Blood gas tension refers to the partial pressure of gases in blood.[1] There are several significant purposes for measuring gas tension;[2] the most common gas tensions measured are oxygen tension[3] (PxO2), the carbon dioxide tension[3] (PxCO2) and carbon monoxide tension[3] (PxCO). The subscript x in each symbol represents the source of the gas being measured; "a" meaning arterial,[3] "A" being alveolar,[3] "v" being venous,[3] "c" being capillary.[3]