Aerospace Physiology: Gas Laws, Oxygen, Pressure, and Acceleration
The physical foundation
Flight physiology is built on four gas relationships. Boyle's law explains barotrauma and trapped-gas expansion with altitude. Dalton's law explains partial pressures and why alveolar oxygen falls with altitude. Henry's law explains dissolved nitrogen and decompression sickness. Graham's law and diffusion explain oxygen transfer at the alveolus. Candidates must distinguish hypoxic hypoxia (low alveolar oxygen), hypemic hypoxia (anemia, carbon monoxide), stagnant hypoxia (circulatory failure), and histotoxic hypoxia (cellular poisoning).
Oxygen and pressure
Know time of useful consciousness (TUC) as a range, not a constant: roughly 20-30 minutes at FL180, a few minutes near FL250, and only seconds above FL400, shortened by cold, exertion, and rapid decompression. Recognize that hyperventilation can mimic hypoxia, and that 100% oxygen does not correct every hypoxic cause. Pressure breathing, positive-pressure masks, and cabin altitude limits all follow from these principles. Barotrauma of ear, sinus, and lung, plus altitude decompression sickness, are predictable consequences of pressure change.
Acceleration and orientation
Understand +Gz effects, G-suit function, the anti-G straining maneuver, and G-induced loss of consciousness from cerebral hypoperfusion. Spatial disorientation arises when vestibular and visual cues conflict; somatogyral and somatogravic illusions are classic examples. Night vision depends on rods, dark adaptation, and avoiding bright light.
Traps
- Treating TUC as a fixed table value.
- Confusing hypoxic with hypemic hypoxia.
- Assuming oxygen resolves hyperventilation or CO poisoning.
- Misapplying absolute versus relative pressure in gas-law questions.
- Believing the anti-G straining maneuver is a single sustained Valsalva rather than a cyclic effort.
Worked example
Rapid decompression at FL350: don the mask and establish oxygen first, then initiate emergency descent to FL100 or minimum safe altitude. Rule of thumb: above FL400, assume seconds, not minutes.