A system and method are configured to monitor composition of a flow of breathable gas being provided to a subject. A measurement flow of breathable gas is diverted in order to monitor composition of the flow of breathable gas, and then the measurement flow of breathable gas is returned to the flow of breathable gas. This will tend to conserve the constituent gases within the flow of breathable gas, which may be significant in instances where the flow of breathable gas is being used to deliver medicaments or drugs (e.g., relatively expensive anesthetic, and/or other medicaments or drugs). Since the measurement flow of breathable gas will tend to have contaminants (e.g., mucus blood, medications, or other materials), routing the measurement flow of breathable gas back into the flow of breathable gas constitutes a placement solution for the contaminants. In system 10, the components that contact the measurement flow of breathable gas may be disposable, in order to facilitate reintroduction of the measurement flow of breathable gas back into the flow of breathable gas.
