Answer:
When the blood and the dialysate are flowing in the same direction, as the the dialysate and the blood move away from the region of higher concentration of the urea, to a region distant from the source, the concentration of urea in the blood stream and in the dialysis reach equilibrium and diffusion across the semipermeable membrane stops within the higher filter regions such as II, III, IV or V
However, for counter current flow, as the concentration of the urea in the blood stream becomes increasingly lesser the, it encounters increasingly unadulterated dialysate coming from the dialysate source, such that diffusion takes place in all regions of the filter
Explanation:
Due to difference in the concentration of urea between blood and filter.
With countercurrent flow, diffusion occurs in all regions of the filter because of the difference in concentration of urea between the blood and the filter. All regions of filter has less amount of urea as compared to the blood so that's why diffusion occurs in all regions.
Gases diffuse faster from one area to another when the concentration difference between the two areas is at its highest as compared when the concentration difference is low. In counter flow, there is high difference in the concentration between the blood and the filter.
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