Design: Patients are included in a randomly crossover study design such that each patient receives one HD session with exercise (intervention) and the next session without exercise (control), alternating for six consecutive HD sessions. The blood flow is maintained at around 300mL/minute and the dialysate flow at 500mL/minute. The dialysate calcium concentration used is 3.0 mEq/L. A new dialyzer is used in the first HD session of each patient at the study, and the dialyzer is reprocessed and reused up to 12 times according to its priming in subsequent sessions. During every HD session continuous spent sampling of dialysate is obtained. The study began in August 2010.