The concentration of Urea in blood is higher as compared to concentration of urea in dialysis so the urea from the blood moves from the blood towards dialysis due to concentration gradient. The dialysis fluid contains no concentration of urea, so all of the urea diffuses from the blood into the dialysis fluid due to the high concentration in the blood to the lower concentration in the dialysis fluid. So we can say that diffusion of urea occur due to concentration gradient.
