Revision sheet
Homeostasis and feedback loops
The single idea the whole of physiology is built on, and the two shapes a control loop can take.
The internal environment
42, glycaemia around 1 g/L, osmolarity near 300 mOsm/L. Claude Bernard's phrase — the constancy of the internal environment is the condition of free life — is the whole programme.
Homeostasis is not immobility. Each variable oscillates around a set point; what is regulated is the amplitude of the oscillation, not its existence. Physiology is the study of the mechanisms that keep those oscillations small.
- Cells are bathed in interstitial fluid, not blood — plasma is the reservoir that maintains it.
- A regulated variable is stable on average, never strictly constant.
The four parts of a control loop
Every regulation has the same architecture. A sensor measures the variable; an integrating centre — usually in the hypothalamus, the brainstem or an endocrine gland — compares the measurement with a set point; an effector acts; and the result is fed back to the sensor.
Losing any one of the four loses the regulation.
Recognising the four parts turns an unfamiliar regulation into a familiar one. For blood pressure the sensors are the carotid and aortic baroreceptors, the centre is the medulla, the effectors are the heart and the vessels.
- Sensorintegrating centreeffectorfeedback
Negative feedback: the normal case
In a negative feedback loop the response opposes the change that produced it, which drives the variable back towards its set point. Almost every physiological regulation works this way: glycaemia rises, insulin is secreted, glycaemia falls, insulin secretion falls with it.
The same shape governs thermoregulation, blood pressure, osmolarity and every endocrine axis. If an exam asks you to explain a regulation and you cannot name the negative feedback in it, you have not finished the answer.
- Negative feedback = stability. The response cancels its own cause.
- It is the default: assume it unless there is a clear reason not to.
Positive feedback: the rare, deliberate exception
In positive feedback the response amplifies the change, driving the variable further from where it started. It is destabilising by nature, so the body uses it only where an event must be carried through quickly to completion, and it always ends with an external stop.
There are three classical examples: the depolarising phase of the action potential, where Na⁺ entry opens more Na⁺ channels; childbirth, where the head stretching the cervix triggers oxytocin, which strengthens contractions, which stretches the cervix further; and the LH surge that triggers ovulation, where high oestradiol reverses its own feedback from negative to positive.
- Positive feedback = amplification, and it must be terminated by an outside event.
- Action potential, childbirth, LH surge — know all three by name.