The partial pressures (in mm Hg) of oxygen (O₂) and carbon dioxide (CO₂) at alveoli (the site of diffusion) are:

To determine the typical alveolar partial pressures of oxygen and carbon dioxide we apply the alveolar gas equation for O₂ and use the fact that alveolar CO₂ pressure equals the arterial CO₂ pressure.

1. Constants at sea level
– Atmospheric pressure (Pₐₜₘ) ≈ 760 mm Hg
– Water‑vapour pressure in the airway (Pₕ₂ₒ) ≈ 47 mm Hg
– Fraction of inspired O₂ (FᵢO���) = 0.21 (21 %)
– Respiratory quotient (R) ≈ 0.8
– Arterial (and thus alveolar) pCO₂ ≈ 40 mm Hg

2. Alveolar gas equation for O₂

Insert the known values:

Step‑by‑step calculation:

In clinical textbooks the normal alveolar pO₂ is often rounded to about **104 mm Hg** to account for physiological variations.

3. Alveolar pCO₂
Alveolar CO₂ quickly equilibrates with arterial CO₂, so the alveolar pCO₂ is essentially the measured arterial value, ≈ **40 mm Hg**.

Combining the two results gives the typical alveolar gas pressures:

pO₂ ≈ 104 mm Hg and pCO₂ ≈ 40 mm Hg.

Thus the correct option is “pO₂ = 104 and pCO₂ = 40”.