Abstract: Systems for monitoring fluids such as blood being treated extracorporeally are disclosed, including a monitor for monitoring the fluid flow through a tube system between a fluid inlet, fluid treatment means and a fluid outlet. The monitor disclosed includes a monitor housing which contains at least one pressure sensor, and a cassette which is adapted to be removably coupled to the monitor housing, the cassette including pressure transmission ducts for transmitting the pressure within the tube system to a pressure transmitting cavity adapted to be juxtaposed with the pressure sensor when the cassette is coupled to the monitor housing so that the pressure within the tube system can be directly measured by the pressure sensor when the cassette is coupled to the monitor housing. In one embodiment of the monitor the cassette includes a portion of the tube system, while in another embodiment a conventional tube system can be affixed to the cassette.