Describe the regulation of body temperature. Explain the mechanisms of heat production and heat loss, and the pathogenesis of fever.

Normal Values

• Diurnal variation — lowest at 4–6 a.m., highest at 4–6 p.m.; range about 1 °C
• Rises 0.5 °C in the luteal phase after ovulation (progesterone)
• Rises in exercise, after meals, and in the newborn is unstable

Heat Production

Heat Loss

The Hypothalamic Thermostat

↑ Core temperature → Anterior hypothalamic thermoreceptors → Heat loss centre → Cutaneous vasodilatation + sweating → ↑ Heat loss → Temperature returns to set point

• Behavioural responses — seeking shade, changing clothing, curling up, using a fan — are the most effective of all
• Sweating — up to 1.5 L/hour; sympathetic cholinergic supply, so it is blocked by atropine
• Shivering — from the primary motor centre for shivering in the dorsomedial posterior hypothalamus

Fever

Infection → exogenous pyrogen (bacterial endotoxin) → Macrophages release endogenous pyrogens — IL-1, IL-6, TNF-α → Act on the OVLT of the hypothalamus → ↑ Prostaglandin E2 → set point raised → Body behaves as if cold → shivering, vasoconstriction → Temperature rises to the new set point

• The chill phase occurs while the temperature is climbing; the flush and sweating phase when the set point returns to normal
• Antipyretics inhibit cyclo-oxygenase → ↓ PGE2 → reset the thermostat. They do not lower a normal temperature
• Fever is not merely harmful — it impairs bacterial growth and enhances immune function

Skin Circulation in Thermoregulation

• Skin blood flow can vary from 50 mL/min to 3 L/min — a 60-fold range, the widest of any tissue
• Arteriovenous anastomoses in the hands, feet, ears, nose and lips are richly supplied by sympathetic vasoconstrictor fibres and act as radiators
• Counter-current heat exchange in the limbs — in the cold, venous return through the deep venae comitantes conserves heat; in the warm, blood returns through superficial veins to lose it
• Piloerection traps a layer of air; effective in furred animals, vestigial in man ("goose flesh")

Applied Aspects

• Heat stroke — the set point is normal but the mechanisms are overwhelmed; core > 40 °C with dry hot skin and altered consciousness. Treated by physical cooling, not antipyretics
• Hypothermia — core < 35 °C; shivering ceases below 30 °C and consciousness is lost; used deliberately in cardiac surgery to protect the brain
• Malignant hyperthermia — ryanodine receptor mutation; triggered by halothane and succinylcholine; treated with dantrolene
• The newborn has a large surface area to weight ratio, little insulating fat and cannot shiver → very prone to hypothermia
• Therapeutic hypothermia after cardiac arrest or birth asphyxia reduces cerebral metabolic demand and improves neurological outcome
• Fever in the elderly may be absent despite serious infection, because thermoregulatory responses blunt with age
• Antipyretic versus cooling — antipyretics for fever, physical cooling for hyperthermia; using the wrong one wastes critical time
• Measuring temperature — rectal and tympanic reflect core best; axillary is least reliable
• Sweat composition — hypotonic; NaCl content falls with acclimatisation under the influence of aldosterone
• Cold injury — frostbite from freezing; trench foot from prolonged wet cold above freezing
• Rewarming must be gradual in severe hypothermia — rapid peripheral rewarming causes afterdrop and arrhythmia
• Malignant hyperthermia and neuroleptic malignant syndrome are drug-induced hyperthermias, not fevers