An HME device, used in a closed breathing circuit of a ventilation system, has a housing with an inlet opening and with an outlet opening, an HME chamber (50a; 50b; 50c; 50d; 50e; 50f; 50g; 50h; 50i) arranged between the inlet opening and the outlet opening for receiving an HME medium and a switching mechanism (70a; 70b; 70c; 70d; 70e; 70f; 70g; 70h; 70i). The HME device can be switched over between an HME mode (M1), in which an HME fluid passage is provided from the inlet opening through the HME chamber to the outlet opening, and a bypass mode (M2), in which a fluid bypass passage is provided from the inlet opening past the HME chamber through a bypass channel (80a; 80b; 80d; 80e; 80f; 80h) in the housing to the outlet opening. The bypass channel is blocked with respect to the HME chamber in the bypass mode (M2).