A patient interface system configured for sealed delivery of a flow of breathable gas at a continuously positive pressure with respect to ambient air pressure to an entrance to a patient’s airways including at least the patient’s nares, wherein the patient interface system is configured to maintain a therapeutic pressure in a range of about 4 cmH2O to about 30 cmH2O above ambient air pressure in use, throughout the patient’s respiratory cycle, while the patient is sleeping, to ameliorate sleep disordered breathing, said patient interface system comprising: a patient interface comprising: a seal forming structure configured to seal with an area around the entrance to the patient’s airways including at least the patient’s nares, the seal forming structure being constructed from a soft, flexible, resilient material； a positioning and stabilising structure configured to maintain the seal forming structure in sealing contact with the area surrounding the entrance to the patient’s airways while maintaining the therapeutic pressure at the entrance to the patient’s airways； a plenum chamber configured to be pressurised at the therapeutic pressure in use； a connection port configured to connect to an air circuit； and a gas washout vent configured to allow a flow of patient exhaled gas to escape to ambient air to minimise rebreathing of exhaled carbon dioxide by the patient； and a heat and moisture exchanger (HME) comprising: a moisture-absorbent HME material； and a rigid frame releasably attached to the patient interface to support the HME material relative to the patient interface such that during use: the flow of breathable gas travels in a first direction from the connection port, through the HME material, and to the patient’s airways； and the flow of patient exhaled gas travels in a second direction, opposite the first direction, from the patient’s airways, through the HME material, and exits to atmosphere through the gas washout vent, wherein a patient-facing side of th
