Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Blood gas tension refers to the partial pressure of gases in blood. There are several significant purposes for measuring gas tension; the most common gas tensions measured are oxygen tension (PxO2), the carbon dioxide tension (PxCO2) and carbon monoxide tension (PxCO). The x factor in the symbol for each would be replaced by the source of the gas being measured; "a" meaning arterial, "A" being alveolar, "v" being venous, "c" being capillary.