The<unk>  pO2 is not routinely measured but is calculated from blood<unk>  measurements by the<unk> :
p
A
O
2
=
p
I
O
2
−
p
A
C
O
2
/
R
+
F
{\displaystyle p_{A}O_{2}=p_{I}O_{2}-p_{A}CO_{2}/R+F\,}
where:
R is the Respiratory quotient (normally about 0.8)
pAO2 is the<unk>  pO2
pIO2 is the Inspired pO2, equal to about 150 mm Hg (0.21 x 713 mmHg at sea level).  The given pressure at sea level is due to atmospheric pressure (760 mmHg) minus the partial pressure of water vapor (47 mmHg), as<unk> <unk>  is completely saturated with water.  The mole fraction of oxygen is about 0.21 in dry atmospheric<unk> .
pACO2 is the<unk>  pCO2 (assumed to be equal to the measured arterial pCO2)
F is a correction factor (usually less than 2 mmHg)
Alveolar gas equation
Ventilation/perfusion ratio
Alveolar-arterial gradient
Arteriovenous oxygen difference
Volumetric flow rate
Diffusion capacity
Oxygen Enhancement Ratio
Pulmonary compliance