**1. Introduction**

Modern dialysis devices such as the Fresenius FMC 5008 allow the relationship between the flow rates of blood QB and dialysate QD to be fixed at a set ratio (e.g. QD=1.2 QB oder QD=1.5 QB). The FMC-Genius®-Therapy System, which uses a double-sided peristaltic pump for both the patient's blood and the dialysis fluid (50…350 ml/min), even allows a ratio of QB=QD.

The relationship between the effectiveness of haemodialysis on the one hand, and the flows of blood and dialysate on the other, is guided by physical principles. These principles have been known for some time and have been widely discussed in the literature [1]. However, the fact that dialysis machines are completely automated makes it easy to neglect the underlying principles, so much so that knowledge of these may at times be lacking. There are also financial pressures that result in a desire to economise on water and dialysate concentrate, and a desire to work with as low a dialysate flow rate as possible.

The following sections will provide a brief summary of the theory behind the principles involved. In vitro and in vivo testing results from some of the more commonly available dialysers are used to provide a clearer picture of the theoretical principles involved. These measurements show that in spite of modern dialysis membranes and structural changes, it is not in fact possible to reduce the dialysate flow rate without also reducing the dialysis dose.
