Stress Hormones: Adrenaline, Cortisol and Feedback

Acute stress: the quick alarm

An acute stress is a short, sudden challenge: a barking dog, a near-miss on the road, a surprise question. The brain's alarm centre (the amygdala) calls the hypothalamus. The body prepares to fight or run: fight-or-flight. Breathing quickens, pupils widen, digestion pauses, muscles get ready. When the danger goes, the body returns to calm.

The autonomic nervous system and the heart

The autonomic nervous system runs the organs without thinking. Its sympathetic part speeds you up (fast heart, wide airways); its parasympathetic part calms you down (slow heart, digestion). Under stress the sympathetic nerves reach the adrenal glands and the heart. Cardiac output = heart rate × stroke volume (blood pumped per beat). If both rise, more blood goes to muscles and the brain.

Adrenaline and cortisol

Adrenaline (from the inner part of the adrenal glands) acts in seconds: faster heart, more blood to muscles, more glucose released from the liver. Cortisol (from the outer part) acts in minutes to hours: it keeps blood sugar high, helps the body use energy and damps swelling and the immune system. Both are hormones, chemical messengers that travel in blood.

The adrenaline and cortisol loop

The cortisol chain: hypothalamus releases CRH, the pituitary releases ACTH, the adrenal cortex releases cortisol. Adrenaline follows a faster nerve route. The two together form a loop: nerves start adrenaline at once; the hormone chain keeps the body ready for longer; both stop when the threat ends.

Feedback control of stress

Cortisol in the blood is sensed by the hypothalamus, the pituitary and the hippocampus. When it is high enough, they reduce CRH and ACTH, so less cortisol is made. This is negative feedback: the product switches off its own making, like a thermostat. It keeps the level steady and ends the stress response.

Chronic stress and the hippocampus

Chronic stress lasts for weeks or months: heavy study pressure with no rest, bullying, money worries. The feedback brake weakens and cortisol stays high. The hippocampus (memory centre) can shrink, so learning and memory suffer. Other effects: high blood pressure, higher blood sugar, poor sleep, weaker defence against infections, anxiety and low mood.

Resilience, disease and stress management

Resilience is the ability to recover from stress. It grows with sleep, exercise, friends, healthy food, hobbies and knowing when to ask for help. Slow breathing, stretching, short breaks and talking to someone trusted all calm the sympathetic system and let the brake work. Seek a doctor or counsellor if stress is long or heavy. This lesson is for learning and is not medical advice.

Try it

In the 3D: go to the last step and slide the days of stress from 0 to 30. Watch the brake, the cortisol bar and the purple memory centre.

At home: measure your pulse at rest for 30 seconds and double it. Jump on the spot for one minute and measure again. Then breathe slowly for two minutes and measure a third time. Which hormone-driven state does each represent?

Key formulas and definitions

• Cardiac output = heart rate × stroke volume
• Heart rate (per minute) = pulse in 30 s × 2
• Stress chain: stress → hypothalamus → CRH → pituitary → ACTH → adrenal cortex → cortisol
• Negative feedback: high cortisol → less CRH and ACTH → less cortisol

Worked examples

Common mistakes

• Thinking stress is always bad. Short, acute stress helps us react and perform.
• Mixing the order: adrenaline is fast (seconds), cortisol is slower (minutes to hours).
• Confusing positive and negative feedback. In stress, cortisol shuts down its own production: that is negative feedback.
• Believing chronic stress is just 'in the mind'. It changes hormones and body organs.