A branching unit for delivering a respiratory gas of a subject is disclosed herein. The branching unit includes a first limb for delivering an expiratory gas during an expiratory phase and a second limb for delivering an inspiratory gas during an inspiratory phase. The branching unit also includes a third limb for delivering both the expiratory gas and the inspiratory gas and a common branching point for the first limb, the second limb and the third limb. The first limb, the second limb and the third limb include a volume for the respiratory gas and which volume includes both an active volume with the gas exchanging between the inspiratory phase and the expiratory phase and a dead volume for the respiratory gas with insufficient gas exchange from the inspiratory phase to the expiratory phase and the dead volume being less than 1 ml.