Blood gas

Arterial oxygen partial pressure. Directly measured from arterial blood.
Broadly speaking, it equals the blood plasma oxygen concentration.
Arterial oxygen saturation. Directly measured from arterial blood.
Broadly speaking, it equals the fraction of available haemoglobin sites binding oxygen.
Arterial oxygen saturation. Measured by a pulse oximeter. Like SaO2, but may erroneously mistake carboxyhaemoglobin, methaemoglobin and sulphaemoglobin for oxyhaemoglobin. Cannot be reliable measured when there is no pulse!
Fraction of inspired oxygen. This is 21% for room air. The maximum dose of oxygen that can be given on a medical ward is 60% (via a rebreathing mask with 15 litres of oxygen passing through it every minute). Concentrations higher than 60% require artificial ventilation on the intensive care unit.
For the sake of examiners, the alveolar gas equation is
PAO2 = FiO2 x (PB - PH2O) - PACO2 x (FiO2 + (1 - FiO2)/RQ)
Alveolar oxygen partial pressure. Cannot be measured directly.
Atmospheric pressure.
Saturated vapor pressure of water. This is 6.3 kPa at 37 deg C.
Alveolar carbon dioxide partial pressure. Assumed to be equal to arterial carbon dioxide partial pressure.
Respiratory quotient. The ratio of the volume of carbon dioxide produced to the volume of oxygen consumed. The value is normally assumed to be 0.8 (i.e., less carbon dioxide is produced than oxygen consumed).
For practical purposes, the more important relationship is
FiO2/PaO2 = constant.
PAO2/PaO2 = constant, is more accurate, but more difficult to calculate. Normal range is 0.74 to 0.90.

The other useful equation is oxygen content:

CaO2 = (SaO2 x Hb x 1.34) + 0.022 x PaO2.
The normal range for CaO2 is 16 to 22 ml/dl blood.