The top GCA is the Mix Gas GCA, containing three gas tanks. In the Neurolab configuration, these tanks hold O2 (oxygen), HVR Mix (used for hypoxic ventilatory response test), and CO2 Mix (used for carbon dioxide rebreathe test). The bottom GCA is the In/Bolus GCA, which contains three tanks, two large and one small. For Neurolab, the two large tanks hold Lo Mix, a rebreathing gas used for the measurement of cardiac output and lung diffusing capacity.