Coaching Centre
Homeostasis and feedback
Why the body keeps its internal conditions steady, negative feedback loops, how the hypothalamus controls body temperature, fever, heat stroke and hypothermia, positive feedback, and how other animals control their temperature.
In an Adelaide heatwave the air can reach 46 °C. In a Thredbo winter it can drop below −10 °C. Through all of that, the temperature deep inside your body hardly moves from about 37 °C. If it climbs to 42 °C or falls to 30 °C, you are in serious danger. Keeping it steady is a full-time job.
Keeping things steady
Your cells live in a liquid environment: the tissue fluid around them and the blood that feeds it. The enzymes that run every reaction in your cells only work well within a narrow range of temperature and pH, and cells need a steady supply of glucose, water and oxygen. Homeostasis is keeping this internal environment fairly constant, even when conditions outside change.
| Condition | Normal range | Main control |
|---|---|---|
| Core body temperature | about 36.5–37.5 °C | Hypothalamus, nerves to skin and muscles |
| Blood glucose | about 4–8 mmol/L | Insulin and glucagon from the pancreas |
| Water in the blood | kept steady | Hypothalamus, ADH, kidneys |
| Carbon dioxide (blood pH) | pH about 7.35–7.45 | Brain stem changes breathing rate |
"Constant" doesn't mean the value never changes. It moves up and down a little around a set point, and the body keeps pulling it back.
Negative feedback
Homeostasis works by negative feedback: any change from the set point triggers a response that reverses the change. A home air conditioner works the same way. If the room gets too warm, a sensor detects it and the cooling switches on; when the room is cool enough, it switches off.
Every negative feedback loop has the same parts:
- a stimulus: a change away from the set point
- receptors that detect it
- a control centre, often the hypothalamus, which compares the value with the set point
- effectors, muscles or glands, that make a response to bring the value back.
Messages in a loop can be carried by nerves, hormones or both. Temperature control uses mainly nerves; blood glucose and water balance use mainly hormones. You met the blood glucose and water balance loops in Year 8, so this page focuses on temperature.
Controlling body temperature
Your body makes heat all the time, as cellular respiration in every cell releases energy. Working muscles make far more. Heat is lost from the skin by radiation, by conduction and convection to the air, and by evaporation of sweat. Temperature control is about balancing heat gained against heat lost.
Temperature receptors in the skin detect changes outside; receptors in the hypothalamus itself detect the temperature of the blood flowing through the brain. The hypothalamus then sends nerve messages to the effectors.
- Vasodilation. When you are hot, blood vessels supplying the skin widen. More warm blood flows near the surface and loses heat to the air. This is why you go red when hot.
- Sweating. Sweat glands release sweat. As the water evaporates it takes heat energy from the skin. On a humid day the air is already full of water vapour, so sweat drips off instead of evaporating and you cool down much less.
- Vasoconstriction. When you are cold, skin blood vessels narrow, keeping warm blood deeper in the body. Your skin goes pale and your fingers feel cold, but your core stays warm.
- Shivering. Muscles contract rapidly, and the extra respiration releases heat.
- Goosebumps. Tiny muscles raise your hairs. In furry mammals this traps a thick layer of insulating air; in humans it makes almost no difference, a leftover from our ancestors.
- Behaviour. You also respond by choice: taking off a jumper, finding shade, having a cold drink.
- Set the air to 40 °C while resting, then switch to running. What happens to core temperature, and which responses are working hardest?
- Keep it hot and turn on humid air. Why does core temperature now rise even though the body is sweating?
- Turn off the body's control system at 5 °C and at 40 °C. How long does it take to reach a dangerous temperature? What does this show about negative feedback?
- Turn on fever at a comfortable air temperature. Why does the body shiver even though it is not cold outside?
When control fails
Heat stroke happens when core temperature rises above about 40 °C. Enzymes and cell membranes start to be damaged, the person becomes confused, may stop sweating, and can collapse. It is a medical emergency. Heatwaves kill more Australians than any other natural hazard, and the people most at risk are older people, babies, outdoor workers and people with heart disease. Drinking water, resting in shade or air conditioning, and wetting the skin all help.
Hypothermia happens when core temperature falls below 35 °C, often after being in cold water or wet and windy weather. Shivering becomes violent, then stops; the person becomes clumsy and confused, and the heart can stop.
Fever is different: the body raises its temperature on purpose. Chemicals released during an infection make the hypothalamus reset its set point, perhaps to 39 °C. Now normal body temperature feels "too cold", so you shiver and feel chilly while your temperature climbs. A higher temperature slows the growth of many pathogens and speeds up some immune cells (4I). When the fever "breaks", the set point drops back and you sweat heavily.
Positive feedback
In positive feedback, a change triggers a response that makes the change bigger. That would be disastrous for homeostasis, so it is only used for events that need to build to a finish. During childbirth, pressure of the baby's head on the cervix triggers the release of the hormone oxytocin, which makes the uterus contract harder, which pushes the head down more, and so on until the baby is born. Blood clotting also builds up by positive feedback.
Other animals
Mammals and birds are endotherms: they make most of their own heat and keep a steady body temperature. That lets them stay active at night and in cold weather, but costs a lot of food. Reptiles, amphibians, fish and insects are ectotherms: their temperature mostly follows their surroundings, and they control it by behaviour. A bearded dragon basks in the sun in the morning, turns side-on to soak up heat, and hides under rocks when it gets too hot. Ectotherms need far less food, so a lizard can live on a few insects a week. Kangaroos can't sweat much while resting, so on hot days they lick their forearms, where blood vessels run close to the skin, and the evaporating saliva cools the blood.
Test yourself
Which part of the brain acts as the control centre for body temperature?
The hypothalamus compares blood temperature with the set point.
Why does sweating cool you less on a humid day?
Cooling comes from evaporation, which is slow when the air is already full of water vapour.
On a cold day your fingers go pale because
Vasoconstriction reduces blood flow to the skin to save heat.
Which of these is an example of positive feedback?
Each contraction triggers more oxytocin, which makes the next contraction stronger.
Label the parts of the negative feedback loop.
Which response happens when the body is too hot, and which when it is too cold?
Keeping the internal environment constant is called . The normal value a condition is kept close to is the . A response that reverses a change is feedback. Sweat cools the body when it . A core temperature below 35 °C is called .
A student runs a 5 km race on a hot day. Using the idea of a negative feedback loop, explain how her body stops her core temperature rising too far, and why the risk is greater if the air is humid.
Running increases respiration in her muscles, which releases heat, and the hot air adds more, so her core temperature starts to rise. This is the stimulus. Temperature receptors in her skin and in the hypothalamus detect the change. The hypothalamus, the control centre, compares her temperature with the set point of about 37 °C and sends nerve impulses to the effectors. Blood vessels supplying the skin widen (vasodilation), so more warm blood flows near the surface and loses heat to the air. Sweat glands release sweat, which takes heat from the skin as it evaporates. As her temperature falls back towards the set point, these responses ease off; that is negative feedback. In humid air, the air already holds a lot of water vapour, so sweat evaporates slowly and drips off without cooling her. Heat builds up faster than she can lose it, and she is at risk of heat stroke.