Abstract: The present invention relates to a system for sampling arterial blood from a patient. The system includes a reservoir suitable for containing fluid connected to a first end of a conduit. A connector suitable for connection to an artery of a patient is attached to the second end of the conduit. The system for sampling also includes a fluid displacement system which has a second conduit having first and second ends. A controller is positioned in the second conduit to permit fluid flow through the second conduit. The displacement system also includes a junction at the first end of the second conduit where the second conduit connects to the first conduit. A flow regulator is operative in a first mode to convey fluid to a patient through the first conduit from the reservoir and operative in a second mode to remove blood from a patient and convey it in the first conduit to a limit location.