Countercurrent multiplication is the process whereby energy is used to generate an osmotic gradient in which water is reabsorbed from the tubular fluid to produced concentrated urine.
Explanation:
Countercurrent multiplication moves sodium chloride from tubular fluid into interstitial space deep within the kidneys. The concurrent multiplication process build up osmotic gradient interstitial fluid in a single effect. Here, sodium chloride is driven by active transport out of the tubular fluid in the thick ascending limb into the interstitial fluid, which becomes hyperosmotic. This result to water moving passively down its concentration gradient out of the tubular fluid in the descending limb into the interstitial space to a point where it attains a state of equilibrium.
Therefore,the correct option is A, where the ions leaves the ascending limb to make water comes out of descending limb.
