<prompt> Gas used to measure the diffusion capacity of lungCONOCO2Nitrogen  <response>: Diffusion Capacity. Diffusion capacity assesses how well a tracer gas in inspired air can cross from the air into the blood. The test measures the absorption of a low concentration of carbon monoxide in inhaled air by hemoglobin in red blood cells that circulate through pulmonary capillaries. Ref: guyton and hall textbook of medical physiology 12 edition page number: 348,349,350 