Which part of the nephron is primarily responsible for sodium and water balance?

• A. Distal convoluted tubule
• B. Loop of Henle
• C. Glomerulus
• D. Bowman's capsule

Answer: (B)

Sodium and water balance in the nephron hinges on the loop of Henle’s role in creating the medullary osmotic gradient through countercurrent multiplication. As filtrate moves down the descending limb, water exits into the hyperosmotic medullary interstitium, concentrating the filtrate. The ascending limb, being impermeable to water, actively reabsorbs Na+, K+, and Cl−, diluting the filtrate and increasing the interstitial osmolarity. This setup yields a strong osmotic gradient that drives water reabsorption in the collecting ducts when ADH is present and allows sodium reabsorption to be modulated further by hormones later in the nephron. In short, the loop of Henle establishes the conditions that enable the kidney to regulate sodium and water balance efficiently. The glomerulus and Bowman's capsule mainly handle filtration, while the distal tubule and collecting duct perform final adjustments under hormonal control, using the gradient the loop creates. Disrupting the loop’s function (as with loop-diuretics) reduces the kidney’s ability to concentrate urine and regulate Na+ and water, underscoring its foundational role.