The hypothalamus compares core temperature Tc against a setpoint Tset ≈ 37 °C and drives effectors proportionally to the error, with a small dead-band so effectors don't chatter right at the setpoint — this is a textbook negative-feedback control loop. When Tc rises above Tset, sweating and vasodilation switch on to dump heat; when Tc falls below Tset, shivering and vasoconstriction switch on to conserve and generate it. Each effector pushes Tc back toward Tset, which is exactly what makes it a *negative* feedback loop rather than a runaway one.
e = Tc − Tset
sweat = sweatGain·max(0, e−0.15) (evaporative loss)
shiver = shiverGain·max(0, −e−0.15) (metabolic heat)
bloodFlow = clamp(0.5 + e/4, 0, 1) (vasomotor tone)
dryLoss = G(bloodFlow,clo)·(Tc − Tamb)
C·dTc/dt = M·exercise + shiver·1.2 − dryLoss − sweat·L
- Ambient temperature — the environment Tc exchanges dry heat with; above Tc it becomes a heat source, which is why sweating alone must carry the load on a hot day.
- Exercise / heat load — extra metabolic heat production, exactly like muscle activity during exertion, that the loop has to shed.
- Clothing insulation — clo value that throttles dry heat exchange with the environment in both directions.
- Sweat / shiver gain — effector sensitivity; too low and the body drifts off setpoint under load, too high and it overshoots and oscillates.
- Fever offset — shifts Tset up by 1.5 °C, showing how a fever forces the same feedback loop to defend a higher target (shivering to "heat up to" the new setpoint, then sweating once achieved).
Real-world relevance: this proportional control loop with saturating effectors is the same model clinicians reason about for heatstroke, hypothermia and fever, and the one exercise physiologists use to predict sweat and cardiovascular strain during hot-weather exertion.