Question

Assertion : In the descending limb of loop of Henle, the urine is hypertonic, while in ascending limb of loops of Henle, the urine is hypotonic. 

Reason : Descending limb is impermeable to Na⁺ while ascending limb is impermeable to H₂O.

(a) Both A and R are true and R is the correct explanation of A.

(b) Both A and R are true but R is not correct explanation of A

(c) A is true but R is false

(d) A and R are false

Answer 1

The correct option is (a) Both A and R are true and R is the correct explanation of A.

Explanation:

The first wide part of the descending limb is impermeable to ions, urea and water. The second (narrow) part of the descending limb is freely permeable to water. The interstitial fluid around it has a high osmotic pressure due to a high concentration of sodium chloride and urea in it. Therefore, water is drawn out of the filtrate by osmosis. The exit of water makes the filtrate hypertonic by the time it reaches the turn of the loop to enter the ascending limb. The ascending limb is impermeable to water along its entire length. Its first (narrow) part is permeable to inorganic ions (Na⁺, K⁺, Cl^- ) and urea. The cells of the second (wide) region of the ascending limb pass inorganic ions out of the filtrate by active transport into the renal, medulla which becomes concentrated. The filtrate becomes hypotonic to plasma due to loss of inorganic ions.