Diving Physics and Physiology: Gas Laws, Bubbles, and Thermal Stress
Core concepts
This domain is the physics and physiology substrate for the clinical domains. Master the gas laws: Boyle (P×V constant - barotrauma), Dalton (total pressure = sum of partial pressures - gas toxicity and hypoxia), Henry (dissolved gas proportional to partial pressure - inert-gas loading and DCS), Charles/Gay-Lussac (temperature–pressure–volume), and Fick (diffusion). Understand absolute vs gauge pressure, ATA vs bar, and salt vs fresh water (10.3 m vs 10 m per atm).
Physiology: immersion causes a central fluid shift (increased preload, natriuresis, decreased vital capacity), cold increases gas solubility and inert-gas loading, and immersion diuresis is normal. Breathing gas density rises with depth and increases work of breathing; V/Q mismatch and shunt worsen with immersion. Hypothermia is a leading cause of diving death; cold incapacitation precedes hypothermia in immersion.
Traps
Questions test whether you know that gas solubility increases as temperature falls (a diver chilled after a dive off-gasses more slowly and is at higher DCS risk), that partial pressure, not percentage, drives toxicity, and that fresh water drowning differs from salt water in electrolyte effect (both are primarily hypoxic, not hyper/hyponatremic, in practice).
Worked rule of thumb
At 30 m (4 ATA), a 21% oxygen mix delivers a PO2 of 0.21 × 4 = 0.84 ATA - near the 1.4 ATA working limit. Use PO2 = FiO2 × ATA to screen any gas mix for hypoxia (<0.16 ATA) or toxicity (>1.4 ATA working, >1.6 ATA resting).