A respirator comprises a respiratory air circulation unit with an adjustable displacement volume and a respiratory air supply line and a respiratory system comprising an inspiration loop, an expiration loop and valve elements mounted in these loops. The valve elements are controllable by means of a control unit, so that small as well as large displacement volumes can be executed by means of a single circulation unit with the smallest space requirements possible. For this purpose the inspiration phase is divided into several sub-phases, during which the control unit controls the valve elements and the respiratory air circulation unit 4 so that several fillings of the adjustable displacement volume are supplied to the inspiration loop and only then is the expiration phase initiated. Thus the displacement volume of the circulation unit can be kept small. One or a few inspiration sub-phases are sufficient for the artificial respiration of infants and premature infants, while a multitude of sub-phases can be executed for the artificial respiration of adults with a large respiratory volume.