IGCSE Biology / Homeostasis and excretion
Homeostasis and negative feedback
Edexcel International GCSE Biology (4BI1), chapter 8: Homeostasis and excretion, specification point 2.81. Free lesson.
Read the revision notes · IGCSE Biology past papers
Homeostasis keeps the body's internal conditions within narrow limits, rather than holding them at one fixed value. Negative feedback detects and reverses changes, helping cells and enzymes work properly.
What homeostasis means
Homeostasis is the maintenance of internal conditions within narrow limits. Conditions still change during the day, such as body temperature during exercise or blood glucose after a meal. They fluctuate around a normal value because negative feedback detects changes and reverses them. Homeostasis does not mean that conditions stay completely constant. It means keeping conditions close to their normal levels.
How negative feedback restores a condition
A negative feedback loop reverses a change. The response opposes the original change, which brings the condition back towards normal. Once the change has been reversed, the receptor detects no change, so the correction switches off.
The conditions homeostasis controls
Four conditions to know are body temperature, blood glucose, water content of the blood and carbon dioxide in the blood. Each is controlled in a particular way. Body temperature: about 37 °C; controlled by sweating, shivering and changes to blood flow in the skin. Blood glucose: controlled by insulin and glucagon from the pancreas, acting on the liver. Water content of the blood: controlled by ADH from the pituitary, acting on the kidney. Carbon dioxide in the blood: controlled by changing the breathing rate; the change is detected in the brain.
Why stable conditions matter
Homeostasis matters because enzymes and cells need a stable environment to work properly. Enzymes work within a narrow range of temperature and pH. Outside that range, they can denature or work more slowly. Cells also need a steady water concentration, because changes can damage them through osmosis. Conditions change, but negative feedback reverses those changes. This keeps levels fluctuating around normal values rather than fixed at one value.