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Summary 16 Homeostasis
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- AQA
Concise homeostasis summary notes for the new linear A-level biology 7401/7402
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16 HomeostasisHomeostasis is the maintenance of a constant internal environment.
What is maintained?
® Temperature
® Water levels
® Blood pH levels
® Blood glucose concentration
Homeostasis ensures body cells get what they need to function, despite external changes.
® Temperature – too high and enzymes will denature, too low and chemical reactions
will be too slow
® Water content – not in equilibrium could cause cells to burst (lysis) or shrivel up
(crenation)
Control mechanisms
1. Set point (optimum condition) is monitored by receptor
2. Receptor detects changes in set point and tells controller
3. Controller relays message to effector
4. Effector carries out appropriate changes, creating a feedback loop
5. Feedback loop tells receptor what changes were made
Control mechanisms are mostly negative feedback systems
® Negative feedback: when feedback causes the corrective measures to be turned off
® Positive feedback: when feedback causes corrective measures to remain turned on
Thermoregulation
® To warm the body:
1. Vasoconstriction
If temperature falls, blood vessel constricts so less heat is lost from the
surface of the skin
2. Shivering
3. Increased metabolic rate
4. Decrease sweating
5. Behaviour mechanisms e.g. huddling
6. Raising of hair / piloerection
Hairs trap a layer of air next to the skin – insulating layer
® To cool the body:
1. Vasodilation
Diameter of arteries become larger near skin surface, brings blood closer
so more heat can be lost
2. Sweating
Liquid sweat turns into a gas (evaporates) using heat from the skin
3. Lowering body hair
4. Behaviour mechanisms e.g. sitting in shade
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