A patient module (10) is intended for use when ventilating a patient with a pressure source (24) that can be fluidically coupled via the patient module (10) to a patient interface (26), which can be connected to the airways of a patient. The patient module (10) includes a housing (12) and a valve section (14) in the housing (12) as well as an HME filter (30) spaced apart from the valve section (14). The HME filter (30) is located upstream of the valve section (14) in relation to an expiratory volume flow, so that the HME filter (30) divides an interior of the housing (12) into a dry area and an area coming into contact with the moisture carried along by the exhaled breathing gas. The valve section (14) is located in the dry area. A process for operating the patient module (10) includes calibration steps.