Basic Aviation Physiology and the Body's Response to Flight
The physiological environment of flight
Flight exposes the body to reduced barometric pressure, reduced oxygen partial pressure, cold, vibration, noise, low humidity and acceleration. The body responds through respiration, circulation and thermoregulation. The respiratory system exchanges gases in the alveoli by partial-pressure gradients; at altitude the percentage of oxygen in air stays about 21 per cent, but the partial pressure falls, so less oxygen enters the blood. This is why 'air is thinner' questions should be answered in terms of partial pressure, not percentage.
Circulation and oxygen transport
Haemoglobin carries most oxygen. Cardiac output, breathing rate and depth increase as the body compensates. Carbon monoxide binds haemoglobin far more strongly than oxygen, so a leaking cabin heater causes insidious hypoxia with few warning signs. Smoking and anaemia reduce oxygen-carrying capacity and raise susceptibility to hypoxia.
Traps in questions
Candidates often confuse the percentage of oxygen with its partial pressure, or assume that breathing faster always raises blood oxygen. Hyperventilation lowers carbon dioxide and can cause tingling and dizziness that mimic hypoxia. Also distinguish the effects of cold (shivering, reduced dexterity) from those of vibration (blurred vision, fatigue).
Worked example
At 10,000 ft cabin altitude, oxygen still makes up 21 per cent of the air, but the partial pressure is roughly 70 per cent of sea level. Rule of thumb: above about 10,000 ft, supplemental oxygen or descent is required, and the higher the cabin, the shorter the time of useful consciousness.