Exercise and Thermoregulation

Heat Balance in Exercise - Sweaty Science

• Exercise ↑ metabolic heat production (muscles ~75-80% inefficient).
• Heat dissipation vital to prevent hyperthermia.
• Key Heat Loss Mechanisms:
  • Evaporation (Sweating): Primary (~80% of loss), especially in heat.
    • Effectiveness ↓ by high humidity.
    • Sweat rate: 1-2 L/hr (can exceed 3 L/hr).
  • Convection: Airflow over skin.
  • Radiation: To cooler objects.
  • Conduction: Direct contact (minor).
• Heat Balance: $H_{gain} = H_{loss} \pm \Delta S_{body}$ (Heat Storage).
• Core temperature: Can safely rise to 38-40°C; >40°C risks heat illness.

⭐ Evaporation of 1 liter of sweat dissipates approximately 580 kcal of heat.

Physiological Responses to Exercise in Heat - Code Red Systems

Overwhelmed heat dissipation during intense exercise in heat triggers critical system failure, a medical emergency. Key systems affected:

• Cardiovascular System:
  • Extreme vasodilation + dehydration → ↓ effective blood volume.
  • Results in ↓ cardiac output, hypotension, ↑↑ heart rate, potential myocardial ischemia.
• Central Nervous System (CNS):
  • Direct neuronal thermal injury + cerebral hypoperfusion.
  • Symptoms: confusion, delirium, ataxia, seizures, coma.
• Musculoskeletal & Renal Systems:
  • Exertional rhabdomyolysis → myoglobinuria → acute kidney injury (AKI).
  • Direct heat damage to renal tubules also contributes.

⭐ Exertional Heat Stroke (EHS) is characterized by a core body temperature exceeding 40°C (104°F) coupled with significant central nervous system dysfunction.

Exertional Heat Illnesses - Exercise Heatstroke & Co.

• Spectrum: Heat Cramps → Heat Syncope → Heat Exhaustion → Exertional Heat Stroke (EHS).
• Heat Cramps: Muscle spasms; normal Core Body Temperature (CBT). Tx: Electrolyte fluids.
• Heat Syncope: Fainting post-exercise; peripheral vasodilation. Tx: Supine, elevate legs.
• Heat Exhaustion: CBT <40°C; fatigue, nausea, profuse sweating. CNS intact. Tx: Cool, hydrate.
• Exertional Heat Stroke (EHS): Medical emergency! CBT >40°C + CNS dysfunction (e.g., confusion, coma).
  • Rapid cooling (e.g., cold water immersion) is life-saving.
  • Complications: Rhabdomyolysis, multi-organ failure.

  ⭐ EHS is primarily defined by a core body temperature >40°C combined with central nervous system dysfunction. Sweating may still be present.

• Exercise-Associated Hyponatremia (EAH): Due to overhydration with hypotonic fluids. Symptoms: Confusion, nausea. Serum Na+ <135 mEq/L differentiates it from heatstroke if CBT is not significantly elevated or if symptoms persist despite cooling for EHS suspects where EAH might co-exist or be the primary issue.

Acclimatization & Safe Practices - Beat the Heat Hacks

• Heat Acclimatization (HA):
  • Achieved: 7-14 days (daily moderate exercise in heat).
  • Adaptations:
    • ↑ Plasma volume (~10-12%, up to 27%).
    • ↑ Sweat rate (earlier, dilute, wider distribution).
    • ↓ Core temp & HR at same workload.
    • ↓ Salt loss (sweat/urine).
  • 📌 Mnemonic "SWEAT" (sweat glands): Sooner, Wider, Earlier, Amount ↑, Thinner.
  • Lost: 1-4 weeks if no heat exposure.
• Safe Exercise Tips:
  • Hydrate: Pre (~500ml 2-3h prior), during (150-250ml/15-20min), post. Urine: pale yellow.
  • Dress: Light, loose, wicking.
  • Timing: Avoid 10 AM - 4 PM.
  • Pace: Gradual ↑ intensity/duration.
  • Aware: Heat illness signs (dizziness, nausea). Buddy system.
  • Cool: Water spray, wet towels.

⭐ Plasma volume expansion is the earliest HA adaptation (1-3 days), boosting cardiovascular stability and sweat capacity.

High‑Yield Points - ⚡ Biggest Takeaways

• Exercise ↑ core body temperature due to ↑ metabolic heat production.
• Key heat dissipation: evaporative cooling (sweating) and cutaneous vasodilation.
• Significant sweat loss risks dehydration and electrolyte disturbances (Na+, Cl-).
• Cardiovascular drift: ↑ HR, ↓ SV at constant workload, especially in heat.
• Acclimatization: earlier/↑ sweating, dilute sweat, ↑ plasma volume, better heat tolerance.
• Heat stroke: medical emergency, core temp >40°C, CNS dysfunction.
• High ambient humidity severely impairs evaporative heat loss.