Answer: By maintaining amount of ions in the blood.
Countercurrent flow met the goals due to difference in concentration gradient.
Explanation: Diffusion restored homeostasis during the dialysis treatment because during dialysis, the concentration of ion in the blood is maintained that leads to homeostasis. Countercurrent flow met the goals because it flows in opposite direction while on the other hand, parallel flow did not meet the goals because it flows in the same direction. For example. the blood flows in one direction and the dialysate flows in the opposite direction. The countercurrent flow of the blood and dialysate increases the concentration gradient between the blood and dialysate.
The increased gradient helps to remove more potassium, phosphorus and urea from the blood. If we used parallel flow so less concentration gradient of solutes is present and hence, less potassium, phosphorus and urea are removed from the blood