What you need to know
- 7.16(i)negative feedbackA change from the normal level triggers a response that reverses the change, returning conditions to the norm. and positive feedbackA change triggers a response that increases the change further. control.
- 7.16(ii)Negative feedback keeps systems within narrow limits.
Thermoregulation detail, ADH and exercise balance are 7C. The birth illustration of positive feedback is not a reproductive-hormone topic to memorise. Blood glucose hormones are not a required loop on this page.
By the end, you should be able to…
- Define negative and positive feedback by the direction of the response.
- Explain narrow limits without drawing a perfectly flat line.
- Use one illustration of each, and no extra pathway.
Build your understanding
Negative feedback reverses the change
In negative feedbackA change from the normal level triggers a response that reverses the change, returning conditions to the norm., a change away from a set point is detected and the response reduces that change. If body temperature rises, responses increase heat loss until temperature falls back. If it falls, responses increase heat gain. The “negative” means the response is opposite in direction to the disturbance, not that the outcome is harmful.
The system does not sit on a perfectly flat line. It oscillates inside a narrow range: a small rise, a correction, a small fall, a correction. Those narrow limits are the point of 7.16. A wider and wider swing would mean the feedback is failing.
Positive feedback increases the change
In positive feedbackA change triggers a response that increases the change further., the response increases the change that was detected, until some endpoint removes the stimulus. A standard illustration is childbirth: contraction pushes the fetus against the cervix, which leads to more contraction, until delivery ends the stimulus. You do not need the names of every hormone in that pathway. You need the direction: the response amplifies the change.
Positive feedback is a poor way to hold temperature or water potentialThe tendency of water to move out of a solution. Pure water is 0; adding solutes makes it negative. Water moves from higher to lower water potential. steady, because it would drive the value further away. It is useful where a process should run to completion. Do not write that all feedback in the body is negative, and do not write that positive feedback is always a disease.
Name the parts of a control loop
A complete negative-feedback answer has a receptor that detects the change, a coordinator that decides, and an effector that carries out the response which reverses the change. In later lessons the hypothalamusA region of the brain that controls homeostasis, including body temperature and water balance., the medulla and the pituitary play coordinator roles. Here, the principle is enough.
homeostasisKeeping the internal environment within narrow limits, despite changes outside., the maintenance of a steady internal environment, depends on this negative feedback. The definition and the exercise examples are developed in 7C. Do not answer a 7.16 question with only the word homeostasis.
NOT: homeostasis is not a perfectly flat line, and positive feedback is not the usual homeostatic mechanism.
- Negative feedback opposes the change
- The set value can wobble
- Positive feedback amplifies a change
- It continues to an endpoint
Two traps this diagram blocks
- Do not call sweating or shivering positive feedback.
- Do not describe homeostasis as keeping every value perfectly constant.
From the diagram alone
Question
Distinguish negative feedback from positive feedback, using temperature as the negative example. [3]
Show a model answer
Negative feedback reverses a change, so a rise in temperature causes responses that lower it again. Positive feedback increases a change until an endpoint, so it does not restore a set range.
Make your reasoning visible
Say the direction
Negative: response opposes the change. Positive: response increases the change.
Narrow, not fixed
“Within narrow limits” allows small fluctuation. “Constant and unchanging” is too strong.
Key explanations
- homeostasisKeeping the internal environment within narrow limits, despite changes outside. keeps internal conditions in a workable range by negative feedbackA change from the normal level triggers a response that reverses the change, returning conditions to the norm..
- A small departure that is then corrected is success, not failure. The value is not frozen.
- That increase continues until an endpoint, rather than restoring a set range.
- Birth contractions illustrate the direction only. A full birth pathway is not required.
Avoid this misconception
The claim: Negative feedback means something has gone wrong, and a normal graph of temperature is a flat line.
The correction: Negative feedback is the normal correcting process. The graph wobbles inside a band. positive feedbackA change triggers a response that increases the change further. is the one that pushes further from the starting value.
Order a negative feedback loop
Negative feedback keeps a factor within narrow limits: whichever way it changes, the response pushes it back. Order the loop.
Order the steps
What to do
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
Body temperature rises above the norm. Put the negative feedback loop in order.
1 of 6 in the right place
Real systems have separate effectors for rises and falls, and the factor overshoots a little each way, so it fluctuates around the norm rather than staying exactly on it.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Define negative feedback.
Show suggested answer
A change is detected and the response reverses that change, keeping the variable within narrow limits.
Practice question
Retrieval and explanation · suggested answer
How is positive feedback different?
Show suggested answer
The response increases the change until an endpoint is reached.
Practice question
Retrieval and explanation · suggested answer
Why is temperature control described as negative feedback rather than positive?
Show suggested answer
A rise causes responses that lower the temperature, and a fall causes responses that raise it. The response opposes the change.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A variable rises, the body responds, and the variable rises faster. Someone calls this “negative feedback because the body is involved”. Rewrite the classification and say what would have to happen instead for the label negative to be correct.
Show a hint
Ignore the word negative as a value judgement. Use direction.
Explore the reasoning
This is positive feedback: the response amplifies the rise. It would be negative feedback only if the response brought the variable back down toward the set range. Involvement of the body does not decide the name.
Recall this lesson
Lesson recall questions
Question 1 of 5: Define negative feedback control.
Negative and positive feedback
Practice question
Retrieval and explanation · suggested answer
Define negative feedback control.
Lesson: 7B 6: Principles of Homeostasis
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Heat illness is failed balance, not a new definition
7B · Principles of HomeostasisThe NHS distinguishes heat exhaustion, with heavy sweating, from heatstroke, a medical emergency in which the body is dangerously overheated and sweating may have stopped. That is what…Specification 7.16(ii)Read & use the scienceClose article
Why this matters
The NHS distinguishes heat exhaustion, with heavy sweating, from heatstroke, a medical emergency in which the body is dangerously overheated and sweating may have stopped. That is what it looks like when temperature is no longer held in a safe range. It is not the list of thermoregulatory effectors, and it is not advice you should turn into a first-aid essay in the exam. 7.16: negative feedback is what normally keeps a variable inside narrow limits. If the disturbance is too large, the limits are left.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.16(ii) Negative feedback maintains variables within narrow limits.
Use the science
Why is heatstroke not an example of successful negative feedback?
Explore a possible response
Successful negative feedback would increase heat loss and bring temperature back down. Heatstroke is the situation in which that control has been overwhelmed. It is a medical emergency on the NHS page, not a model answer for a healthy loop.
Explore the source
Feedback is measured over time
7B · Principles of HomeostasisA physiological control system can be recognised by a time series: a disturbance changes a variable, sensors detect it, and responses tend to bring it back toward a…Specification 7.16(ii)Read & use the scienceClose article
Why this matters
A physiological control system can be recognised by a time series: a disturbance changes a variable, sensors detect it, and responses tend to bring it back toward a range. That does not mean the graph is flat. The NIH account of the hypothalamus and pituitary describes multiple feedback signals, reminding us that a real body has interacting loops rather than a single cartoon arrow. For 7.16, identify the variable and the response before calling it negative feedback. An oscillation around a narrow range may be normal; a graph that drifts away after each response suggests the control is not compensating.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.16(ii) Negative feedback maintains variables within narrow limits.
Use the science
A graph shows temperature rising, then falling after sweating increases. What extra pattern would support negative feedback rather than coincidence?
Explore a possible response
Repeated disturbances followed by compensating responses and return toward a narrow range would support the feedback interpretation. One rise and fall alone is less persuasive.
Explore the source
- Functional anatomy of the hypothalamus and pituitaryEndotext, NCBI Bookshelf