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By the end of this lesson you should be able to explain and apply each part of this topic — The Generic Feedback Loop, Negative Feedback, Positive Feedback and Negative Versus Positive Feedback — and use these ideas accurately in exam-style questions.
Spec Mapping — OCR H420 Module 5.1.1 — Communication and homeostasis, content statements covering negative feedback as the principal mechanism of homeostatic control and positive feedback as a less common amplifying mechanism, with named physiological examples of each (refer to the official OCR H420 specification document for exact wording).
Homeostatic control relies on feedback: the output of a system is fed back to its input so that the system can correct itself. In mammals, feedback comes in two contrasting forms. Negative feedback reverses a change and is the dominant mechanism of homeostasis, keeping variables like body temperature and blood glucose close to a set point. Positive feedback amplifies a change and drives a process to completion — it is far less common, but plays crucial roles in childbirth, blood clotting and the generation of action potentials. This lesson explains both systems in the detail required for OCR A-Level Biology A specification module 5.1.1(d)–(e).
The conceptual heritage matters here. Claude Bernard's nineteenth-century insistence on the constancy of the milieu intérieur and Walter Cannon's twentieth-century coining of the word homeostasis both relied implicitly on negative-feedback control as their mechanism, although neither used the term feedback in the engineering sense. The mathematical theory of feedback control came later, from cyberneticists such as Norbert Wiener in the 1940s, and is paraphrased here only to make clear that the biology you are learning maps directly onto a wider scientific framework that runs from thermostats to climate models. Positive feedback in physiology has a different historical lineage — Alan Hodgkin and Andrew Huxley's celebrated 1952 paper on the squid giant axon described the regenerative Na⁺ cycle that creates an action potential, demonstrating that nature deploys positive feedback wherever a rapid, decisive, all-or-nothing switch is needed (paraphrased).
Key Definitions:
- Negative feedback — a control mechanism in which a change in a variable triggers a response that reverses the change, restoring the variable to its set point.
- Positive feedback — a control mechanism in which a change in a variable triggers a response that amplifies the change, moving the variable further from its starting value.
- Set point — the normal or target value of a controlled variable.
Every feedback loop in a mammal has the same basic architecture: receptors detect a change; a coordinator (often a region of the brain or an endocrine gland) interprets the signal; effectors carry out the response; and the result is detected again by the receptors, closing the loop.
flowchart LR
A[Stimulus: change in variable] --> B[Receptor]
B --> C[Coordinator / central control]
C --> D[Effector]
D --> E[Response]
E -.feedback.-> A
If the response reverses the stimulus, the feedback is negative. If the response reinforces the stimulus, it is positive.
Negative feedback is the basis of almost all homeostatic control in mammals. It keeps variables oscillating gently around a set point rather than drifting uncontrollably.
The set point is not a single fixed number but a narrow range around which the variable oscillates. For human core temperature, the set point is ~37 °C with daily fluctuations of ±0.5 °C. For blood glucose, fasting levels sit around 4–6 mmol dm⁻³ and rise to around 7–8 mmol dm⁻³ after a meal.
flowchart TB
A[Rising variable] --> B[Receptor detects increase]
B --> C[Coordinator signals effector]
C --> D[Effector reduces variable]
D --> E[Variable returns to set point]
F[Falling variable] --> G[Receptor detects decrease]
G --> H[Coordinator signals opposing effector]
H --> I[Effector increases variable]
I --> E
Positive feedback is rare in mammals because it is inherently destabilising — a small change is amplified until the system reaches a new end-point. It is used when rapid, all-or-nothing responses are required.
Positive feedback here ensures that the action potential is all-or-nothing: once threshold is reached, the depolarisation runs to completion.
| Feature | Negative feedback | Positive feedback |
|---|---|---|
| Effect on change | Reverses it | Reinforces it |
| Role in homeostasis | Central — keeps variables stable | Unusual — used only where amplification is needed |
| Outcome | Returns to set point | Moves further from starting point |
| Examples | Body temperature, blood glucose, blood water potential | Childbirth, action potentials, blood clotting |
| End of response | Self-limiting as variable normalises | Ends only when an external factor stops it |
A dehydrated runner experiences a rise in blood solute concentration.
This is a classic negative feedback loop — the change in blood water potential is reversed by the response.
Feedback questions are usually qualitative, but the strongest candidates can quantify the corrective capacity of an effector. Consider the thermoregulatory loop responding to a mild exercise-induced heat load.
Part A — how fast can sweating remove heat? The latent heat of vaporisation of water is L=2.4 kJ g⁻¹. If sweat evaporates from the skin at a rate of m˙=0.60 g s⁻¹, the rate of heat removal is:
Q˙=m˙×L=0.60 g s−1×2.4 kJ g−1=1.44 kJ s−1=1440 W
A resting adult produces only ~80–100 W of metabolic heat, so even modest evaporative sweating (provided the sweat actually evaporates) has more than enough capacity to reverse a resting heat load — which is exactly what a negative-feedback effector must be able to do.
Part B — why does the corrected variable oscillate rather than sit still? Suppose 1 g of sweat carries 2.4 kJ, and the body's specific heat capacity gives a temperature change of about 0.30 °C per 100 kJ removed from a 70 kg person. There is always a lag between the thermoreceptors detecting a rise and the effectors delivering the correction. During that lag the temperature continues to climb slightly (overshoot); the correction then over-shoots in the opposite direction before the receptors detect the reversal and reduce their signalling. The system therefore settles into a small damped oscillation about the ~37 °C set point rather than clamping to a single value. The size of the oscillation depends on the loop gain — informally,
G=Δdetected errorΔeffector response
A high-gain loop corrects quickly but tends to overshoot more; a low-gain loop is stable but sluggish. Real physiological loops are tuned toward moderate gain — enough to correct promptly, not so much that they hunt wildly. This is the quantitative reason the OCR mark scheme rewards "the variable oscillates around a set point" rather than "the variable is held constant": constancy is impossible in any real loop with a finite response lag.
Beyond spec — accept and use: the formal control-theory treatment of loop gain and damping is undergraduate material. At A-Level you only need the conceptual consequence: finite lag + corrective response = damped oscillation about a set point.
The same oscillation logic explains a range of real physiological rhythms that students often mistake for "failures" of homeostasis. Blood glucose rises after a meal and is corrected by insulin, overshooting slightly into a mild post-prandial dip before glucagon nudges it back up — a two-effector damped oscillation, not a fault. Core temperature runs a shallow circadian oscillation of roughly half a degree because the set point itself is gently reset across the day. And during a fever the set point is deliberately raised by pyrogens: the same negative-feedback machinery then defends the new, higher set point, which is why a feverish patient shivers even though they already feel hot. In every case the corrected variable oscillates about a regulated set point, and in every case the underlying reason is the finite lag between detection and correction quantified above. Recognising that oscillation is an inevitable and designed feature — rather than evidence of poor control — is one of the clearest markers of an A*-standard understanding of feedback.
When describing any feedback loop, structure your answer using the five steps: stimulus → receptor → coordinator → effector → response → feedback. OCR mark schemes almost always give a mark for correctly identifying each component, and candidates who use a clear framework score much more reliably than those who write general paragraphs.
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