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6.4.2Organisms Respond to Changes in their Environments (A Level only)

Negative Feedback — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 34 of 44 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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6.4.2 Negative Feedback — AQA A Level Biology 7402 notes: Explain how negative feedback restores a variable to its set point and contrast it with positive feedback.Negative feedback is a control mechanism in which a change in a variable is detected and a response is triggered tha…

  • Vasodilation
  • Vasoconstriction
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Hypothalamus

Section 1 of 9The principle

Negative feedback is a control mechanism in which a change in a variable is detected and a response is triggered that reverses that change, bringing the variable back towards the set point.

The principle
When the variable returns to normal, the response is reduced and eventually switched off.
Negative feedback therefore keeps the variable fluctuating around the set point, and it is the basis of most homeostatic systems.

The sequence isvariable moves from set point, receptor detects the change, coordinator processes the information, effector acts, the response reverses the change, and the receptor detects that the variable has returned to normal.

Section 2 of 9Body temperature as an example

The hypothalamus in the brain contains thermoreceptors that monitor the temperature of the blood, and thermoreceptors in the skin send information as well. If the core temperature rises above 37 °C, the heat loss centre responds by:

Vasodilation
  • Vasodilation arterioles near the skin surface widen, so more blood flows through skin capillaries and more heat is lost by radiation.
Sweating
  • Sweating sweat glands secrete sweat, and evaporation takes latent heat from the skin.
Hairs lie flat, reducing insulation by trapped air.

If the core temperature falls, the heat gain centre triggers:

Vasoconstriction
  • Vasoconstriction arterioles near the skin narrow, so less blood flows through skin capillaries and less heat is lost.
Shivering
  • Shivering involuntary contraction of skeletal muscles releases heat from respiration.
Hair erector muscles contract, raising hairs to trap an insulating layer of air (not very effective in humans).
Release of adrenaline and thyroxine increase metabolic rate.
Sweating decreases.

Section 3 of 9Why separate mechanisms are an advantage

Having separate mechanisms for correcting a change in each direction gives a more precise control than a single mechanism.

1
If the response to a rise were only to reduce a heating mechanism, the temperature would fall slowly and might overshoot.
2
With two opposing mechanisms, the system can respond to a fall and a rise, and the variable is held near the set point.
3
Antagonistic effectors also allow a faster response.
4
A good answer to a question about this advantage says that control is more precise, because it allows corrections in both directions.

Section 4 of 9Positive feedback

In positive feedback the response increases the change that triggered it, so the variable moves further away from its starting value.

Positive feedback
It does not maintain homeostasis and is generally short-lived.
Examples include the rising Na+ permeability during depolarisation in an action potential, the secretion of oxytocin during labour (contractions cause more oxytocin release,
  • which causes stronger contractions), and blood clotting.
Positive feedback is only stopped by an external event such as the end of the stimulus or the birth of the baby.
FeatureNegative feedbackPositive feedback
Effect of responseReverses the changeAmplifies the change
ResultStability around a set pointRapid change away from the start
RoleHomeostasisCompleting a process quickly

Section 5 of 9Using graphs

Graphs of body temperature or glucose concentration against time show oscillation around the set point with a delay between the change and the response.

Using graphs
Explain these oscillations by describing the time taken for receptors to detect a change and for effectors to respond.
If a graph shows no return to normal after a disturbance, suggest that the feedback system has failed, as in diabetes.
When asked to describe a graph, quote the highest and lowest values and the time taken to return to the set point.

Section 6 of 9Control of heart rate as another example

Negative feedback is also used outside temperature and glucose control.

1
In the control of blood pressure, baroreceptors in the carotid arteries and aorta detect a rise in pressure and send impulses to the cardiac centre in the medulla,
  • which reduces the heart rate through the vagus nerve;
    • a fall in pressure increases the heart rate through the sympathetic nerve.
2
Control of carbon dioxide in the blood works in a similar way using chemoreceptors.
3
Whatever the example, the answer should name the receptor, the coordinator, the effector and the corrective action.
4
The idea of a set point is useful, but note that the body does not have a single device holding it; it is the result of many interacting responses.

Section 7 of 9Exam-style questions

  1. Explain how negative feedback controls body temperature when the core temperature rises. [4]4 marks

  2. Explain the advantage of having separate mechanisms for raising and lowering a variable. [2]2 marks

  3. Distinguish between negative and positive feedback. [2]2 marks

Section 8 of 9Key terms

TermMeaning
Negative feedbackResponse that reverses a change and restores the set point
Positive feedbackResponse that amplifies the original change
VasodilationWidening of arterioles, increasing blood flow
VasoconstrictionNarrowing of arterioles, decreasing blood flow
HypothalamusPart of the brain that monitors and regulates body temperature
ThermoreceptorReceptor that detects temperature changes

Section 9 of 9Exam technique

1
Saying negative feedback means a bad outcome; it means the response opposes the change.
2
Describing vasoconstriction as constriction of capillaries; arterioles change diameter, not capillaries.
3
Saying sweating cools the body because sweat is cold; evaporation takes heat energy from the skin.
4
Saying shivering warms through muscle contraction itself; heat comes from respiration in the muscles.
5
Giving an example of positive feedback as a homeostatic mechanism.

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