A patient interface for delivery of a flow of breathable gas to an entrance of a patient's airways including at least an entrance of the patient's nares, comprising: a seal-forming structure arranged to surround at least the entrance of the patient's nares and constructed from a soft, flexible, resilient material; a plenum chamber; a positioning and stabilising structure configured to hold the seal forming structure in sealing position in use; a connection port for connection to an air circuit; a vent constructed and arranged to allow for washout of exhaled carbon dioxide; a heat and moisture exchanger for removable engagement with the patient interface, wherein the HME has an inwardly curved portion to avoid contact with the patient's nose or mouth; wherein the HME is configured to separate the plenum chamber of the patient interface into a first anterior chamber and a second posterior chamber, and wherein the second posterior chamber comprises the seal-forming structure and the first anterior chamber comprises the connection port and the vent; wherein the HME is positioned in the plenum chamber to such that the flow of breathable gas flows from the connection port through the HME to the patient; wherein the HME is positioned in the plenum chamber such that expiratory gases flow from the patient through the HME and are washed out through the vent, characterized in that the HME comprises a rigid frame circumferentially surrounding a peripheral surface of the HME, wherein the frame is configured to removably engage to an inner surface of the plenum chamber of the patient interface to position the HME in a flow path of the flow of breathable gas. a seal-forming structure arranged to surround at least the entrance of the patient's nares and constructed from a soft, flexible, resilient material; a plenum chamber; a positioning and stabilising structure configured to hold the seal forming structure in sealing position in use; a connection port for connection to an air circuit; a vent constructed and arranged to allow for washout of exhaled carbon dioxide; a heat and moisture exchanger for removable engagement with the patient interface, wherein the HME has an inwardly curved portion to avoid contact with the patient's nose or mouth; wherein the HME is configured to separate the plenum chamber of the patient interface into a first anterior chamber and a second posterior chamber, and wherein the second posterior chamber comprises the seal-forming structure and the first anterior chamber comprises the connection port and the vent; wherein the HME is positioned in the plenum chamber to such that the flow of breathable gas flows from the connection port through the HME to the patient; wherein the HME is positioned in the plenum chamber such that expiratory gases flow from the patient through the HME and are washed out through the vent, wherein the HME has an inwardly curved portion to avoid contact with the patient's nose or mouth; wherein the HME is configured to separate the plenum chamber of the patient interface into a first anterior chamber and a second posterior chamber, and wherein the second posterior chamber comprises the seal-forming structure and the first anterior chamber comprises the connection port and the vent; wherein the HME is positioned in the plenum chamber to such that the flow of breathable gas flows from the connection port through the HME to the patient; wherein the HME is positioned in the plenum chamber such that expiratory gases flow from the patient through the HME and are washed out through the vent,