A system comprising: a blood fluid removal device comprising (i) an inlet for receiving blood from a patient, (ii) an outlet for returning blood to the patient, (iii) a medium for removing fluid and contaminants from the blood, the medium being positioned between the inlet and the outlet, (iv) a source of dialysate, and (v) a fluid path carrying the dialysate from the source to the medium; a concentrate source (410, 420, 430) housing a concentrate solution comprising concentrated electrolyte or pH buffer; a concentrate flow control element (415, 425, 435) configured to control the rate at which the concentrate solution enters the fluid path from the concentrate source; characterized by: a first sensor configured to monitor an indicator of blood electrolyte concentration or blood pH in the blood before the blood enters the medium; a second sensor configured to monitor an indicator of blood electrolyte concentration or blood pH in the blood after the blood exits the medium; and control electronics in operable communication with the first and second sensors and the concentrate flow control element, wherein the control electronics are configured to compare data acquired from the first sensor to data acquired from the second sensor and wherein the control electronics are configured to adjust the rate at which the concentrate solution enters the fluid path based on the comparison of the data acquired from the first sensor and the second sensor. a blood fluid removal device comprising (i) an inlet for receiving blood from a patient, (ii) an outlet for returning blood to the patient, (iii) a medium for removing fluid and contaminants from the blood, the medium being positioned between the inlet and the outlet, (iv) a source of dialysate, and (v) a fluid path carrying the dialysate from the source to the medium; (i) an inlet for receiving blood from a patient, (ii) an outlet for returning blood to the patient, (iii) a medium for removing fluid and contaminants from the blood, the medium being positioned between the inlet and the outlet, (iv) a source of dialysate, and (v) a fluid path carrying the dialysate from the source to the medium; a concentrate source (410, 420, 430) housing a concentrate solution comprising concentrated electrolyte or pH buffer; a concentrate flow control element (415, 425, 435) configured to control the rate at which the concentrate solution enters the fluid path from the concentrate source; characterized by: a first sensor configured to monitor an indicator of blood electrolyte concentration or blood pH in the blood before the blood enters the medium; a second sensor configured to monitor an indicator of blood electrolyte concentration or blood pH in the blood after the blood exits the medium; and control electronics in operable communication with the first and second sensors and the concentrate flow control element, wherein the control electronics are configured to compare data acquired from the first sensor to data acquired from the second sensor and wherein the control electronics are configured to adjust the rate at which the concentrate solution enters the fluid path based on the comparison of the data acquired from the first sensor and the second sensor.