Inhaled vs. Exhaled Air: Understanding the Key Differences

Compositional Differences Between Inspired and Expired Air

Gas exchange in the alveoli alters the partial pressures of O₂, CO₂, and H₂O. Typical values at sea level, rest:

Component	Inhaled (Dry Air)	Exhaled (Alveolar)	Change
O₂	20.95% (~160 mmHg)	13–16% (~100 mmHg)	↓ 4–7%
CO₂	0.04% (~0.3 mmHg)	4–5.3% (~40 mmHg)	↑ ~100×
N₂	78.08% (~596 mmHg)	74–75% (~565 mmHg)	Slight ↓ (dilution by H₂O/CO₂)
H₂O vapor	Variable (0–4% RH)	6.2% (47 mmHg, saturated at 37 °C)	↑ to saturation
Temperature	Ambient	37 °C (core)	Warmed
Particulates / Pathogens	Present	Filtered by respiratory mucosa	Reduced

Physiologic Basis

• O₂ diffuses into pulmonary capillary blood; CO₂ diffuses out.
• Air is humidified to 100% RH and warmed to body temperature in conducting airways.
• N₂ is physiologically inert (except at high pressure → narcosis).

Clinical Relevance

• End‑tidal CO₂ (EtCO₂) monitoring approximates arterial CO₂ in healthy lungs.
• Expired O₂ fraction used in indirect calorimetry (metabolic cart).
• Inspired gas composition manipulated in anesthesia, hyperbaric therapy, altitude simulation.