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Breathing circuit. Fresh gas consisting of air or 100% oxygen is supplied through the fresh gas inlet (FGI) at a rate equal to the subject's resting ventilation and flows into the spirometer, from which the subject breathes. If ventilation increases, the reservoir empties during the course of the breath and the remainder of the breath is drawn through the demand regulator. Normocapnia is maintained when 6% carbon dioxide is supplied through the demand gas inlet (DGI).
