Partial pressure = Total pressure x concentration (%)
Dry air has 20.93% O2.
Sea level pressure = 760mmHg
Water vapour pressure = 47mmHg
Thus,
=> Pressure exerted by gas in saturated moist air = 760-47 = 713mmHg
=> Partial pressure of O2 in saturated moist air = 713 x 0.2093 = 149.23mmHg
i.e. PiO2 = 149mmHg
PaO2 decreases with increasing age from about 20 years old
PAO2 = PIO2 - (PACO2/R) + F
NB: