A portable hemodialysis system is provided including a disposable cartridge and a reused dialysis machine. The disposable cartridge includes a dialysate flow path and a blood flow path which flow in opposite directions through a dialyzer. The disposable cartridge includes a filter for removing waste products from the dialysate, and pressure and fluid flow sensors for measuring the pressure and fluid flow in the dialysate flow path and blood flow path. Preferably, the filter has a vapor membrane for releasing gases including ammonia, but not liquids such as the dialysate. The reused dialysis machine possesses a reservoir for dialysate, an ammonia sensor adjacent to the vapor membrane, a level sensor, a blood leak sensor, a venous blood line pressure sensor, a venous blood line bubble detector, pump motors, and a processor connected to the motors and sensors for controlling and monitoring hemodialysis treatment.
