Nitrogen Levels in Inhaled vs. Exhaled Air: Why the Slight Increase?

Atmospheric air composition is remarkably stable; the primary respiratory gases (O₂, CO₂) change significantly between inhalation and exhalation, while nitrogen (N₂) remains largely inert.

Typical Composition

Gas	Inhaled Air	Exhaled Air (Alveolar)
Nitrogen (N2)	78.08%	78.60% (approx.)
Oxygen (O2)	20.95%	13.6–16%
Carbon Dioxide (CO2)	0.04%	4–5.3%
Water Vapor (H2O)	Variable (0–4%)	6.2% (saturated at 37°C)
Argon, trace gases	~0.93%	~0.93%

Why Nitrogen % Increases Slightly

• N₂ is metabolically inert (not consumed or produced in significant amounts).
• O₂ is consumed (~250 mL/min at rest) and CO₂ is produced (~200 mL/min).
• Water vapor is added in the airways (humidification to 44 mg/L at 37°C).
• The removal of O₂ and addition of CO₂ and H₂O change the denominator, causing the fractional concentration of N₂ to rise slightly even though the absolute amount of N₂ exchanged is near zero.

Physiological Note

• N₂ partial pressure equilibrates between alveolar air and blood; no net diffusion.
• In decompression sickness ("the bends"), rapid pressure reduction causes dissolved N₂ to form bubbles—proof that N₂ does dissolve in tissues under pressure.
• N₂ washout (breathing 100% O₂) is used to denitrogenate before surgery or hyperbaric treatment.