A molecular flow sensor includes a flowmeter integrated with an in airway laser gas analyser. The laser gas analyser spectroscopically analyses the concentrations of gases in a measurement space. The gas flow is measured by measuring the pressure drop across the flow-sensing mesh screens using a differential pressure sensor. Helical baffles are provided upstream and downstream of the measurement space to provide a low resistance gas flow path while blocking ambient light from entering the measurement space, and flow-conditioning elements of mesh or metal foam are provided between the helical baffles and the flow-sensing meshes. Pressure averaging is conducted circumferentially around the gas flow path. At least two independent pressure sensors may be provided, switchable out independently to allow regular calibration while the other is still measuring. The flow sensor is particularly useful for measuring respiratory gas exchange such as oxygen consumption or carbon dioxide production by a respiring subject.