A disposable cartridge for use in a hemodialysis machine has a blood flow path for carrying a volume of blood to be treated in a dialyser and a dialysate flow path, isolated from the blood flow path, for delivering a flow of dialysate solution through the dialyser. The cartridge is received in an engine section of the machine. The engine section has first and second platens which close when the cartridge is inserted to retain the cartridge. Actuators and sensors arranged on the second platen control operation of the cartridge.