Oxygen Exchange During Breathing: Why Exhaled Air Has Less O₂

The difference between inhaled and exhaled oxygen concentrations reflects pulmonary gas exchange.

Gas Concentrations (Approximate)

Gas	Inhaled Air	Exhaled Air
O₂	21%	15–16%
CO₂	0.04%	4–5%
N₂	78%	78% (inert)
H₂O	Variable	Saturated (~6% at 37°C)

Physiological Basis

1. Ventilation: Air reaches alveoli (dead space ~150 mL excluded).
2. Diffusion: O₂ crosses alveolar-capillary membrane (driven by PₐO₂ – PᵥO₂ gradient) → binds hemoglobin.
3. Cellular Respiration: Mitochondria use O₂ as final electron acceptor → ATP + CO₂ + H₂O.
4. CO₂ Transport: Dissolved, bicarbonate (70%), carbaminohemoglobin → lungs → exhaled.

Why Exhaled O₂ Is Not Zero

• Hemoglobin is not fully desaturated in venous blood (SvO₂ ~70–75%).
• Anatomic dead space (trachea, bronchi) — no gas exchange.
• Ventilation-perfusion mismatch.
• Oxygen extraction fraction ~25% at rest; up to 70–80% in maximal exercise.

Exercise Effect

During exercise: O₂ consumption ↑ → mixed venous O₂ ↓ → exhaled O₂ ↓ (can reach 12–13%). CO₂ production ↑ → exhaled CO₂ ↑. Ventilation increases disproportionately (hyperpnea).