The invention relates to a device for supplying gas to a patient, in particular a patient breathing spontaneously. Said device comprises a) a metering device (A) which is used to meter the gas to be inhaled by the patient (P) into the gas tank (B), b) a gas tank (B) which is used to store the gas to be inhaled by the patient (P), c) a demand valve (D) which regulates the volume flow of the gas inhaled by the patient (P), d) a reservoir (F) which is used to hold the gas exhaled by the patient (P), e) a carbon dioxide separating unit (G) which is used to separate carbon dioxide from the exhaled gas, f) a pump (H) which is used to return the exhaled gas purified of carbon dioxide into the gas tank (B), and g) an analysis/control unit (J) which analyzes the gas returned to the gas tank (B) and controls the metering device (A) according to the composition of said gas.
