Chemical analysis of the pH, carbon dioxide and oxygen concentrations, and oxygen saturation of the blood. This analysis is used to diagnose serious metabolic and respiratory disorders. It may be performed using arterial or venous blood, although only arterial blood gas analysis evaluates lung function; the specimen may be obtained from numerous sites. Mixed venous samples may be obtained from the right atrium of the heart. The blood sample is usually collected in a heparinized syringe, with care being taken to ensure that the specimen is immediately placed on ice (to avoid misinterpretations caused by metabolism) and not exposed to air (to prevent oxygenation of the sample.