Thermoregulation during exercise

Heat Balance - The Body's Thermostat

• Core principle: To maintain thermal balance, heat production must equal heat loss. The body tightly regulates core temperature around 37°C (98.6°F).
• Heat balance equation: $S = M - W \pm R \pm C \pm K - E$
  • S: Heat Storage
  • M: Metabolic Heat Production
  • W: Work Done
  • R, C, K: Radiation, Convection, Conduction
  • E: Evaporation
• During exercise, metabolic heat (M) is the primary heat source. Evaporation (E) via sweating becomes the dominant mechanism for heat dissipation.

⭐ The anterior hypothalamus is the central coordinating center for thermoregulation, responding to input from central and peripheral thermoreceptors to initiate heat loss or heat gain mechanisms.

Physiological Response - The Cooling Crew

• Primary Goal: Dissipate metabolic heat generated by muscles to maintain core temperature homeostasis (setpoint ~37°C).
• Central Control: The anterior hypothalamus detects increased blood temperature and orchestrates the cooling response.

• Key Mechanisms:
  • Evaporation: The most significant mechanism for heat loss during exercise. Its efficacy is reduced in high humidity.
  • Cutaneous Vasodilation: Increases blood flow to the skin, bringing heat from the core to the surface.

⭐ Sympathetic innervation of eccrine sweat glands is an exception to the rule: it is cholinergic (using acetylcholine), not adrenergic.

Heat Acclimatization - Training for the Tropics

• Timeline: Occurs over ~10-14 days with daily exercise in the heat (~90 min).
• Primary Adaptations:
  • Plasma Volume: ↑ Plasma volume expansion (~10-25%), improving cardiovascular stability.
  • Sweating:
    • Earlier onset at a lower core temperature.
    • ↑ Sweat rate, maximizing evaporative cooling.
    • ↓ Sweat [NaCl] (more dilute) via ↑ aldosterone, conserving electrolytes.
  • Cardiovascular: ↓ Heart rate and ↑ stroke volume at any given workload.
  • Thermoregulation: ↓ Core temperature response to exercise.

⭐ The initial and most important adaptation is plasma volume expansion, which supports stroke volume and allows for increased skin blood flow and sweating.

Heat Illnesses - When the System Fails

• Heat Cramps: Painful muscle spasms due to salt loss from heavy sweating. Core temperature is normal.
• Heat Exhaustion: Systemic symptoms (nausea, headache, weakness) from significant water & salt loss.
  • Core temperature is elevated but < 40°C (< 104°F).
  • Mental status remains intact.
• Heat Stroke: Medical emergency from failed thermoregulation.
  • Core temperature > 40°C (> 104°F).
  • Classic triad: Hyperthermia, altered mental status (delirium, coma), and often, anhidrosis (dry skin).

⭐ The hallmark of heat stroke is CNS dysfunction. Unlike heat exhaustion, patients are confused, delirious, or comatose.

High‑Yield Points - ⚡ Biggest Takeaways

• Exercise ↑ metabolic heat production, raising core body temperature.
• Evaporation of sweat is the primary mechanism for heat dissipation.
• Cutaneous vasodilation shunts blood to the skin, aiding heat loss.
• The hypothalamus is the central thermoregulatory center.
• Heat acclimatization results in earlier, more profuse sweating with less salt loss.
• Dehydration impairs thermoregulation by reducing sweat volume and skin blood flow.
• Failure can lead to exertional heatstroke, a life-threatening emergency.