Salt and water excess is an essential mechanism of hypertension. This is particularly relevant to patients with end stage kidney disease (ESKD) on dialysis. We have demonstrated that individualization of the sodium concentration in the dialysate as to match the patient's own serum sodium concentration leads to less thirst, interdialytic weight gain, and better BP control in hypertensive patients. In this study we will evaluate the mechanisms underlying this response by measuring systemic hemodynamics, body volume spaces, and biochemical marker of volume status.