Staying balanced in a changing world
Whether you’re in a snowstorm or a heat wave, your body temperature stays close to 37 °C. Your blood sugar stays fairly steady whether you just ate cake or skipped lunch. This stability is called homeostasis: the maintenance of a stable internal environment.
Homeostasis is essential because enzymes and cells only work well within narrow ranges.
The control loop
Every homeostatic system has the same basic parts:
| Part | Role | Temperature example |
|---|---|---|
| Stimulus | A change from normal | Body temperature rises |
| Receptor | Detects the change | Temperature sensors in skin and hypothalamus |
| Control centre | Compares to the set point and decides a response | Hypothalamus in the brain |
| Effector | Carries out the response | Sweat glands, blood vessels |
| Response | Counteracts the change | Body cools down |
Negative feedback
Most homeostasis uses negative feedback: the response reverses the original change. It works like a home thermostat.
Room too cold → thermostat detects it → furnace turns on → room warms → thermostat turns furnace off.
Body temperature
| If too hot | If too cold |
|---|---|
| Sweat glands release sweat; evaporation cools skin | Muscles shiver to make heat |
| Blood vessels near skin widen (vasodilation) to release heat | Blood vessels near skin narrow (vasoconstriction) to keep heat in |
| Behaviour: seek shade, remove clothing | Behaviour: put on a coat, curl up |
| Hair stands up (goosebumps) |
Blood glucose
The pancreas controls blood glucose using two hormones:
- When glucose is high (after a meal), the pancreas releases insulin. Insulin helps cells take in glucose and tells the liver to store it as glycogen. Blood glucose falls.
- When glucose is low, the pancreas releases glucagon. Glucagon tells the liver to break glycogen into glucose. Blood glucose rises.
Insulin and glucagon are antagonistic hormones: they have opposite effects.
Water balance
When you’re dehydrated, the hypothalamus detects concentrated blood. The pituitary gland releases antidiuretic hormone (ADH), which makes the kidneys reabsorb more water. You produce less, darker urine and feel thirsty.
Blood CO₂ and breathing
When CO₂ builds up during exercise, sensors in the brainstem and blood vessels detect it, and you breathe faster and deeper to remove it.
Positive feedback
Positive feedback amplifies a change instead of reversing it. It’s less common and usually drives a process to completion.
| Example | How it works |
|---|---|
| Childbirth | Contractions push the baby against the cervix, which triggers release of oxytocin, which causes stronger contractions, until birth |
| Blood clotting | Platelets at a wound release chemicals that attract more platelets, quickly forming a clot |
| Fever (above a dangerous level) | High temperature speeds metabolism, producing more heat; this can become dangerous |
When homeostasis fails
| Condition | What goes wrong |
|---|---|
| Type 1 diabetes | The immune system destroys insulin-producing cells; blood glucose stays high |
| Type 2 diabetes | Cells stop responding properly to insulin |
| Hypothermia | Body temperature drops below about 35 °C; heat loss exceeds heat production |
| Heat stroke | Body temperature rises above about 40 °C; cooling systems fail |
| Dehydration | Water loss exceeds intake; blood volume and pressure fall |
Homeostasis in other organisms
- Plants close their stomata to reduce water loss on hot, dry days.
- Desert kangaroo rats produce very concentrated urine to save water.
- Reptiles regulate temperature by behaviour, basking in the sun or hiding in shade.
Common mistakes
- Thinking "negative" means bad. Negative feedback is the main way the body stays healthy.
- Mixing up insulin and glucagon. Insulin lowers blood glucose; glucagon raises it.
- Thinking the set point never changes. During a fever, the body’s temperature set point is raised.
Try it yourself
- Draw a negative feedback loop for blood glucose after eating a large meal. Label stimulus, receptor, control centre, effector and response.
- Explain why positive feedback could be dangerous if it didn’t stop.
- Why do you get pale when you’re cold and flushed when you’re hot?
Quick recap
- Homeostasis keeps internal conditions stable around a set point.
- Feedback loops use receptors, a control centre and effectors.
- Negative feedback reverses changes and is the most common type.
- Insulin lowers blood glucose; glucagon raises it.
- Positive feedback amplifies change and drives processes like childbirth and clotting.