Atmosphere & physiology

040 Human PerformanceChapter 2

The atmosphere and basic physiology explain why altitude affects pilots — through partial pressure of oxygen, gas expansion, and blood circulation.

Atmospheric composition

Key fact

At sea level: ~78% nitrogen, ~21% oxygen, ~0.03% CO₂, trace gases. Proportions remain constant up to ~100 km.

Altitude	Pressure (approx)	Notes
MSL	760 mmHg	PP O₂ ≈ 103 mmHg
10,000 ft	500 mmHg	PP O₂ ≈ 55 mmHg — critical for mental function
18,000 ft	380 mmHg	½ sea-level pressure
33,700 ft	190 mmHg	100% O₂ ≈ sea-level O₂ partial pressure
40,000 ft	140 mmHg	100% O₂ ≈ 55 mmHg (10,000 ft equivalent)

Atmospheric zones

Key fact

Physiological zone (0–10,000 ft) — sufficient oxygen for normal function. Physiological deficient zone (10,000–60,000 ft) — oxygen deficiency begins. Space equivalent zone (60,000 ft+) — even 100% O₂ under pressure insufficient without special protection.

Gas laws (exam essentials)

Formula — Boyle's law

Volume inversely proportional to pressure (constant temperature)
P × V = constant

Trapped gas expands on ascent — dysbarism, barotrauma.

Formula — Charles's law

Volume proportional to absolute temperature (constant pressure)

Formula — Dalton's law

Total pressure = sum of partial pressures

Explains hypoxia at altitude — lower PP O₂.

Formula — Henry's law

Gas dissolved in liquid proportional to partial pressure of that gas

Explains DCS — nitrogen coming out of solution on rapid decompression.

Exam trap

Mnemonic: “Henry’s sick (DCS), Dalton’s hypoxic” — know which law applies to which condition.

Key fact

Fick’s law — rate of gas diffusion across a membrane proportional to partial pressure difference. Governs O₂/CO₂ exchange in alveoli.

Respiratory system

Key fact

Path: nose/mouth → pharynx → larynx → trachea → bronchi → alveoli. Nose moistens, filters, warms air.

Key fact

Breathing rate controlled by CO₂ level in blood (respiratory centre in brain), not primarily by O₂. Too much CO₂ → faster breathing. Too little CO₂ → slower breathing.

Key fact

Normal breathing at rest: 12–20 cycles/min (often cited 16–18). Tidal volume ≈ 500 ml per breath.

Lung volume	Approx value
Tidal volume (TV)	500 ml
Inspiratory reserve	3,300 ml
Expiratory reserve	1,000 ml
Residual volume	1,200 ml
Total (male)	~5.5–6 L

Key fact

Alveolar partial pressures (approx): O₂ 102 mmHg, CO₂ 40 mmHg, water vapour 47 mmHg. CO₂ diffuses from blood to alveoli because alveolar PP CO₂ is lower than in venous blood.

Circulatory system

Key fact

Arteries carry blood from the heart. Veins carry blood to the heart. Coronary arteries supply the heart muscle itself.

Formula — Cardiac output

Cardiac output = stroke volume × heart rate
Typical: 70 ml × 70 bpm ≈ 4.9–5.2 L/min

Key fact

Healthy adult norms: pulse 60–80 bpm, blood pressure 120/80 mmHg, breathing 10–15/min at rest.

Key fact

Blood: ~55% plasma, ~45% cells. Red cells carry haemoglobin for O₂ transport. Anaemia = insufficient functioning haemoglobin → hypaemic hypoxia.

Key fact

Pressoreceptors in carotid and aortic vessels sense blood pressure — constrict vessels, increase heart rate and cardiac output when pressure is low (e.g. after blood donation).

Key disorders (brief)

Condition	Key point
Hypertension	High BP — increases heart attack risk
Angina	Narrowed coronary artery — reduced O₂ to heart muscle
Heart attack (MI)	Blocked coronary artery — tissue death; common >40 years
Stroke	Blood supply to brain cut off
Faint/syncope	Reduced blood/oxygen to brain — low BP