A dialysis bloodline set includes an arterial chamber with upper, lower and central portions, in which the central portion forms a vertical riser tube having a maximum section area less than that of the lower portion. The lower portion has a shoulder which is positioned to deflect blood entering the lower portion to flow across the aperture at the lower end of the riser tube. The bloodline is prepared by filling the arterial chamber with a priming liquid which extends to a level part way up the riser tube and which is partially displaced by the blood to leave a plug of priming liquid within the base of the riser tube above the blood. The plug of priming liquid separates the blood from a gas in the upper part of the arterial chamber while the blood flowing across the aperture forms a dynamic interface.
