Question 22

College Answer

Most answers gave a good account of counter currents in the kidney but few described the 
generation of the counter current multiplier arrangement. Many candidates did not discuss 
counter current mechanisms in the skin but instead, focussed on thermal control by the skin

Discussion

• Renal countercurrent mechanism:
  • The removal of water from the thin descending limb of the loop of Henle, making the fluid there also hyperosmolar
  • Solutes extracted from the thick ascending limb in the Loop of Henle are transferred to the renal medulla, which then becomes hyperosmolar (1200 mOsm/kg).
  • The movement of increasingly hyperosmolar fluid up into the thick ascending limb continuously delivers more solute, which allows the medullary interstitium to increase in osmolality
  • The concentration gradient maintained in this way reduces the energy cost of extracting solutes from the thick ascending limb.
  • The countercurrent exchange of solutes between the descending vasa recta, ascending vasa recta prevents the washout of concentrated inner medullary solutes

Pallone, Thomas L., et al. "Countercurrent exchange in the renal medulla." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 284.5 (2003): R1153-R1175.

Sands, Jeff M., and Juha P. Kokko. "Current concepts of the countercurrent multiplication system." Kidney International Supplement 57 (1996).

Sands, Jeff M., and Harold E. Layton. "The physiology of urinary concentration: an update." Seminars in nephrology. Vol. 29. No. 3. WB Saunders, 2009.