A breathing tube for use in lung function diagnostics comprises a breathing tube body having first and second windows. The first window allows ultrasonic waves to pass from an exterior to an interior of the breathing tube body and vice versa, and the second window allows ultrasonic waves to pass from an interior to an exterior of the breathing tube body and vice versa. First and second sealing elements circumferentially extend around the breathing tube body on its outside and define a first breathing tube body section located between the first and second sealing elements, in which the first and second windows are located. At least one of the first and second sealing elements includes a first groove, a second groove, and a sealing lip located between the first groove and the second groove, which protrudes over a surface of the breathing tube body.
