The Alveolar-arterial gradient<unk> , is used to compare the causes of hypoxemia.
A - a gradient = PAO2 - PaO2
Where:
PAO2 = alveolar PO2 (calculated from the alveolar gas equation)
PaO2 = arterial PO2 (measured in arterial blood)
PAO2 = PIO2 - PACO2/R
Where:
PIO2 = FIO2 x (760 mmHg - 47mmHg)  = inspired O2
       - FIO2 = 21% at sea level atmospheric pressure = 0.21
       - 760 mmHg = atmospheric pressure at sea level
       - 47 mmHg = H2O pressure
PACO2 = arterial PCO2 measured in plasma
R = respiratory exchange ratio or respiratory quotient = 0.8 under normal conditions
The normal<unk>  is < 10 mmHg, but can range from 5-20 mmHg in a normal individual. An increased<unk>  suggests a diffusion defect, V/Q (ventilation/perfusion) defect, or right-to-left shunt.
A-a gradient
Pressure gradient 
Volumetric flow rate
Alveolar-arterial gradient
Polarization density
Electrochemical gradient
Collapsing pulse
E/a ratio