How does the renal countercurrent multiplier mechanism allow the creation of a concentrated urine?
A. By pumping NaCl and urea into the ascending limb of the loop of Henle, it raises the solute load, which turns into a concentrated urine once water is extracted from the collecting duct.
B. When anti-diuretic hormone is present, it stimulates the pumping of NaCl from the medullary interstitial fluid and water follows, concentrating the urine.
C. By concentrating NaCl in the renal medullary interstitial fluid, it allows water to be reabsorbed from the collecting ducts when vasopressin is present.
D. It transports NaCl from the medullary interstitial fluid into the collecting duct, which directly increases the osmolarity of the urine.
E. It transports urea from the medullary interstitial fluid into the collecting duct, which directly increases the osmolarity of the urine.
Answer: C
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