Correct Option
The correct option is 1, 2 and 3 only.
Explanation
Mammals have the ability to produce concentrated urine to conserve water. This process is primarily driven by the counter-current mechanism involving the Loop of Henle and the vasa recta. This mechanism establishes and maintains a high osmolarity gradient in the medullary interstitium, facilitating water reabsorption from the collecting duct.
Statement-wise Analysis
- Statement 1 is Correct: The flow of filtrate in the two limbs of Henle’s loop is in opposite directions (descending limb moves fluid down, ascending limb moves fluid up). Similarly, the flow of blood through the two limbs of the vasa recta is also in a counter-current pattern. The proximity and opposite flow directions between the loop and the vasa recta are fundamental to the counter-current multiplier and exchanger systems.
- Statement 2 is Correct: There is a distinct osmolarity gradient within the kidney. The osmolarity increases progressively from the cortex towards the inner medulla. It ranges from approximately 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla.
- Statement 3 is Correct: The gradient in the medullary interstitium is primarily maintained by NaCl and urea. NaCl is transported out of the ascending limb of the Loop of Henle, while urea is recycled between the collecting duct and the thin ascending limb. These solutes accumulate in the medulla, increasing its osmolarity.
- Statement 4 is Incorrect: Human kidneys can produce urine nearly four times more concentrated than the initial filtrate formed. The initial filtrate has an osmolarity of roughly 300 mOsmol/L, whereas the final concentrated urine can reach an osmolarity of approximately 1200 mOsmol/L.
Key Takeaway
Key Takeaway: The counter-current mechanism allows human kidneys to concentrate urine up to four times the initial filtrate concentration (from 300 to 1200 mOsmol/L). This gradient is established principally by the movement of NaCl and urea in the renal medulla.