A breathing circuit includes a flexible hollow tube having a generally circular cross section and a flexible printed circuit board assembly disposed in the flexible hollow tube. The flexible printed circuit board assembly defines part of at least one inspiratory passage within the flexible hollow tube and part of at least one expiratory passage within the flexible hollow tube. The flexible printed circuit board assembly is configured to heat airflow within one or more of the at least one inspiratory passage and the at least one expiratory passage, and monitor a first property of the airflow within one or more of the at least one inspiratory passage and the at least one expiratory passage at multiple locations along the axial length of the flexible hollow tube.