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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Adrenaline is a hormone produced by the adrenal glands in response to fear, stress or excitement. It increases heart rate, breathing rate, blood glucose level and blood flow to muscles, preparing the body for rapid physical action: the fight-or-flight response.
Adrenaline is the fastest-acting hormone in the body. It links the nervous system (which triggers its release) to the hormonal system (which distributes it via the blood). Questions ask you to explain how each effect of adrenaline prepares the body for action.
What triggers adrenaline release
When the brain perceives a threat, danger, excitement or stress, it sends nerve impulses to the adrenal glands (located on top of each kidney). The adrenal glands respond by secreting adrenaline directly into the bloodstream.
Adrenaline is sometimes called the "emergency hormone" because its release prepares the body for immediate physical action: either fighting the threat or running away from it.
Effects of adrenaline on the body
Effect of adrenaline
How it prepares the body
Heart rate increases
More blood pumped per minute, delivering oxygen and glucose to muscles faster
Breathing rate and depth increase
More oxygen absorbed in the lungs and more CO₂ removed
Blood glucose increases
Liver converts glycogen to glucose, providing more fuel for respiration in muscles
Blood directed to muscles
Blood vessels to muscles dilate while those to the gut constrict, prioritising muscle function
Pupils dilate
More light enters the eyes, improving vision for detecting threats
Linking adrenaline to respiration and homeostasis
The effects of adrenaline all serve one purpose: to increase the rate of aerobic respiration in muscles, providing more energy for physical action. By increasing both oxygen delivery (heart rate + breathing rate) and glucose availability (glycogen breakdown), adrenaline ensures muscles have the substrates they need.
Adrenaline acts quickly but its effects are temporary. Once the perceived threat has passed, adrenaline levels fall and the body returns to its normal state. This is an example of how the body coordinates rapid short-term responses to maintain survival.
Try a focused question (4 marks)
Explain how adrenaline prepares the body for physical activity.
Show the answer and marking guidance
Adrenaline increases heart rate, so more oxygenated blood is pumped to the muscles per minute.
Breathing rate increases, so more oxygen enters the blood at the lungs.
The liver converts glycogen to glucose, increasing blood glucose to fuel respiration.
More blood is directed to muscles and away from non-essential organs, providing substrates for increased aerobic respiration.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying adrenaline is produced by the brain: it is produced by the adrenal glands.
Listing effects without explaining how each one prepares the body for action.
Confusing adrenaline with insulin: insulin lowers blood glucose; adrenaline raises it.
Describing adrenaline as a permanent effect: its effects are temporary.