The P_aO₂ is measured in a blood gas analyzer using a heparinized arterial blood sample.

It only measures the PO₂ in plasma, and adequate oxygenation cannot be assured until the Hb concentration is known. Even then the relationship with C_aO₂ is non-linear.

Oxygen cascade

The normal P_aO₂ (13 kPa) is reached after a three stage fall from the atmospheric level (21 kPa), known as the oxygen cascade (Fig 1):

1. In the upper airway, humidification adds water vapour
2. In the alveoli, O₂ is taken up in exchange for CO₂
3. In the circulation, from the small physiological shunt caused by the bronchial circulation and thebesian veins