The hormonal system

The endocrine system plays an important role in responding to our internal and external environments. Hormones such as insulin, adrenaline and thyroxine keep things inside our bodies running smoothly..

The endocrine system

The endocrine system is a group of glands that secrete chemicals called hormones. Hormones travel in the bloodstream to their target organs. Compared to the nervous system, the effects of hormones are much slower but are longer lasting.

The pituitary gland in the brain is known as the ‘master gland’ — it releases hormones which stimulate other glands to release hormones. These include:

• Pancreas – located behind the stomach. It releases the hormones insulin and glucagon which control blood glucose levels.

• Thyroid – found in the neck. It releases thyroxine which controls metabolism and is involved in maintaining body temperature and heart rate.

• Adrenal gland – located just above the kidneys. The adrenal gland releases adrenaline which is involved in the ‘fight or flight’ response. Adrenaline causes changes in the body, such as increased breathing rate, increased blood glucose, dilated pupils and increased blood flow to muscles. These changes help to prepare our bodies for action.

• Ovary – found in the female reproductive system either side of the uterus. It releases oestrogen and progesterone which are involved in the menstrual cycle and the development of female secondary sexual characteristics (e.g. breast development, growth of pubic hair).

• Testes – found in the male reproductive system next to the penis. The testes release testosterone which is involved in sperm production and secondary sexual characteristics in males (i.e. voice lowering, growth of facial hair and pubic hair).

Control of blood glucose concentration

Blood glucose concentration is one of the internal conditions that is kept constant with our bodies (homeostasis).

• If glucose is too high, the pancreas releases insulin. Insulin causes glucose to move from the bloodstream into target cell, reducing blood glucose.

• In the liver and muscle, glucose is stored as glycogen.

• If glucose is too low, the pancreas releases glucagon.

• Glucagon stimulates the breakdown of glycogen into glucose.

The action of insulin in reducing blood glucose levels and glucagon in increasing blood glucose are examples of negative feedback loops.

Diabetes is a condition in which blood glucose is not properly controlled.

• Type 1 diabetes is caused when the pancreas does not secrete enough insulin. As a result, liver and muscle cells do not take up glucose and convert it into glycogen for storage. Type 1 diabetes is treated with insulin injections.

• In type 2 diabetics, insulin is still secreted by the pancreas but the individual’s cells no longer respond to it. It is more common in people with obesity — it’s thought that a sugary diet and an insulin overload in the bloodstream can ‘desensitise’ cells to insulin. Type 2 diabetes can be treated with a carefully controlled diet and exercise programmes.

Hormones in human reproduction

During puberty, levels of reproductive hormones increase, resulting in the development of secondary sexual characteristics. Examples of secondary sex characteristics are basically the changes which occur during a person’s teenage years – things like breast development in females, growth of pubic hair and acne. In females, the main reproductive hormone is oestrogen. It is secreted by the ovary and its main role is to ensure that the lining of the uterus thickens properly so that a fertilised egg cell can be implanted where it will grow into an embryo. In males, the main reproductive hormone is testosterone which is produced in the testes. The role of testosterone is to stimulate sperm production.

Follicle stimulating hormone (FSH) is produced by the pituitary gland in the brain and causes the maturation of an egg in the ovary. As the egg matures, it releases oestrogen which stops FSH being produced so that only one egg matures per cycle. Oestrogen also causes thickening of the uterus lining and stimulates the pituitary gland to release luteinising hormone (LH), which causes release of the egg from the ovary. If the egg is fertilised, it will implant itself into the uterus lining, which is maintained by the hormone progesterone. If the egg is unfertilised, the uterus lining breaks down, resulting in menstruation.

Contraception

Contraception is any method which prevents fertilisation of an egg by sperm. It includes hormonal methods, such as the contraceptive pill, and non-hormonal methods like condoms.

Hormonal methods of contraception:

• Oral contraceptives – these contain hormones to inhibit FSH production so that no eggs mature

• Injection, implant or skin patch – these release progesterone slowly to inhibit the maturation and release of eggs

Hormonal methods can be extremely effective (99%) when used correctly, however oral contraceptives require that the person taking the pill remembers to take it on a daily basis. Forgetting to take the pill reduces its effectiveness. There are also side effects associated with the use of hormonal contraceptives, including mood swings and changes to blood pressure.

Non-hormonal methods of contraception:

• Barrier methods such as condoms and diaphragms – these prevent the sperm from reaching the egg

• Intrauterine device – these prevent the egg from implanting in the uterine lining

• Spermicidal agents – these kill or disable sperm

• Abstaining from intercourse when an egg may be in the oviduct

• Surgical methods of female or male sterilisation such as a vasectomy

Condoms are cheap and simple to use but there is a risk of tearing, which will increase the likelihood of pregnancy. Diaphragms are only effective if they are left in the body for several hours after sexual intercourse. Some people may have allergic reactions to spermicidal agents which means that this is an unsuitable method of contraception for some people. Abstinence isn’t always effective as it can be difficult to work out the exact time of ovulation during a woman’s menstrual cycle – if the timings are off then the chances of getting pregnant are greatly increased. Surgical methods are permanent and irreversible and involve surgery which carries the risk of infection.

Using hormones to treat infertility

Hormones are not only used to reduce the chances of pregnancy, but are used as a fertility treatment to increase the chances of fertilisation. This can be done with ‘fertility drugs’ or IVF treatment.

• Fertility drugs involve giving a woman FSH and LH to stimulate the maturation and release of multiple eggs from her ovaries. She may then become pregnant in the normal way.

• In Vitro Fertilisation (IVF) involves giving a women FSH and LH to stimulate the maturation and ovulation of multiple eggs. The eggs are collected from the woman and fertilised by sperm from her partner in a petri dish. The fertilised eggs will grow by mitosis and develop into embryos. When the eggs have grown into tiny balls of cells, one or two embryos will be inserted into the woman’s uterus.

Although fertility treatment may allow a couple to have a child who may otherwise be unable to conceive naturally, there are downsides:

• It is emotionally and physically stressful

• It is expensive

• It has a low success rate

• Sometimes 2-3 embryos are inserted into the uterus to increase the success rate, but this can increase the likelihood of twins or triplets. This is a risk to both the babies and the mother.

Negative feedback

Adrenaline is a hormone which is released by the adrenal glands and stimulates the ‘fight or flight’ response. If we encounter something scary, whether it’s a mugger or a spider, our body will release adrenaline which will prepare our bodies for action by increasing the delivery of oxygen and glucose to our cells. This increases the amount of respiration taking place, providing more energy for muscle contraction – either to run away or to fight the threat. Adrenaline has the following effects on the human body:

• Increases heart rate

• Vasodilation (blood vessel become wider)

• Pupils dilate

• Glycogen is broken down into glucose

• Blood flow is diverted away from the digestive system and directed to skeletal muscle

• Breathing rate increases

Thyroxine is a hormone which is secreted by the thyroid gland which increases metabolic rate (i.e. how fast respiration and other chemical reactions are happening). Levels of thyroxine are controlled by a negative feedback mechanism:

• When thyroxine levels in the blood fall too low, the hypothalamus is stimulated, causing it to release thyrotropin releasing hormone (TRH).

• TRH acts on the pituitary gland, causing it to release thyroid stimulating hormone (TSH).

• TSH stimulates the thyroid gland to secrete thyroxine, bringing thyroxine levels within the normal range.

• If thyroxine levels become too high, the release of TRH from the hypothalamus is inhibited, so less TSH is released from the pituitary gland. This causes levels of thyroxine to fall back to their normal range.

Nervous system vs hormonal system

Both systems allow us to respond to the environment and maintain constant internal conditions (homeostasis). But there are important differences between them:

1. The nervous system carries information in the form of electrical impulses (until a synapse is reached, at which point it relies on chemicals) whereas the hormonal system only uses chemical signals.

2. The nervous system transmits information along neurons whereas hormones travel in the bloodstream.

3. The effects of nerve impulses are much more short-lived compared to the effects of hormones.

4. The nervous system produces a faster response, whereas hormones act on target organs more slowly.

Plant hormones

Plants also use hormones to respond to their environment, allowing plants to grow in response to light (phototropism) and gravity (geotropism).

• Shoots grow towards light (positive phototropism) - this allows them to maximise the amount of light they absorb for photosynthesis

• Shoots grow against the force of gravity (negative geotropism) - this ensures they grow upwards, towards the light

• Roots grow away from light (negative phototropism) - ensuring that they grow deeper into the soil, anchoring the plant into the ground

• Roots grow in the direction of gravity (positive geotropism) - this also helps to anchor the plant into the ground.

• These responses are controlled by a hormone called auxin:

• Auxin accumulates in the shaded side of the plant, away from light, and causes cells to grow longer (cell elongation).

• The longer cells on the shaded side make the shoot heavier on this side, causing it to bend towards the light.

 Other hormones are involved in plant growth, including:

• Gibberellins – important for initiating seed germination

• Ethene – controls cell division and fruit ripening

Uses of plant hormones

Humans use plant hormones to control the growth of plants in gardening and agriculture.

• Ethene is used in the food industry to control the fruit ripening during storage and transport. It allows seasonal fruit and veg to be available all year round.

• Auxins are used as weed killers, rooting powders and for stimulating the growth of plant cells in tissue culture.

• Gibberellins can be used to promote flowering, end seed dormancy and increase fruit size.