Answer: Option D
:
D
The only true statement here is that urine is isotonic in the proximal convoluted tubule (PCT).
One of the important aspects of elimination of wastes from the body is the maintenance of body fluid and electrolyte balance. The body cannot afford to lose all the salts and electrolytes.Therefore the kidneys have a special mechanism to concentrate urine, it is called countercurrent mechanism, which occurs in the loop of Henle. The animals which excrete hypertonic urine havelonger loops of Henle. The loop of Henle dips into the medullary region of the kidneys, which has aconcentration gradient ranging from about 300mOsmols to 1200mOsmols from the cortex to the inner medulla. This osmolarity gradient in theinterstitial fluidis maintained by the countercurrent mechanism. The urine before it reaches the loop of Henle, that is in the PCT is isotonic. However, as it travels down the descending loop of Henle the urinegets increasingly concentrated, because it loses water to the highly concentrated interstitium,through the water permeable channels of the descending duct. Thus urine at the U-turn of the loop is highly concentrated, at about 1200mOsmols. Now the ascending loop is completely impermeable to water and highly permeable to sodium. Therefore as the urine travels up the ascending limb, the sodium and chloride ions diffuse out of the ascending limb through the sodium and calcium-permeable channels. Thus urine loses most of its salt as it travels up the ascending loop becoming more hypotonic until it is isotonic once again at the DCT.
Therefore urine becomes more and more hypertonic as it passes down the Henle's loop, but becomesisotonic once again as it gets tothe DCT, and when it enters the collecting tubule.