Apparatus for the respirating of patients with means for the circulating in one direction in a line system of gas, with supplies for the various components of the gas and with a line system part in which a carbon dioxide absorber device is provided, whereby the line system further is provided with a by-pass line for leading of the respiratory gas outside of the line system part with the absorber device, characterized in that a control unit is provided by means of which the operating of the closing means takes place in dependence on one or more measurements of the concentration of carbon dioxide in the respiratory gas in the line system, and wherein the line system is provided with a circulation blower.