Your turn
Each lab is one step from the video. Drag, tap and see what the body does.
Each lab isolates one control loop. The models are simplified to show the idea; they are not medical tools.
Tap the buttons and slide the sliders.
Drag the weather from freezing to scorching. How much does the inside temperature move?
The outside swings by tens of degrees; core temperature stays within about 36.5–37.5 °C.
Slide from snow to beach. Inside, you stay just right!
Make the body too hot or too cold. Watch the signal go sensor → brain → effector, then switch off.
Sensor → control centre (hypothalamus, set point ~37 °C) → effector; the response reverses the change and shuts off.
Make it too hot or too cold. Watch the body fix it.
A simplified loop: the real body runs many at once.
Drag the weather. Which effectors switch on when it's hot? Which when it's cold?
Heat: vasodilation + sweating. Cold: vasoconstriction, piloerection (goosebumps), shivering.
Slide it! Hot makes sweat. Cold makes goosebumps and shivers.
Give a snack or go for a run. Can you keep the line in the green? Try switching the hormones off.
Insulin lowers glucose after a meal; glucagon raises it when it falls. Turn the hormones off to see an unregulated system.
Tap snack, then run. Your body keeps the line in the green.
A toy model, not real data.
Tap a dot on the body. What does each system keep steady?
Nervous, endocrine, circulatory, respiratory, digestive and urinary systems each hold different variables.
Slide the temperature. When do the enzymes slow down? When do they stop?
Activity roughly halves per 10 °C drop; proteins begin to denature around 50 °C.
Slide it. Too cold: slow. Too hot: they stop!
The curve is a simplified model. This page isn't medical advice.
Feel very sick, or much too hot or cold? Tell a grown-up.
Homeostasis is a big word for a simple idea: your body keeps things just right inside, even when the outside changes.
On a snowy hill it is freezing. On a sunny beach it is hot. But deep inside you, it stays just right.
Your skin feels if you get too hot or too cold. It sends a message to your brain. Your brain says: fix it!
Too hot? You sweat, and sweat helps you cool down. Too cold? You get goosebumps and you shiver. Shivering makes heat!
Sugar in your blood is fuel. After a snack, your body tucks some away. After a long run, it lets some out.
Your body has tiny helpers called enzymes. They work best when it is just right. Too cold, and they go slow. Too hot, and they stop working.
Homeostasis is your body keeping its inside steady while the outside keeps changing. Temperature, blood sugar, water, salt, oxygen and the acidity (pH) of your blood are all held inside narrow ranges.12 It matters because the tiny machines that run your cells, called enzymes, only work well inside those ranges. Too cold and they slow down; much too hot and they lose their shape and stop working.29 When homeostasis can't keep up, you get ill.11
The word comes from Greek words meaning "equal" and "holding still".11 But "still" doesn't mean frozen: values wobble a little around a middle value called the set point, and the body keeps nudging them back. Biologists call this a dynamic equilibrium.2
This page explains how the body works; it isn't medical advice.
| What's kept steady | Normal range | Main controllers |
|---|---|---|
| Core body temperature | about 36.5–37.5 °C (97.7–99.5 °F)3 | hypothalamus, skin, sweat glands, muscles |
| Blood sugar (glucose) | about 70–100 mg/dL when fasting (higher for a while after meals)4 | pancreas (insulin, glucagon), liver |
| Blood acidity (pH) | 7.35–7.456 | lungs, kidneys, chemical buffers |
| Water and salt | kept balanced without you thinking about it1 | kidneys, hypothalamus (thirst) |
Most homeostasis works like the thermostat in a house.3 Every loop has three parts:1
As the value returns to normal, the sensor notices and the effector switches off. Because the response reverses the change, it's called negative feedback. "Negative" doesn't mean bad; it means "undo". Negative feedback is the main way the body keeps itself steady.211
change → sensor → control centre → effector → change undone
Your body's set point for temperature is about 37 °C (98.6 °F).1 Here's what the hypothalamus switches on:18
| Too hot | Too cold |
|---|---|
| Blood vessels in the skin widen, so warm blood can give its heat to the air | Skin vessels narrow, keeping warm blood deep in the body |
| Sweat glands release sweat; as it evaporates it takes heat away | Tiny muscles raise your hairs: goosebumps2 |
| Breathing gets deeper, losing extra heat from the lungs | Muscles shiver; each contraction releases heat |
At rest, most of your heat leaves by radiation (about 60%), then evaporation (about 20%) and moving air (about 15%). During exercise, sweating becomes the main way to cool down.3 Where does the heat come from in the first place? About 60% of the energy your cells release while making their fuel molecule, ATP, comes out as heat.3
After a meal, glucose rises. Cells in the pancreas release insulin, which tells muscle, fat and liver cells to take glucose in, so the level falls back. If glucose drops, say during a long run, other pancreas cells release glucagon, which tells the liver to turn its stored sugar (glycogen) back into glucose.14 Two hormones, pushing in opposite directions. Both are still negative feedback, because each one reverses a change.2 When the body makes too little insulin or stops responding to it, blood sugar stays too high: that's diabetes; our explainer on what causes diabetes shows how.
Almost every system helps. The quick version, one row per system:
| System | What it keeps steady | How |
|---|---|---|
| Nervous system | Temperature, heart rate, breathing | Sensors report to the brain; nerve signals reach their targets within milliseconds. The hypothalamus runs many loops automatically, and the autonomic nerves speed up or slow down the heart.512 |
| Endocrine (hormone) system | Blood sugar, water, metabolism | Glands release hormones into the blood. Slower than nerves (up to hours), but they reach the whole body.5 |
| Circulatory system | Delivery of heat, fuel and oxygen | The heart pumps blood everywhere; skin vessels widen to lose heat or narrow to keep it.1 |
| Respiratory system | Oxygen, carbon dioxide, blood pH | Sensors track CO2; breathing faster blows off extra CO2, which makes the blood less acidic.6 |
| Digestive system | The supply of fuel and nutrients | Breaks food down and absorbs the nutrients into the blood, so there's glucose to regulate in the first place.13 |
| Urinary system (kidneys) | Water, salt, waste, acidity | Filter about 150 quarts of blood a day and remove extra fluid and acid. When blood gets too concentrated, the hypothalamus makes you thirsty and sends the hormone ADH out through the pituitary gland, telling the kidneys to save water.710 |
| Muscular system | Body temperature | Muscles make heat; shivering is rapid contractions that warm you up.111 |
| Skin (integumentary system) | Body temperature | Sweat glands, skin blood vessels and the tiny hair muscles that make goosebumps.8 |
| Cells | What goes in and out | Each cell's membrane is selectively permeable: it lets some substances through and blocks others.14 |
Because of enzymes. Enzymes speed up the chemical reactions that keep you alive, and each one works best within a certain range of temperature and pH. Outside that range, the shape of the enzyme's active site changes and it works less well; extreme heat or acidity can denature it, which means it loses the shape it needs to work (often for good).9 In mammals, enzyme activity roughly halves for every 10 °C drop in temperature, and body proteins start to denature at around 50 °C.2 Our explainer on how enzymes work goes deeper.
So when homeostasis can't keep up, things go wrong quickly. Cleveland Clinic puts it plainly: every disease or medical condition involves a disruption of homeostasis.11
Even "too much of a good thing" is a problem. Drinking far too much water too quickly, or breathing pure oxygen for too long, can be harmful too.11 Homeostasis is about just right, not "more is better".
No. During an infection, substances called pyrogens reset the hypothalamus's thermostat to a higher temperature, and the body then works to reach and hold that new set point. That's why you can shiver at the start of a fever, even though you're warm.2 A fever is part of the body's defence against infection.12
Positive feedback does the opposite: it makes a change bigger until something ends it. In childbirth, stretching triggers the hormone oxytocin, which causes stronger contractions and more stretching, until the baby is born. Blood clotting works the same way: each step triggers more clotting until the wound is sealed.1 Positive feedback is rare in the body; the normal examples have a clear end point, and positive feedback that doesn't stop can be harmful.12
Homeostasis is your body keeping its inside conditions steady, such as temperature, blood sugar, water and salt, even when the world outside changes. The word comes from Greek words meaning "equal" and "holding still".
Your cells depend on enzymes, and enzymes only work well within a narrow range of temperature and acidity. If conditions drift too far, reactions slow down or enzymes lose their shape and stop working. Serious failures of homeostasis, like heatstroke or severe dehydration, can be life-threatening.
Body temperature is the classic example. When you get too hot, blood vessels in your skin widen and you sweat; when you get too cold, the vessels narrow, you get goosebumps and you shiver. Blood sugar is another: insulin brings it down after a meal and glucagon raises it when it falls.
Negative feedback is a loop that reverses a change. A sensor detects the change, a control centre compares it with a set point, and an effector pushes the value back. "Negative" doesn't mean bad; it means the response undoes the change. Most homeostasis works this way.
Positive feedback makes a change bigger instead of reversing it, until something ends it. Examples are childbirth, where the hormone oxytocin strengthens contractions, and blood clotting, where each step triggers more clotting. It's rare in the body. The normal examples, like childbirth and clotting, have a clear end point; positive feedback that doesn't stop can be harmful.
You become ill. Examples include heatstroke (body temperature can rise above 41 °C), hypothermia (below 35 °C), dehydration and diabetes, where blood sugar stays too high. Anyone with worrying symptoms should see a doctor.
Each cell is wrapped in a selectively permeable membrane that lets some substances in and keeps others out. The body around the cells helps too: blood delivers fuel and oxygen and carries away waste, and the kidneys keep the water and salt around cells balanced.