Question

Which part of nephron is impermeable to salts?
A. DCT
B. PCT
C. Ascending limb of the loop of Henle
D. Descending limb of the loop of Henle

Answer 1

The impermeability of a part of the tubule towards salt contributes indirectly to the interstitium concentration. As the salts cannot move inside due to impermeability, the interstitium of the renal medulla becomes hypertonic due to high salt concentration. So, as the filtrate moves down into the interstitium of the renal medulla, osmosis takes place and water flows out freely to maintain the filtrate tonicity and equilibrate the interstitium.

Complete answer: Nephrons helps in eliminating wastes from the body, Each nephron comprises a renal corpuscle which is the initial component for filtration and a renal tubule which performs function of reabsorption and secretion.
The proximal convoluted tubule (PCT) is the site where maximum reabsorption of water and salts takes place. The collecting duct and distal convoluted tubule (DCT) are the final reabsorption sites in the nephron. DCT is different from other components of nephron as its permeability to water and ions vary with hormone stimulus. In a normal case, it shows impermeability to water and permeability to ions. However, some hormones secreted from the pituitary gland such as antidiuretic hormone contribute to increasing water reabsorption by them.
The loop of Henle is a U-shaped tubular structure consisting of a descending limb and an ascending limb. It helps in transferring of fluids from the proximal tubule to the distal tubule. The descending limb of the loop of Henle forms the initial part of the loop of Henle which helps in maintaining the urine concentration. It is entirely impermeable to the sodium and chloride ions but it is permeable to the water which accounts for reabsorption of water from this tubule through osmosis. However, the ascending limb of the loop of Henle is highly permeable to ions but impermeable to water.
So, the correct answer is option “D” that the descending limb of the loop of Henle is impermeable to salts.

Note:
A nephron is the structural and functional unit of the kidney. Its main function is regulation of water and soluble substances which is carried out through blood filtration, reabsorption of what is needed and excretion of the remaining as urine.