Countercurrent flow maximizes the concentration gradient between blood and dialysate throughout the length of the dialyzer (see Box 22.1). When blood flow and dialysate flow are in the same direction (cocurrent), small solute clearance decreases by about 10%.

Why is countercurrent flow important?

Countercurrent flow produces the maximum concentration difference over the entire length of the membrane and allows recovery of a substantial portion of the most highly diffusive solute while minimizing the transport of the less diffusive solutes.

Why is countercurrent flow used in dialysis?

The counter-current flow of the blood and dialysate maximizes the concentration gradient of solutes between the blood and dialysate, which helps to remove more urea and creatinine from the blood.