(4) 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 blood glucose, heart rate (pulse rate) and breathing rate to increase the supply of oxygen and glucose for respiration in 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!

 

Reminder: Homeostasis the function of self-regulating processes by which biological systems (organisms) tend to maintain physical and chemical stability while adjusting to conditions that are optimal for survival i.e. counteracting unfavourable conditions.

These process can involve hormones secreted from the glands of the endocrine system.

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Key points Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Key points concerning adrenaline

Hormonal Control: Adrenaline and the Fight or Flight Response

What is Adrenaline?

• Adrenaline (also called epinephrine) is a hormone secreted by the adrenal glands, located just above the kidneys.

• It is released in response to stressful or threatening situations, triggering the fight or flight response.

• Adrenaline is part of the endocrine system and is not regulated by negative feedback.

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When is Adrenaline Released?

• The hypothalamus in the brain detects a threat and activates the sympathetic nervous system.

• This sends signals to the adrenal medulla (inner part of the adrenal gland), which releases adrenaline into the bloodstream.

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Effects of Adrenaline on the Body

Adrenaline prepares the body for rapid action by triggering several physiological changes:

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Why is This Important in Biology?

• Demonstrates how the nervous and endocrine systems work together.

• Shows how the body maintains homeostasis during acute stress.

• Highlights the role of hormonal coordination in survival.

• Provides a real-world example of rapid physiological adaptation.

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• Summary of the action of adrenaline

• Adrenaline is released in response to fear or stress.

• It is secreted by the adrenal medulla.

• It causes increased heart rate, breathing rate, and blood glucose.

• It prepares the body for fight or flight.

• It is not controlled by negative feedback.

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Summary of learning objectives and key words or phrases

Be able to describe and understand the function of the hormone adrenaline produced in the adrenal gland to enable the body's to response to a stress situation (fight or flight), and relate this function to the endocrine system.

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