The Endocrine System — PT Direct

Endocrine system new terms

When it comes to the endocrine system there are some commonly used terms that may be new to you.  To help you some of these new terms and their definitions are shown in the following table.

What is the endocrine system?

The endocrine system is a control system of the human body much like the nervous system.  The endocrine system produces chemical messages in the form of hormones, whereas the nervous system produces electrical messages.

The endocrine system is made up of lots of specialised endocrine glands that secrete hormones into the bloodstream.

Some of the major endocrine glands are shown on this diagram. 

What does the endocrine system do?

The hormones that are secreted by the endocrine glands are chemical messengers which are carried by the bloodstream to other tissues or organs in the body.  The messages they deliver tell these tissues or organs to either increase or decrease their activity.

Hormones act only on target tissues or organs that have the appropriate receptor sites for that given hormone.  In this way hormonal messages are delivered to, and act on, only the areas of the body they are intended for.

The endocrine system is linked to the nervous system by effects of the hypothalamus on the pituitary gland, as seen in the adjacent image. 

The pituitary gland is known as the ‘master gland’ because its secretions control the activity of other endocrine glands.  The activity of the pituitary gland is however controlled by the hypothalamus which as well as being an endocrine gland, is also part of the nervous system.

Along with the nervous system, the endocrine system coordinates the body’s functions to maintain homeostasis during rest and exercise.  The nervous and endocrine systems also work together to initiate and control movement, and all the physiological processes movement involves. 

Where the nervous system acts quickly (virtually instantly) delivering messages by nerve impulses, the endocrine system has a slower but longer lasting response which compliments the nervous system.  The endocrine system regulates growth, development and reproduction and augments the body’s capacity for handling physical and psychological stress.

Why is the endocrine system important?

The endocrine system and the hormones it releases influence almost every cell, organ and function of our body. In general these hormones are in charge of body processes that occur slowly such as mood regulation, growth and development, tissue function, metabolism, sexual function and reproductive processes.  Other body processes that require a faster response such as breathing and body movements are controlled by the nervous system.

When training clients you are essentially trying to elicit the best possible hormone response in order to help them achieve their goals.  For example if a client wants to increase muscle mass it is important that they undertake the correct type of training to release more growth hormone.  This will help them repair and grow more muscle after the training session.

It’s therefore critically important to have a good understanding of the endocrine system and some of the key hormones, as this will help to ensure your training doesn't over stress your clients endocrine system and instead provides the ideal training stimulus to achieve the desired hormonal response.

The endocrine system and fitness

The endocrine system and fitness go hand in hand.  Whether you are eating your last big meal before running a marathon, playing a game of football or recovering after a training session in the gym the endocrine system is constantly working.

An accompanying page in this folder summarises the major endocrine glands, the hormones they release, the tissues and organs they target and the effects they have with exercise.  We will briefly cover some of these hormones now as they relate to exercise.

Key hormones and the regulation of hormone activity

During exercise several hormones (adrenaline, nor adrenaline, growth hormone and cortisol) function together to mobilise fuel for the production of ATP (energy) for the exercise

These hormones affect the cells on three primary target tissues during exercise, these are:

1.      Fat

2.      Liver

3.      Skeletal muscle

When they bind to receptors on fat cells, fat storage is inhibited and fat mobilisation is enhanced for energy.  When they bind to receptors on the liver, glycogen is broken down into glucose for readily available energy.  When they bind to receptors on skeletal muscle, stored glycogen is broken down into glucose and the uptake and utilisation of fatty acids for energy is increased.

Hormones – glucagon and insulin

Glucagon and insulin are both secreted by the pancreas and act antagonistically to each other in order to maintain blood glucose levels, as shown in the adjacent diagram.

With exercise there is a greater demand from muscle tissue for blood glucose for fuel, causing blood glucose levels to drop.  Glucagon levels consequently increase with exercise and insulin levels are simultaneously suppressed.

This is vital as it ensures that blood glucose levels do not drop too low as the brain (neural tissue) can only use glucose to produce energy.  If there wasn’t any glucose in the blood the brain would be starved of fuel…which in case you’re wondering is definitely NOT a good thing – in fact it can be downright fatal!

Hormones  – adrenaline and nor adrenaline

Adrenaline and nor adrenaline also help to enhance cardiac (heart) function by increasing heart rate, constricting blood vessels and increasing blood pressure.  This in turn helps to distribute blood flow to active tissues, providing these tissues with the energy and oxygen they need. 

How is activity at the ‘target tissues’ regulated?

As we previously mentioned hormones are designed to affect specific target tissues that have the correct receptors.  Think of the receptors as a lock and the hormone as a key, if the key fits the lock then the hormone can affect the target tissue.

However there are three things that influence the activity of hormones on the target tissue, these are:

1.      Number of receptors:  The number of receptors for a specific hormone can be altered to meet the demands placed upon the body over time.  Up regulation means more receptors have developed.  Down regulation means there are less receptors.  Up and down regulation changes the cells sensitivity to hormones.

This is one reason exercise is beneficial for people with type II diabetes or at risk of developing type II diabetes. 

This is because it can help to increase the sensitivity of the tissues receptor cells to the hormone insulin, this in turn helps to control blood glucose (sugar) levels and even increase the total number of receptor cells on the target tissue.

Up regulation of receptor cells can also be seen when we look at growth hormone and the effect of exercise.  Completing a well structured resistance training programme aimed at muscle growth stimulates an increase in the number of receptor cells within muscle tissue.

2.      Chemical bond:  The chemical bonding between a hormone and its receptor can be weak or strong.  A strong bond will result in a greater reaction in the target tissues than a weak bond.

For example, with exercise greater demand is placed on the body for fuel for the working muscles.  Glucagon is one hormone that ensures that there is a regular supply of glucose in the blood for use by working muscles.  With exercise over time the bonds between glucagon and its receptors strengthen to meet the increased demands for glucose from the working muscle.

Completing a well structured resistance training programme aimed at muscle growth will also increase the sensitivity of the receptor cells to growth hormone and result in a stronger bond.

3.      Blood hormone levels:  Lastly the level of the hormone in the blood at any one moment will affect the response of the tissues.

The greater the stress that is put on the body the greater the hormonal response will be in order to meet the demands of the body and to maintain homeostasis. 

We see this with growth hormone in the adjacent diagram, which is released by the pituitary gland under the control of the hypothalamus.

Growth hormone acts on most tissues in the body, with two of its key roles being to stimulate cell growth and mobilise fat for energy.

Intense resistance training (heavy weights) stimulates the release of greater quantities of growth hormone than less intense forms of exercise.  Consequently intense resistance training has a greater response (growth in skeletal muscle) than other less intense forms of exercise.  This is due to the larger quantities of growth hormone that are released into the bloodstream.

How is the level of hormones in the blood regulated?

Hormones are released into the blood in brief spurts.  Having the correct amount of hormones in the blood when needed is vital in order for them to effectively carryout their functions.  Too much or not enough of certain hormones can put stress on systems and have negative effects like weight gain or depression.

For this reason it is important that the amount of hormones circulating in the blood at any given time is regulated and controlled, this is achieved through the following three methods:

1.      Neural stimulation:  Neural stimulation of hormones occurs when a stress is present.  The hypothalamus and sympathetic nervous system activate the adrenalin gland to release adrenalin and noradrenalin. 

This in turn increases heart rate and metabolic rate to allow enough blood sugars to be circulating to provide adequate fuelling of muscle and nerve tissue.  An example of this response, which is rapid and can be significant, is the ‘fight or flight’ reaction.

2.      Hormonal secretions (hormones that affect other hormones):  Hormones also influence the release of other hormones.  The hypothalamus makes and secretes hormones that act on the pituitary gland to promote or inhibit its secretion of other hormones. 

Regulation of hormone release in this form is usually achieved through a ‘negative feedback loop’.  An example would be that high levels of hormone ‘A’ causes hormone ‘B’ to be secreted.  Hormone ‘B’ acts on the gland that secretes hormone ‘A’, inhibiting further secretion of ‘A’.  Once hormone ‘B’ levels begin to drop the gland begins to secrete ‘A’ again. 

3.      Blood conditions (what is in the blood at any time):  Blood conditions refers to the levels of ions, nutrients, bile and other body compounds circulating in the blood.    

For example, high blood glucose levels signal the pancreas to release the hormone insulin.  The higher the levels of blood sugar the greater the amount of insulin released by the pancreas.  Insulin helps glucose to enter muscle and liver cells, thereby lowering blood glucose levels and removing the stimulus for further insulin release from the pancreas.

How negative feedback loops work – a closer look at the hormone cortisol

Out of the three hormonal control systems previously mentioned, the most predominant is the negative feedback loop (hormones that affect hormones).  It works in a similar way to how a thermostat in a heat pump works.

The thermostat detects the temperature is too low and turns the hot air on.  As the hot air raises the temperature to the set optimal level the thermostat turns the heat pump off.  

In the endocrine system, receptors in the body register that the amount of hormone or compounds in the blood are not optimal and cause a release or inhibition of hormones to manage the situation.

A good example of this concerns the hormone cortisol.  Cortisol is known as the ‘stress’ hormone as it is secreted in higher levels when higher levels of stress (physical and psychological) are present. 

Cortisol works on target tissues to increase blood glucose levels.  It does this by aiding the metabolism of carbohydrate reserves from the liver as well as breaking down fat and protein to help provide quick bursts of energy.

As protein is primarily used in the body to build and repair tissue, cortisol is known as a ‘catabolic’ or breakdown hormone, as it interferes with the ‘anabolic’ or building functions of protein.

Cortisol is very much a ‘fight or flight’ hormone, akin to the sympathetic division of the nervous system.  Small increases have positive effects such as providing a quick burst of energy and lowering sensitivity to pain via an anti-inflammatory action.  It is important that the levels of cortisol are regulated as prolonged elevated levels have negative effects. 

We see how cortisol is regulated via a negative feedback loop in the adjacent diagram. 

A physical or psychological stressor causes the hypothalamus to secrete corticotrophin releasing hormone (CRH) which causes the anterior pituitary gland to secrete adrenocorticotropin (ACTH) which in turn tells the adrenal glands to release cortisol. 

As well as acting on its target tissues (liver, fat, muscle etc) cortisol also sends ‘negative feedback’ to the hypothalamus and anterior pituitary telling them to stop secreting CRH and ACTH, and by virtue of this limiting the further release of cortisol.

Unfortunately this regulatory response is not as effective if the stressors that caused cortisol to be released in the first place remain. 

Intense exercise, financial stress, business stress, relationship stress, work stress etc are increasingly prevalent in today’s society.  If the stressors remain then so do the stimulants for elevated cortisol levels, irrespective of the negative feedback loop.

If cortisol remains elevated for a prolonged period of time the positive effects it can have become negative.  As the body stays in a catabolic ‘breakdown’ state there is a loss in muscle tissue, immunity is lowered, people become more vulnerable to illness, there is an increase in the storage of abdominal fat and imbalances in blood sugar levels develop. 

It is important to understand that cortisol is not responsible for this; rather the stressors that have caused cortisol to remain elevated are responsible.  As a trainer you need to be able to recognise when clients are highly stressed and make sure you don’t add to their stress levels by destroying them in the gym with long hard workouts.  The stressors must be removed or minimised through various means of relaxation if positive results are to occur.