Basic Aviation Physiology and Health
Basic Aviation Physiology and Health
The exam tests how the body responds to altitude and pressure, and whether you can separate hypoxia from hyperventilation, or explain why a decompression demands oxygen first and descent second. Questions are often short clinical scenarios.
Respiration, circulation and hypoxia
- Alveolar oxygen partial pressure falls with altitude; at FL350 ambient pressure is about 238 hPa and without oxygen time of useful consciousness is roughly 30-60 seconds; at FL400 it can be 15-20 seconds.
- Hypoxic hypoxia: low alveolar PO2 (altitude, smoke, oxygen failure). Stagnant hypoxia: poor circulation. Anaemic hypoxia: low haemoglobin or carbon monoxide poisoning. Histotoxic hypoxia: cells cannot use oxygen.
- Hyperventilation lowers CO2, causing dizziness, tingling, cramps and carpopedal spasm; it can mimic hypoxia. Treatment: control breathing, and if altitude is a factor, give oxygen and descend.
- Blood: haemoglobin carries oxygen; carbon monoxide binds about 200-250 times more readily than oxygen, so CO poisoning is insidious.
Ears, eyes and decompression
- The Eustachian tube equalises middle-ear pressure; during descent, a blocked tube causes barotitis. Valsalva and Toynbee manoeuvres help, but never fly with a severe cold.
- The eye: rods are for night vision and are in the periphery; cones give colour and detail in the fovea. Dark adaptation takes about 20-30 minutes; use off-centre viewing at night and avoid bright white light.
- Decompression: slow decompression may cause subtle hypoxia; rapid decompression produces a loud bang, fog, temperature drop and lung expansion. Do not hold your breath; don oxygen, establish communication and descend to below FL100.
Worked example
At FL350 a cabin window seal fails and the cabin altitude climbs rapidly. The first action is oxygen mask on and establish communication, then descend. The reasoning: time of useful consciousness at FL350 is only about 30-60 seconds, so the mask buys time; descent reduces the pressure gradient and improves alveolar PO2. Attempting to diagnose the problem first is the classic error.
Traps
Do not say hyperventilation is treated by breathing faster; do not confuse time of useful consciousness with the time to complete loss of consciousness; do not claim oxygen is unnecessary after decompression until the cabin altitude is actually below FL100; and do not forget that CO poisoning may present with cherry-red skin only late, so headache and confusion matter more.