Exercise in environmental extremes

Exercise in the Heat - Sweating Bullets

• Primary Challenge: Dissipate metabolic heat to prevent dangerous hyperthermia.
• Physiological Response:
  • Blood flow shunted to skin for evaporative cooling (sweating).
  • ↓Effective circulating volume → ↓stroke volume.
  • Compensatory ↑heart rate to maintain cardiac output.
• Acclimatization (Adaptations over 10-14 days):
  • ↑Plasma volume.
  • Earlier onset of sweating at a lower core temperature.
  • ↑Sweat rate but with ↓[NaCl] (conserves salt).
  • ↓Heart rate response for a given workload.
• Clinical Risk: Heat stroke if core temp >40°C (104°F) with CNS dysfunction.

⭐ High-Yield: The most critical adaptation in heat acclimatization is the expansion of plasma volume, which improves cardiovascular stability and supports the sweating response.

Exercise in the Cold - Frosty Feats

• Primary Goal: Maintain core body temperature against a cold gradient.
• Key Physiological Responses:
  • Peripheral Vasoconstriction: Shunts blood to the core, preserving heat. This ↑ central blood volume & stroke volume.
  • Shivering: Involuntary muscle contractions that ↑ metabolic heat production up to 5x resting rate; heavily relies on glycogen.
• Metabolic Adjustments:
  • ↑↑ Glycogenolysis for shivering & exercise.
  • ↓ Free fatty acid (FFA) mobilization due to subcutaneous vasoconstriction.
• Major Risks:
  • Hypothermia: Core temperature <35°C (95°F).
  • Frostbite: Freezing of peripheral tissues.
  • Exercise-Induced Bronchospasm: Triggered by cold, dry air.

⭐ Paradoxically, despite ↑ metabolic rate from shivering, maximal oxygen uptake ($VO_2$ max) is reduced in the cold due to impaired enzyme function and reduced maximal heart rate.

Exercise at Altitude - Thin Air Acclimation

• Primary Challenge: Decreased atmospheric pressure → ↓ partial pressure of inspired oxygen ($P_iO_2$) → alveolar hypoxia.
• Immediate Response (Hours):
  • Hypoxemia stimulates peripheral chemoreceptors → hyperventilation.
  • Leads to respiratory alkalosis (↓ $PaCO_2$).
• Acclimatization (Days to Weeks):
  • Renal: Kidneys excrete bicarbonate ($HCO_3^−$) to correct alkalosis.
  • Hematologic: ↑ Erythropoietin (EPO) → ↑ RBC mass & $O_2$ carrying capacity.
  • Cellular: ↑ 2,3-BPG shifts O₂-Hb curve right, improving $O_2$ unloading.

⭐ The initial hyperventilation is crucial but causes respiratory alkalosis; renal bicarbonate excretion is the key compensatory step to normalize pH over days.

High‑Yield Points - ⚡ Biggest Takeaways

• Heat acclimatization involves earlier, more dilute sweating and an expanded plasma volume to improve thermoregulation.
• Heat stroke is a medical emergency defined by CNS dysfunction and a core body temperature >40°C.
• Acute altitude exposure triggers hypoxia-driven hyperventilation, resulting in respiratory alkalosis.
• Long-term altitude acclimatization is marked by EPO-stimulated erythrocytosis, increasing oxygen-carrying capacity.
• At altitude, increased 2,3-BPG levels shift the oxygen-hemoglobin curve to the right, facilitating oxygen unloading to tissues.