Adrenaline and the fight or flight response

Respiration, Muscles and the Internal Environment · The heart, ventilation and exercise · note 4 of 5

Spec 7.14

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Adrenaline and the fight or flight responseSpec 7.14

In short

Adrenaline is a hormone released from the adrenal glands in response to stress or danger. It binds to receptors on target cells and prepares the body for fight or flight by increasing heart rate and stroke volume, widening the airways, redirecting blood to muscles and stimulating the liver to release glucose.

In a threatening situation, the brain stimulates the adrenal glands (above the kidneys) to release adrenaline into the blood. It reaches its target cells, which have specific receptors on their cell surface membrane. This prepares the body for fight or flight.

Heart: increases heart rate and stroke volume, so cardiac output rises and more oxygen and glucose reach the muscles.
Lungs: widens the bronchioles and increases ventilation rate and depth, so more oxygen enters the blood.
Liver: stimulates the conversion of glycogen to glucose, raising blood glucose concentration as a substrate for respiration.
Blood vessels: dilates arterioles supplying skeletal muscles and constricts arterioles to the gut and skin, redirecting blood to muscles.
Eyes: dilates the pupils so more light enters.
Metabolic rate increases.

Exam tip:

Link each effect to its purpose: for example, 'increases heart rate, so more oxygen and glucose are delivered to muscles for aerobic respiration'.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

How does the heart beat without being told by the brain?

Cardiac muscle is myogenic, so it generates its own excitation. The sinoatrial node in the right atrium sets the pace and sends a wave across the atria. The atrioventricular node delays it, then the bundle of His and Purkyne fibres spread it through the ventricles. Nerves only change the rate.

How do you read an ECG?

The P wave is atrial depolarisation, the QRS complex is ventricular depolarisation and the T wave is ventricular repolarisation. The time between two R peaks gives the heart rate: 60 divided by that time in seconds. Irregular spacing or missing P waves can show abnormal rhythms such as atrial fibrillation.

What controls heart rate and breathing rate during exercise?

The medulla oblongata does. Chemoreceptors detect a rise in carbon dioxide and a fall in pH in the blood. The cardiovascular control centre increases heart rate through the sympathetic nerve to the sinoatrial node, and the ventilation centre speeds up and deepens breathing, to deliver oxygen and remove carbon dioxide.

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