system, this is not the case and any changes in the nervous system will be easily reversed.
Where are the endocrine glands?
There are several main endocrine glands in the human body which have different jobs and release different hormones. The main gland is the pituitary gland which is in the brain and produces several different hormones. The pituitary gland uses neurones that link it to the hypothalamus within the brain, so it is a close link between the nervous and hormonal systems. In the table below the main glands and the hormones they release are listed.
| Gland | Hormone released | Function of the hormone |
|---|---|---|
| Pituitary | Follicle-stimulating hormone (FSH) Luteinising hormone (LH) Anti-diuretic hormone (ADH) Growth hormone (GH) | Stimulates egg development and the release of oestrogen Stimulates egg release in women and testosterone production in men Controls the water levels in the blood Speeds up growth in children and stimulates development |
| Thyroid | Thyroxin | Controls the body’s metabolism |
| Pancreas | Insulin Glucagon | Lowers blood glucose levels Raises blood glucose levels |
| Adrenals | Adrenaline | Release a hormone that prepares the body for physical action |
| Testes | Testosterone | Increases male sexual characteristics |
| Ovaries | Oestrogen Progesterone | Increases female sexual characteristics Regulates the menstrual cycle |
Adrenaline
The adrenal glands release adrenaline when you are scared, angry or excited. This hormone acts on a lot of organs and muscles and is used to prepare the body for physical action. When you are threatened or in a dangerous situation, adrenaline is released so that you are alert and ready to deal with an attacker. Adrenaline is also seen by many as a ‘fight or flight’ hormone. When released, adrenaline produces a number of changes in the body including:
- Deeper breathing to ensure a high amount of oxygen
- A faster heart rate to ensure plenty of blood flows to muscles
- A diversion of blood from the intestine to the muscles to again ensure plenty of blood is ready to be used in the muscles
- Body hair will stand on end
- Pupils will dilate to increase vision
- Mental awareness is increased
Insulin
Insulin is a hormone that is used to maintain the level of glucose in the blood. Glucose is used to build glycogen in the liver and this glycogen is essentially a bunch of glucose molecules attached together. When we then don’t eat for a little while, this glycogen is broken back down into glucose in order to fuel our bodies without needing to eat. The glycogen therefore acts as a sort of reserve power supply. The main hormone to control this is insulin which is made by special cells in the pancreas. It stimulates the liver cells to take up more glucose and convert this to glycogen and therefore lowers the levels of glucose in the blood. Diabetes is a disease where the pancreas cannot keep the levels of insulin constant. Instead the glucose level rises to a very high concentration and the symptoms of this can be detected in a diabetic’s urine.
Testosterone
Testosterone is the male hormone which is produced by the testes. It encourages male sexual characteristics as well as reproduction. It is regarded as the most important male sexual hormone and is also found in women, although at a much lower level. The hypothalamus controls the production of testosterone and the hormone directly effects things like sex drive, sperm production, fat distribution and muscle mass.
Oestrogen and progesterone
Oestrogen is the female equivalent to testosterone and is secreted by the ovaries. During puberty it is linked to female characteristics such as enlarging breasts and the widening of the hips. Like testosterone for men, oestrogen is considered to be the most important female hormone. Another hormone is called progesterone which is also secreted by the ovaries. This hormone maintains the lining of the uterus during the menstrual cycle to aid women attempting to conceive a child.
