| PXY | Partial pressure of gas Y in compartment X (mmHg or kPa) | B=barometric | O2 |
| FXY | Fraction of gas Y in compartment X (decimal or %) | I=inspired gas | CO2 |
| CXY | Content of gas Y in compartment X (mL per 100 mL of blood) | E=expired gas | H2O |
| SxO2 | Haemoglobin–oxygen saturation in compartment X (%) | Ē=mixed expired gas | |
| | | ET=end-tidal | |
| | | A=alveolar gas | |
| | | a=arterial blood | |
| | | c=capillary | |
| | | ec=end-capillary blood | |
| | | v̅=mixed venous blood | |
| V′X | Ventilation (mL·min−1) in compartment X | A=alveolar | |
| | | D=dead space | |
| Q′X | Blood flow (mL·min−1) in compartment X | t=total | |
| | | s=shunt | |
| V′A/Q′ | Regional ratio of alveolar ventilation to perfusion (unitless) | | |
| VX | Volume (mL) in compartment X | T=tidal | |
| | | D=dead space | |