Oxygen cascade

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

Ideal alveolar gas equation

PAO2 = PIO2 - (PACO2/R) + F

• PIO2
  = dry barometric pressure x inspired O2 concentration
  = (Atomspheric pressure - water vapour pressure) x FIO2
  = (760 - 47) x 0.2093
  ~ 149 mmHg
  * Assuming 37C and sea level
• R = Respiratory ratio
  = VCO2/VO2 (V with a dot on top, i.e. flow)
  * Normal = 0.8
• F = Correction factor
  = PACO2 x FIO2 x ((1-R)/R)
  = 40 x 0.21 x 0.2/0.8
  ~ 2

NB:

• PACO2, not PaCO2
  * Although they are really the same value