The invention involves a device and method for ascertaining the functioning of the respiratory system and, in particular, for measuring the efficiency of alveolar ventilation of a patient. The device comprises an apparatus containing sensors for measuring the oxygen and carbon dioxide concentrations as well as the volume of air inhaled and exhaled by a patient. The patient is provided a mouthpiece for breathing into and out of the device, which subsequently measures the oxygen and carbon dioxide partial pressures of inhaled and exhaled air. From these measurements, the efficiency of alveolar ventilation during each tidal volume breath may be calculated.