Question

Which of the following statements about the excretory system is CORRECT?

• A. The vessel entering the glomerular capillaries is an arteriole while the vessel coming out of the glomerular capillaries is a venule.
• B. Minimum reabsorption in the nephron takes place in the distal convoluted tubule.
• C. The counter-current and proximity between the Henle’s loop and vasa recta maintain an increasing osmolarity towards the inner medullary interstitium.
• D. As the filtrate passes through the ascending limb of Henle's loop it becomes hypertonic.

Answer 1

Answer: (C)

Mammals have the ability to produce a concentrated urine. The Henle’s loop and vasa recta play a significant role in this. The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of vasa recta is also in a counter current pattern. The proximity between the Henle’s loop and vasa recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium, i.e., from 300 mOsmolL-1 in the cortex to about 1200 mOsmolL-1 in the inner medulla. This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule. The above described transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the counter current mechanism.