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This lesson covers two important hormones in the Edexcel GCSE Biology specification: adrenaline and thyroxine. You will learn how adrenaline prepares the body for a fight or flight response, and how thyroxine controls your metabolic rate through a classic example of negative feedback.
Adrenaline is a hormone produced by the adrenal glands, which are located on top of each kidney. Adrenaline is released in response to fear, stress, or excitement — situations where the body needs to react quickly.
Adrenaline is released when the brain perceives a threat or a stressful situation. This triggers the fight or flight response — a set of rapid physiological changes that prepare the body to either confront the danger or escape from it.
Examples of situations that trigger adrenaline release:
Adrenaline causes a wide range of changes that prepare the body for intense physical activity:
| Effect | Purpose |
|---|---|
| Heart rate increases | Pumps blood faster to deliver more oxygen and glucose to muscles |
| Breathing rate increases | Takes in more oxygen for aerobic respiration in muscles |
| Blood vessels to muscles dilate | More blood (carrying oxygen and glucose) reaches the muscles |
| Blood vessels to skin and digestive system constrict | Diverts blood away from non-essential organs towards muscles |
| Pupils dilate | Allows more light in, improving vision for detecting threats |
| Glycogen in liver is converted to glucose | Releases stored glucose into the blood, providing extra energy for muscles |
| Mental alertness increases | Brain becomes more focused on the perceived threat |
| Sweating increases | Prepares the body for cooling during physical exertion |
Exam Tip: A common 6-mark question asks you to describe the effects of adrenaline. Structure your answer clearly: state the effect, then explain why it helps the body deal with the threat. For example: "Adrenaline increases heart rate so that blood is pumped faster, delivering more oxygen and glucose to the muscles for respiration."
One key effect of adrenaline is its action on the liver:
This ensures the body has a rapid supply of energy during the fight or flight response.
Thyroxine is a hormone produced by the thyroid gland, which is located in the neck, wrapped around the front of the trachea (windpipe).
Thyroxine controls the basal metabolic rate — the speed at which chemical reactions occur in the body's cells. These reactions include respiration, protein synthesis, and growth.
| Function | Detail |
|---|---|
| Controls metabolic rate | Determines how fast cells carry out chemical reactions |
| Important for growth | Necessary for normal growth and development, especially in children |
| Regulates energy release | Influences how quickly the body converts food into energy |
| Affects body temperature | Higher metabolic rate generates more heat |
If thyroxine levels are too high, the metabolic rate increases — the person may lose weight, feel hot, and have a rapid heart rate.
If thyroxine levels are too low, the metabolic rate decreases — the person may gain weight, feel cold, and feel tired.
The level of thyroxine in the blood is controlled by a negative feedback mechanism involving the pituitary gland and the thyroid gland.
Negative feedback is a process in which a change in a condition triggers a response that reverses the change, bringing the condition back to its normal level.
Here is the step-by-step process:
When thyroxine levels are too high:
When thyroxine levels are too low:
Exam Tip: Negative feedback always works in a loop. When drawing or describing it, make sure you show the cycle: thyroxine level changes → pituitary detects the change → TSH production adjusts → thyroid responds → thyroxine level returns to normal. The key word is reverses — the response always opposes the original change.
graph TD
A["Normal thyroxine level"] --> B["Thyroxine rises above normal"]
B --> C["Pituitary gland detects increase"]
C --> D["Pituitary releases LESS TSH"]
D --> E["Thyroid produces LESS thyroxine"]
E --> A
A --> F["Thyroxine falls below normal"]
F --> G["Pituitary gland detects decrease"]
G --> H["Pituitary releases MORE TSH"]
H --> I["Thyroid produces MORE thyroxine"]
I --> A
style A fill:#2ecc71,color:#fff
style B fill:#e74c3c,color:#fff
style F fill:#3498db,color:#fff
style E fill:#27ae60,color:#fff
style I fill:#27ae60,color:#fff
The same loop works in reverse when thyroxine drops below normal.
| Feature | Adrenaline | Thyroxine |
|---|---|---|
| Gland | Adrenal glands (on top of kidneys) | Thyroid gland (in the neck) |
| Trigger | Stress, fear, excitement | Low thyroxine detected by pituitary |
| Speed of action | Very fast (within seconds) | Slower (gradual changes over time) |
| Duration | Short-lived (minutes) | Long-lasting (continuous regulation) |
| Main effect | Prepares body for fight or flight | Controls basal metabolic rate |
| Controlled by negative feedback? | No — released in response to stress | Yes — regulated by TSH from the pituitary |
Exam Tip: A common exam question asks you to explain why adrenaline is not controlled by negative feedback. The answer is that adrenaline is released in response to an external stimulus (stress or danger) rather than being maintained at a set level. Once the stress passes, the adrenal glands simply stop releasing adrenaline.
Negative feedback is essential for homeostasis — maintaining a stable internal environment. Without it, hormone levels could spiral out of control:
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