A system for controlling fluid movement between a subject and a controlled compliant flow path, comprising: a controlled compliant flow path having a substantially constant fluid volume, in operation; an extracorporeal flow path in fluid communication with the controlled compliant flow path across a semi-permeable membrane wherein the extracorporeal flow path has a first terminal end and a second terminal end; a bi-directional control pump operating to selectively meter in and meter out fluid into and from the controlled compliant flow path such as to cause a net fluid flow between the controlled compliant flow path and the extracorporeal flow path that maintains the substantially constant fluid volume in the controlled compliant flow path; and a common reservoir in fluid communication with the control pump, wherein the common reservoir contains any of a dialysate, a filtrate, a volume of a physiologically compatible priming solution, a volume of a physiologically compatible solution to provide a bolus of fluid to a subject receiving treatment, a volume of physiologically compatible solution to provide solution for return blood from the extracorporeal flow path to a subject receiving treatment, a volume of solution returned to the common reservoir from the extracorporeal flow path when blood from a subject is introduced to the extracorporeal flow path; wherein the controlled compliant flow path has a buffer source arranged to increase a buffer concentration of fluid passing through the controlled compliant flow path, characterised in that the controlled compliant flow path further comprises a sodium chloride source arranged to increase a sodium chloride concentration of fluid passing through the controlled compliant flow path. a controlled compliant flow path having a substantially constant fluid volume, in operation; an extracorporeal flow path in fluid communication with the controlled compliant flow path across a semi-permeable membrane wherein the extracorporeal flow path has a first terminal end and a second terminal end; a bi-directional control pump operating to selectively meter in and meter out fluid into and from the controlled compliant flow path such as to cause a net fluid flow between the controlled compliant flow path and the extracorporeal flow path that maintains the substantially constant fluid volume in the controlled compliant flow path; and a common reservoir in fluid communication with the control pump, wherein the common reservoir contains any of a dialysate, a filtrate, a volume of a physiologically compatible priming solution, a volume of a physiologically compatible solution to provide a bolus of fluid to a subject receiving treatment, a volume of physiologically compatible solution to provide solution for return blood from the extracorporeal flow path to a subject receiving treatment, a volume of solution returned to the common reservoir from the extracorporeal flow path when blood from a subject is introduced to the extracorporeal flow path; wherein the controlled compliant flow path has a buffer source arranged to increase a buffer concentration of fluid passing through the controlled compliant flow path, characterised in that the controlled compliant flow path further comprises a sodium chloride source arranged to increase a sodium chloride concentration of fluid passing through the controlled compliant flow path.