What is homeostasis?
Cells need steady conditions for their enzymes to work. The liquid around them (blood plasma and tissue fluid) is the internal environment.
Homeostasis means keeping this internal environment within a narrow normal range. Each factor has a set point:
- Body temperature: about 37 °C
- Blood glucose: about 4–6 mmol/L (roughly 70–110 mg/dL)
- Water and salt balance (the concentration of blood)
- Blood pH: about 7.35–7.45
Levels are never perfectly still. They wobble a little around the set point.
Negative and positive feedback
A control system has three parts:
- Receptor: detects a change (a stimulus).
- Control centre (coordination centre): the brain, spinal cord or a gland compares it with the set point.
- Effector: a muscle or gland that makes a response.
Negative feedback: the response reverses the change. Too high → bring it down; too low → bring it up. Most homeostasis works this way.
Positive feedback: the response makes the change bigger, until an event ends it. Examples: contractions during childbirth, blood clotting.
Nerves send fast, short signals. Hormones travel in blood; they are slower but last longer.
Controlling body temperature
The hypothalamus in the brain is the thermostat. Receptors in the skin and brain tell it the temperature.
Too hot
- Sweat glands release sweat; evaporation takes heat from the skin.
- Blood vessels near the skin widen (vasodilation), so more heat is lost. Skin looks red.
Too cold
- Muscles shiver: quick contractions release heat.
- Skin vessels narrow (vasoconstriction), keeping warm blood deep inside.
- Hairs stand up and sweating stops.
Fever, heatstroke and hypothermia happen when this control fails or is overwhelmed.
Blood glucose, water and pH
Blood glucose
The pancreas checks glucose in the blood.
- High glucose (after a meal): insulin is released. Cells take in glucose; the liver and muscles store it as glycogen.
- Low glucose (fasting, exercise): glucagon is released. The liver breaks glycogen back into glucose.
Diabetes: Type 1, the pancreas makes almost no insulin (treated with insulin injections). Type 2, cells stop responding well to insulin (helped by diet, exercise and medicine).
Water balance (osmoregulation)
If blood becomes too concentrated, the brain triggers the pituitary gland to release ADH. The kidneys then put water back into the blood, so urine is small and dark. Too much water: less ADH, more dilute urine.
pH
Lungs remove carbon dioxide (an acid-forming gas) and kidneys remove extra acid, keeping blood pH near 7.4.
Key formulas and definitions
- Key terms: internal environment, set point, homeostasis
- Negative feedback: change → receptor → control centre → effector → change reversed
- Temperature: hypothalamus; sweating, vasodilation / shivering, vasoconstriction
- Glucose: insulin lowers (glucose → glycogen); glucagon raises (glycogen → glucose)
- Water: high ADH → less, darker urine; low ADH → more, paler urine
- Normal values: 37 °C, glucose 4–6 mmol/L, pH 7.35–7.45
Worked examples
1. You run a race on a hot day. Describe the feedback loop that controls your temperature.
Change: temperature rises. Receptors: in skin and hypothalamus. Control centre: hypothalamus. Effectors: sweat glands release sweat and skin arterioles widen. Response: heat is lost and temperature falls back to about 37 °C.
2. Why does blood glucose rise and then fall after lunch?
Digested starch enters blood as glucose, so it rises. The pancreas releases insulin; cells take in glucose and the liver stores it as glycogen, so it falls back to normal.
3. Why is a person's urine pale after drinking 2 litres of water?
Blood becomes dilute, so less ADH is released. The kidneys return less water to the blood and make a large volume of dilute (pale) urine.
4. Is labour contraction during childbirth negative or positive feedback?
Positive. Contractions trigger more of the hormone oxytocin, which causes stronger contractions, until the baby is born.
Common mistakes
- Saying glucagon and glycogen are the same. Glucagon is a hormone; glycogen is the stored carbohydrate in the liver.
- Thinking skin blood vessels move up and down. They widen or narrow; they do not move.
- Saying shivering happens when you are hot. Shivering makes heat, so it happens when you are cold.
- Calling every feedback negative. Childbirth and clotting use positive feedback, which pushes the change further.