Which nephron segment is most responsive to antidiuretic hormone in terms of water permeability?

Which nephron segment is most responsive to antidiuretic hormone in terms of water permeability?

• A. Proximal tubule
• B. Collecting ducts
• C. Loop of Henle
• D. Distal convoluted tubule

Answer: (B)

Antidiuretic hormone mainly controls water reabsorption by regulating aquaporin channels in the kidney. In the collecting ducts, especially the principal cells, ADH binds to V2 receptors and triggers a signaling cascade that moves aquaporin-2 channels to the apical membrane. This dramatically increases water permeability, allowing water to flow out of the tubular lumen into the hypertonic medullary interstitium and be reabsorbed, concentrating the urine. Without ADH, these ducts are relatively water-impermeable, so much of the water remains in the filtrate. Other nephron segments are not as responsive to ADH. The proximal tubule reabsorbs water largely in an isosmotic fashion that does not depend on ADH. The loop of Henle, particularly the descending limb, conducts water reabsorption passively via the osmotic gradient rather than hormone-regulated channels. The distal tubule has limited water permeability unless ADH is present, but the strongest, most pronounced ADH-mediated increase in water reabsorption occurs in the collecting ducts.

Antidiuretic hormone mainly controls water reabsorption by regulating aquaporin channels in the kidney. In the collecting ducts, especially the principal cells, ADH binds to V2 receptors and triggers a signaling cascade that moves aquaporin-2 channels to the apical membrane. This dramatically increases water permeability, allowing water to flow out of the tubular lumen into the hypertonic medullary interstitium and be reabsorbed, concentrating the urine. Without ADH, these ducts are relatively water-impermeable, so much of the water remains in the filtrate.

Other nephron segments are not as responsive to ADH. The proximal tubule reabsorbs water largely in an isosmotic fashion that does not depend on ADH. The loop of Henle, particularly the descending limb, conducts water reabsorption passively via the osmotic gradient rather than hormone-regulated channels. The distal tubule has limited water permeability unless ADH is present, but the strongest, most pronounced ADH-mediated increase in water reabsorption occurs in the collecting ducts.