A respiratory assistance apparatus configured to provide a heated and humidified gases stream comprising a binary gas mixture of atmospheric air blended with a supplemental gas, the apparatus comprising: a main housing (26, 28) comprising a gases inlet assembly, the gases inlet assembly comprising: a gases inlet or inlets configured to receive atmospheric air; and a supplemental gas connection inlet configured to receive a supply of the supplemental gas from a supplemental gas supply for blending with the atmospheric air to form a gases stream comprising the binary gas mixture; a blower unit configured to pressurise the gases stream, wherein the gases inlet assembly is upstream of the blower unit such that the atmospheric air and supplemental gas are blended together into the binary gas mixture before entering the blower unit; a humidification unit configured to heat and humidify the gases stream, wherein the blower unit and the humidification unit are integrated into the main housing; a gases outlet following the blower unit and humidification unit for the gases stream; a flow path for the gases stream through the main housing from the gases inlet assembly through the blower unit and humidification unit to the gases outlet; a sensor assembly provided in the main housing, the sensor assembly comprising an ultrasound gas composition sensor system for sensing one or more gas concentrations of the gases stream comprising the binary gas mixture prior to the humidification unit, the ultrasound gas composition sensor system comprising a pair of transmitter-receiver transducers (100, 102) that are configured for transmitting bi-directional acoustic pulses through the gases stream for generating a speed of sound signal indicative of the speed of sound in the gases stream in the vicinity of the sensor assembly; and a controller that is operatively connected to the pair of transmitter-receiver transducers and which is configured to generate one or more gas concentration signals indicative of the gas concentration(s) within the gases stream based at least on the speed of sound signal. a main housing (26, 28) comprising a gases inlet assembly, the gases inlet assembly comprising: a gases inlet or inlets configured to receive atmospheric air; and a supplemental gas connection inlet configured to receive a supply of the supplemental gas from a supplemental gas supply for blending with the atmospheric air to form a gases stream comprising the binary gas mixture; a blower unit configured to pressurise the gases stream, wherein the gases inlet assembly is upstream of the blower unit such that the atmospheric air and supplemental gas are blended together into the binary gas mixture before entering the blower unit; a humidification unit configured to heat and humidify the gases stream, wherein the blower unit and the humidification unit are integrated into the main housing; a gases outlet following the blower unit and humidification unit for the gases stream; a flow path for the gases stream through the main housing from the gases inlet assembly through the blower unit and humidification unit to the gases outlet; a sensor assembly provided in the main housing, the sensor assembly comprising an ultrasound gas composition sensor system for sensing one or more gas concentrations of the gases stream comprising the binary gas mixture prior to the humidification unit, the ultrasound gas composition sensor system comprising a pair of transmitter-receiver transducers (100, 102) that are configured for transmitting bi-directional acoustic pulses through the gases stream for generating a speed of sound signal indicative of the speed of sound in the gases stream in the vicinity of the sensor assembly; and a controller that is operatively connected to the pair of transmitter-receiver transducers and which is configured to generate one or more gas concentration signals indicative of the gas concentration(s) within the gases stream based at least on the speed of sound signal.