Homeostasis is the maintenance of a relatively stable internal environment within narrow physiological limits. Negative feedback maintains this stability by detecting a deviation from a normal value and activating responses that reverse the deviation.
Homeostasis does not mean that internal conditions remain perfectly constant. Variables such as core body temperature, blood glucose concentration, blood pH and water balance fluctuate around a set point or normal range.
A negative feedback loop operates through the following stages:
| Component | Role in negative feedback |
|---|---|
| Stimulus | An internal variable moves away from its set point. |
| Receptor | Detects the change and sends information to a control centre. |
| Control centre | Compares the detected value with the set point and coordinates a response. |
| Effector | Produces a response that opposes the original change. |
| Outcome | The variable returns toward its normal range, reducing the original stimulus. |
For example, exercise increases metabolic activity and heat production, causing core body temperature to rise. Thermoreceptors detect this increase, and the hypothalamus acts as the control centre. It stimulates effectors that increase sweating and cause vasodilation of skin arterioles. Evaporation of sweat and increased heat transfer from the skin lower body temperature toward its set point.
If body temperature falls, the response is reversed: sweating decreases, skin arterioles constrict and shivering may generate heat. Therefore, the response depends on the direction of the deviation.
A common misconception is that negative feedback produces a harmful or “negative” result. In physiology, “negative” means that the response opposes the initial change, not that the effect is undesirable.
In an IB SEHS response, identify the regulated variable, receptor, control centre and effector, then explain how the response reverses the deviation. Use a relevant exercise example and avoid claiming that homeostasis keeps conditions completely constant.
