A level Biology B · Edexcel 9BI0 · Topic 9A
Homeostasis
Key points
- Homeostasis is keeping internal conditions (temperature, blood glucose, water potential, pH) within narrow limits so enzymes and cells work properly.
- Control systems have receptors, a coordinator (often the hypothalamus) and effectors.
- Negative feedback reverses a change back to the set point; positive feedback amplifies a change (e.g. oxytocin during childbirth).
- Thermoregulation in mammals: the hypothalamus responds to blood temperature and skin receptors.
- When too hot: vasodilation of skin arterioles, sweating, hairs lie flat. When too cold: vasoconstriction, shivering, hairs raised, adrenaline and thyroxine increase metabolic rate.
- Ectotherms rely mainly on behaviour to control body temperature; endotherms generate heat internally.
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Cells only work well when their surroundings stay within narrow limits. Homeostasis is the set of control systems that keep the internal environment steady despite changes outside and inside the body.
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