Question 13

College Answer

A good answer included an explanation of the purpose of the countercurrent mechanism followed by a detailed description of the functional and anatomical relevance of the Loop of Henle, vasa recta and urea recycling. A diagram if included required some explanation as it is not sufficient to completely replace text alone

Discussion

• The "single effect":
  • The thick ascending limb of the loop of Henle extracts solutes from the tubule fluid 
  • This transfers the solutes to the renal medulla
  • The renal medulla then becomes hyperosmolar (1200 mOsm/kg)
  • This facilitates the removal of water from the thin descending limb of the loop of Henle
  • Thus, fluid in the thin descending limb also becomes hyperosmolar
• Countercurrent multiplication of the single effect
  • The movement of hyperosmolar fluid up into the thick ascending limb continuously delivers more solute
  • Thus, more solute is transferred to the medullary interstitium
  • The hyperosmolarity of the interstitium then extracts more water from the descending tubule fluid, maintaining its hyperosmolarity
  • The concentration gradient maintained in this way reduces the energy cost of extracting solutes from the thick ascending limb.
• Countercurent exchange in the vasa recta
  • The vasa recta are permeable to water and solutes
  • Solutes diffuse into the descending vasa recta, and then back out again as the blood returns via the ascending vasa recta
    • These vessels also have slower flow because of increased crossection, increasing the efficiency of solute exchange
  • This mechanism prevents the washout of concentrated inner medullary solutes
  • More water returns via the ascending vasa recta, removing reclaimed water from the renal medulla
• Role of intrarenal urea recycling:
  • Proximal cortical collecting duct is permeable to water but not to urea.
  • Water can move out of the cortical collecting duct, but urea cannot, which causes the concentration of urea in the duct
  • Distal collecting duct is permeable to urea
  • Thus, the concentrated urea can move into the renal interstitum
  • From there, it can be absorbed into the ascending limb fluid, and recycled
  • Vasopressin increases the permeability of the collecting duct to urea.
• The osmolalities at different points in the tubule are:
  • Renal interstitial osmolality values:
    • Cortex osmolality: 300 mOsm/kg
    • Outer medulla: 800 mOsm/kg
    • Inner medulla:  1200 mOsm/kg
  • Loop of Henle osmolality values:
    • Proximal tubule, straight part: 300 mOsm/kg
    • Descending limb: 800 mOsm/kg
    • Hairpin turn: 1200 mOsm/kg
    • Ascending thin limb: 800 mOsm/kg
    • Ascending thick limb: 100 mOsm/kg, at the end

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.