Feedback mechanisms and homeostasis
Negative feedback, where a rise in the output switches off its own production, holds temperature, glucose, blood pressure and hormones near their set points.
A thermostat is the model: a sensor measures the variable, a control centre compares it with a set point, and an effector corrects the error. Body temperature is held near 37 °C by the hypothalamus, which orders sweating and skin vasodilation when hot and shivering and vasoconstriction when cold; fever is a deliberate resetting of the set point by inflammatory signals.
Glucose is held by the insulin-glucagon pair, calcium by parathyroid hormone, blood pressure by baroreflexes and the kidneys, oxygen and carbon dioxide by the breathing centres. Diseases are often a broken loop: in type 1 diabetes the effector (insulin) is missing; in Cushing's disease the control centre ignores the feedback.
Positive feedback is rare and used for events that must run to completion: the surge of oxytocin in labour, blood clotting and the sodium rush of an action potential.
Key facts
| Fact | Value |
|---|---|
| Core temperature set point | ≈37 °C |
| Fever threshold | ≥38 °C |
| Positive feedback examples | labour, clotting, action potential |