---

(a) What are hormones?

Hormones are produced in the endocrine gland system and are transported by the blood to their target cells, tissues or organs

Many process within the body are coordinated and controlled by chemical substances called hormones.

You can think of hormones as chemical messengers sent around the bloodstream.

Hormones are often quite large organic molecules.

Hormones and the nervous system send information around the body.

Hormones, being directly released into the blood, are quite rapidly carried to all parts of the body BUT only affect the function of particular cells, tissue or organs - the 'targets''.

Hormones, in acting as 'chemical messages', trigger particular biochemical reactions in various types of tissue and organs.

Hormones control functions in cells, tissue and organs that need constant adjustment and their effect is relatively long-lasting compared to eg the nervous responses of a reflex arc.

The activated cells are called 'target cells' and have a chemical receptor that responds to the hormone - the hormones work on effectors.

You should appreciate that the nervous system and hormones enable us to respond to external changes and also help us to control conditions inside our bodies.

BUT, unlike the nervous system, the hormone response times are slower e.g. minutes or hours, an exception is adrenaline , which is the fastest acting hormone response.

Know that hormones are used in some forms of contraception and in fertility treatments.

---

TOP OF PAGE and sub-index

---

(b) Examples of hormones and which gland or organ produces them

Hormones are produced in, and secreted by, various glands called endocrine glands - hence the overall description - the endocrine system - sources and examples of which are described below.

Endocrine glands secrete hormones directly into the bloodstream.

Pituitary gland

The pituitary is a small gland at the base of the brain

The pituitary gland produces many hormones that regulate conditions in the body and growth hormone is important for the development of the body.

Some hormones have a direct effect on the body, but others have an indirect effect by causing other glands to release hormones.

Which is why the pituitary gland is sometimes referred to as the master gland because these hormones act on other glands causing them to release other hormones to bring about changes somewhere in the body.

The pituitary gland produces the hormones FSH and LH which are important control chemicals in the female menstrual cycle. These act on the ovaries and testes to release reproductive hormones which control the release of eggs from the ovaries and the birth of a baby.

FSH = Follicle stimulating hormone   and   LH = Luteinizing hormone

The pituitary gland secretes the hormone TSH which acts on the thyroid gland - which in turn secretes thyroxine hormone (see next section).

TSH = Thyroid stimulating hormone

It also produces the hormone ACTH which acts on the adrenal gland to secrete the hormone adrenal hormone.

ACTH = Adrenocorticotropic hormone

The pituitary produces the growth hormone STH, which acts on the whole body - if very deficient in STH for a long time you may be of short stature and dwarfism, and, if too much of STH for a long time you have excessive growth in stature, organ enlargement and suffer from functional disorders such as diabetes and heart disease.

STH = somatotropin or somatotropic hormone

ADH is a hormone that is produced in a part of the brain called the hypothalamus. It is then stored and released from the pituitary gland. ADH acts on the kidneys to control the amount of water excreted in the urine.

ADH = Anti-diuretic hormone

See Homeostasis - osmoregulation, ADH, water control and kidney function

Thyroid gland

The thyroid gland is attached to the trachea. The thyroid gland produces thyroxine which takes part in regulating functions such as the rate of chemical reactions in metabolism, heart rate and temperature control - its production is triggered by the hormone TSH produced in the pituitary gland.

See also notes on thyroxine and

Homeostasis - thermoregulation, control of temperature

Adrenal gland

The adrenal gland are the top of the kidneys. The adrenal gland produces adrenaline which is used by the body to prepare for 'fight or flight' e.g. helps your body for action if you suffer trauma or find yourself in danger - its production is triggered by the ACTH hormone from the pituitary gland. (see notes on adrenaline)

The pancreas

The pancreas is situated below the stomach.

The pancreas produces insulin which regulates the glucose concentration ('level') in blood.

See Homeostasis - control of blood sugar level - insulin and diabetes

The ovaries (female only)

The ovaries produce the sex hormone oestrogen which is part of the chemistry of the menstrual cycle.

Oestrogen gives girls their female features such as breasts, soft skin, feminine voice and prepares the womb for a baby.

The testes (male only)

The testes produce testosterone, a hormone that controls puberty and sperm production in males.

Testosterone is a sex hormone that gives boys their male features such as deeper voices and more body hair than females.

---

TOP OF PAGE and sub-index

---

(c) A comparison of the nervous system and the endocrine hormone system

Hormones effectively act as 'chemical messages' to trigger particular biochemical reactions and their effects are slower than nervous system.

The effect of hormones is more general around the body, but tend to affect particular cells in particular organs, and relatively long-lasting effect compared to eg the fast but short-term nervous impulses and responses of a reflex arc.

Generally speaking, if your body's response to a situation is relatively long lasting (e.g. minutes or hours) its probably a function of the hormone system.

Some hormones like adrenaline, can act quite quickly - see notes further down.

Nerves: Compared to the hormone system of response and control in the body, the nervous system, using nerve signals which are electrical in nature (not chemical).

The nervous system of neurones acts very fast e.g. a short burst of a nerve impulse for a short time, acting from one precise area to another in the body.

Generally speaking, if your body's response is fast, its probably a nervous reaction.

There are situations when information needs to be passed to your effectors quickly!

Examples of when nerve signal information has to be passed to effectors quickly include nerve signals from your retina, pain receptors, taste buds warning of danger etc. must be processed in microseconds, NOT minutes! too late!

Hormones act too slowly to be of use in most dangerous 'split second' decision making situations.

Hormone levels and negative feedback

Your body can control the level of hormones in the blood using a negative feedback system.

If the body detects that a level of a substance X is above or below the normal level it triggers a response to bring the level of substance X back to normal again.

A good example is the way thyroxine regulates metabolism (see thyroxine notes and graph below)

See also examples of homeostasis

Homeostasis - introduction to how it functions (negative feedback systems explained)

Homeostasis - control of blood sugar level - insulin and diabetes  gcse biology revision notes

Homeostasis - osmoregulation, ADH, water control, urea and ion concentrations and kidney function

Homeostasis - thermoregulation, control of temperature  gcse biology revision notes

---

TOP OF PAGE and sub-index

---

(d) The function of the hormone adrenaline

When you suddenly feel in danger or get a shock (physical or mental) your adrenal gland quite rapidly releases the hormone adrenaline into your bloodstream and distributed all around your body.

The adrenal glands are found just above the kidneys.

Adrenaline causes, what is often described as, the 'fight or fight' response - in other words your body is quite rapidly (by hormonal standards) being prepared to deal with a threat of some kind.

This happens when your brain detects fear or stress (dangerous situation, confrontation etc.) and immediately sends nerve impulses to the adrenal glands which then secrete the hormone adrenaline into the bloodstream to prepare your body for action!

The initial stimulation might be visual, physical or mental.

Note the interaction between the nervous system (electrical impulses in nerve fibres - neurones) and the endocrine system (secretion of hormone molecules into the bloodstream).

There are nerve connections between the brain and adrenal gland - a part of the adrenal gland called the adrenal medulla responds to the nerve signal from the brain (CNS) by releasing the hormone adrenaline.

The secreted adrenaline is carried round in the blood and acts on various parts of the body.

The effects of adrenaline on the body are described below,

The surge in adrenaline levels triggers an increase in heart rate and breathing rate to increase the supply of oxygen and glucose to the cells of your brain and muscles.

The increase in respiration releases more thermal energy and your body temperature rises - but, if it becomes too high, the thermoregulatory centre in the brain detects this and the adrenaline secretion is blocked.

Note that the body's volume of blood is fairly constant, so heart rate must increases to pump more blood around the body at a greater rate to carry extra glucose and oxygen to the muscle cells.

The adrenaline molecules do this by binding to specific receptors in the heart causing the heart muscles to contract more frequently and more forcefully - this increases your heart rate and blood pressure, hence more glucose and oxygen to your cells through your bloodstream e.g. it gives the cells of the muscle tissue extra energy to contract and prepare to fight or flee!

Adrenaline also binds to receptors in the liver causing the cells to increase in the rate of breakdown of glycogen (chemical potential energy store) to increase the level of glucose in the bloodstream for respiration - particularly muscle cells (in limbs or heart).

To increase the rate of respiration you also need more glucose, so the hormone adrenaline performs two functions to increase energy output.

Note that the metabolism of glucose is controlled by three hormones, here its adrenaline acting on the liver, but there is also the action of insulin and glucagon in maintaining the balanced level of glucose in the blood.

Footnote - above is not quite the full "fight or flight" story - another hormone comes into play too!

When the brain responds to the initial stimulus and triggers the release of adrenaline, this hormone from the adrenal gland, cannot alone do everything required in a 'fight or flight' situation.

Simultaneously, the brain also signals the pituitary gland  to release a hormone (name ?) that acts on a different part of the adrenal gland to release a 2nd hormone called cortisol, and it this steroid hormone that sustains our response to danger - most cells in the body have cortisol receptors.

This is another example of several hormones jointly controlling a situation.

Also note that it is the hypothalamus links the nervous and endocrine systems by way of the pituitary gland - nervous responses working with hormone responses to keep us alive!

---

TOP OF PAGE and sub-index

---

(e) The function of the hormone thyroxine

Thyroxine is a hormone made from iodine and amino acids, it is produced in, and released (secreted) by, the thyroid gland in the neck.

Thyroxine has an important role in regulating the basal metabolic rate, the basic rate (speed) at which the chemical reactions of your body occur while your body is at rest.

Thyroxine increases the rate of metabolism of all the body's cells.

 e.g. increases the rate of respiration, powering the cell's chemistry and releasing thermal energy

Thyroxine is also important for many other biochemical processes including facilitating protein synthesis - essential for growth and development.

Problems with an underactive thyroid gland - symptoms of thyroxine deficiency

Tiredness, sluggishness, increase in weight, slower heart rate,

If a child has too little thyroxine it leads to slower growth and mental development.

This potentially harmful situation begins in the uterus, continues in the embryo, through infancy and into childhood, if there is insufficient thyroxine

A negative feedback system maintains the thyroxine concentration in the blood at the correct level.

So, how does the negative feedback system regulate the level of thyroxine in the blood?

Both the pituitary gland and hypothalamus (a small region at the base of the brain) control the thyroid and it is the hypothalamus, using TRH (thyrotropin releasing hormone), that alerts the pituitary gland to produce TSH (thyroid stimulating hormone).

Please note, from now on I'll just use the abbreviations TRH and TSH.

Thyroxine is produced in the thyroid gland, in response to the actions of two principal hormones:

The hormone TRH (from the hypothalamus), stimulates the production of TSH which is made and secreted from the pituitary gland into the bloodstream.

In turn, the production of TSH stimulates the thyroid gland to make more thyroxine.

TSH binds to receptors located on the cells of the thyroid gland to stimulate production of thyroxine.

We now put these two hormonal actions, 'forward and reverse' into the negative feedback system.

I've also added a graph to go with the text below.

If your body detects that the level of thyroxine has risen above 'normal', the hypothalamus stops releasing TRH.

This tells the pituitary gland to stop producing TSH (blocks secretion) inhibiting the production of thyroxine in the thyroid.

In reducing the amount of thyroxine secreted from the pituitary gland, the thyroxine level falls down to normal (1st half of graph below) and your metabolic rate is reduced to 'normal' i.e. becomes stabilised again.

Apparently, a higher than normal thyroxine level also reduces the secretion of TSH from the pituitary gland (i.e. without the intervention of the level of TRH from the hypothalamus).

If your body detects the level of thyroxine has fallen below 'normal', the hypothalamus is stimulated to release TRH.

The release of TRH stimulates the pituitary gland to release TSH.

The TSH stimulates the thyroid gland to produce more thyroxine, whose level rises back to normal (2nd half of graph) and your metabolic rate increases to 'normal' i.e. becomes stabilised again.

Note:

If the body temperature falls, the body produces more thyroxine to increase the rate of respiration and release more thermal energy.

But, since the increase in respiration releases more thermal energy and your body temperature rises, if it becomes too high, the thermoregulatory centre in the brain detects this and the adrenaline secretion is blocked.

The negative feedback system is illustrated in the graph below.

General comment on the graph and negative feedback systems

Using a negative feedback system, your body controls the levels of hormones, and other substances in the blood.

When your body detects that the level of a substance X is too high above the 'normal' level, or too low below the 'normal' level, it triggers a response to bring the level of substance X back up/down to its normal level.

Thyroid gland problems

e.g. if you have an underactive thyroid gland, it can cause your body to gain unnecessary weight.

This is because too little thyroxine is produced and your metabolic rate falls.

As a result, less of the glucose from your food intake is used up in respiration, so the excess glucose is converted to, and stored as, fat.

Fortunately, the remedy, in most cases, is to take thyroxine tablets every day.

---

TOP OF PAGE and sub-index

---

See also Hormone systems - menstrual cycle

and for plants see Hormone control of plant growth and uses of plant hormones  gcse biology revision notes

General HUMAN BIOLOGY revision notes

Introduction to the organisation of cells => tissues => organs => organ systems (e.g. in humans)

Examples of surfaces for the exchange of substances in animal organisms   gcse biology revision notes

See also Enzymes - section on digestion and synthesis  gcse biology revision notes

The human circulatory system - heart, lungs, blood, blood vessels, causes/treatment of cardiovascular disease

Homeostasis - introduction to how it functions (negative feedback systems explained)  gcse biology revision notes

Homeostasis - control of blood sugar level - insulin and diabetes  gcse biology revision notes

Homeostasis - osmoregulation, ADH, water control, urea and ion concentrations and kidney function, dialysis

Homeostasis - thermoregulation, control of temperature  gcse biology revision notes

The brain - what the different parts do and the dangers if damaged gcse biology revision notes

An introduction to the nervous system including the reflex arc  gcse biology revision notes

Hormone systems - Introduction to the endocrine system - adrenaline & thyroxine hormones  gcse biology revision

Hormone systems - menstrual cycle, contraception, fertility treatments  gcse biology revision notes

Respiration - aerobic and anaerobic in plants and animals.  gcse biology revision notes

Keeping healthy - communicable diseases - pathogen infections   gcse biology revision notes

Keeping healthy - non-communicable diseases - risk factors for e.g. cancers   gcse biology revision notes

Keeping healthy - diet and exercise  gcse biology revision notes

Keeping healthy - defence against pathogens, infectious diseases, vaccination, drugs, monoclonal antibodies

See also Culturing microorganisms like bacteria - testing antibiotics/antiseptics  gcse biology revision

Food tests for reducing sugars, starch, proteins and lipids  gcse biology revision notes

The eye - structure and function - correction of vision defects  gcse biology revision notes

Optics - lens types (convex, concave, uses), experiments, ray diagrams, correction of eye defects (gcse physics)