How the Body Releases Excess Heat: Metabolic Heat Production and Thermoregulation

The body continuously generates heat as a byproduct of metabolism (basal metabolic rate ~60–75% of total energy expenditure). To maintain core temperature (~37°C / 98.6°F), heat must be dissipated.

Heat Loss Mechanisms

• Radiation: Infrared emission to cooler surroundings (~60% at rest).
• Convection: Air/water movement across skin (~15%).
• Conduction: Direct contact with cooler surfaces (~3%).
• Evaporation: Sweating (eccrine glands) + respiratory tract evaporation. Primary mechanism during heat stress/exercise (~80% of heat loss when ambient temp > skin temp). Insensible perspiration ~600 mL/day.

Thermoregulatory Control

Hypothalamic thermostat (preoptic area) integrates core and skin temperature signals → effector responses:

• Heat loss: cutaneous vasodilation, sweating.
• Heat conservation: vasoconstriction, piloerection, shivering thermogenesis, non-shivering thermogenesis (brown fat).

Exercise & Heat Stress

Muscle efficiency ~20–25%; 75–80% of metabolic energy becomes heat. Sweat rates up to 2–3 L/hr. Dehydration impairs sweating → hyperthermia risk (heat exhaustion, heat stroke).

Clinical Relevance

• Fever: reset hypothalamic set-point via prostaglandins.
• Hyperthyroidism: increased basal heat production.
• Anhidrosis: inability to sweat → heat intolerance.