A coupling part of a drainage tube unit for aspirating body fluids by a suction pump includes a patient-side drainage attachment, a pump-side drainage orifice for connection to a drainage tube, and a drainage channel connecting the drainage attachment and the drainage orifice to each other. The coupling part also includes a pump-side first service orifice for connection to a service tube, a second service orifice ending in the interior of the coupling part, a service channel connecting these first and second service orifices, and a device which separates the service channel from the drainage channel. The device permits delivery of fluid from the service channel into the drainage channel and prevents delivery of particles and liquid from the drainage channel into the service channel. The device is arranged in the coupling part at a location that allows a flow through the drainage channel from the drainage attachment to the drainage orifice without passage through the device.