An apparatus for extracorporeal treatment of blood can carry through automatically such functions as dialysis and oxygenation compensating, in the process, for changes in the condition of the patient. A fail-safe monitor receives signals from a number of sensors all of which must provide signals within acceptable ranges in order for the apparatus to continue to function. Deviation of a signal outside the corresponding acceptable range for longer than a preset period causes the fail-safe monitor to stop the flow of blood taken from the subject and to activate an alarm. The apparatus may be so constructed that the ducting through which blood and other fluids flow can be caused to act as valves and pumps by suitably applied pressure. Also, such portions of the apparatus can be made in a form which is sufficiently economical so that they can be regarded as disposable. The apparatus may be designed so that it can be programmed for specific patients and for treatment of specific conditions.