Which nephron segment is primarily responsible for countercurrent multiplication and water/solute handling?

• A. Glomerulus
• B. Proximal tubule
• C. Loop of Henle
• D. Collecting duct

Answer: (C)

Countercurrent multiplication starts in the loop of Henle, where the two limbs establish a medullary osmotic gradient that drives water reabsorption and solute handling along the nephron. The descending limb is highly permeable to water, so as filtrate moves into the hyperosmolar medulla, water exits and the filtrate becomes more concentrated. The ascending limb, especially the thick segment, actively pumps Na+, K+, and 2Cl− out into the interstitium while being water-impermeable, which dilutes the tubular fluid and further amplifies the medullary gradient. This continuous looping cycle creates the gradient that the collecting duct uses to adjust final urine concentration under hormonal control. The glomerulus focuses on filtration; the proximal tubule reabsorbs most filtered water and solutes but does not generate the countercurrent gradient; the collecting duct fine-tunes water balance but relies on the gradient established by the loop of Henle.