Abstract: A respiratory apparatus, which receives exhaled gas from a user, removes carbon dioxide from and introduces oxygen into the exhaled gas, to present a breathing gas to a user. A respiratory flow transducer in the respiratory apparatus is subjected to the breathing gas demand of the user. Oxygen is introduced into the breathing system by an oxygen flow regulator connected to an oxygen supply inlet. The respiratory flow transducer and the oxygen flow regulator are connected by a linkage. The linkage constrains the oxygen flow regulator and respiratory flow transducer to operate together whereby there is a substantially constant ratio between the breathing gas flow rate and the oxygen flow rate.An air bleed system can be connected to an air inlet to introduce air into the respiratory apparatus and to displace a portion of the gas from the respiratory apparatus.