Describe how the kidney maintains the medullary concentration gradient.

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College Answer

A useful introduction could include a definition of medullary concentration gradient
and its function. The answer was expected to describe the roles of sodium, chloride
and urea in the countercurrent multiplier and the features of the vasa recta
countercurrent exchange system.
The 3 main areas that needed to be addressed to pass this question included:
[1] The loops of Henle with their water permeable descending limbs and water
impermeable ascending limbs, which actively remove solutes from the tubular
lumen. The counter current multiplier system.
[2] The vasa recta which run parallel to the loops of Henle and are permeable to
water and solute and have low flow. This allows the medullary concentration
gradient to be maintained. The counter current exchange mechanism.
[3] The role of urea which is concentrated in the medulla by mechanisms which
involve changes in permeability to urea in different regions of the tubules partly
influenced by the effects of antidiuretic hormone. `
Some candidates elected to draw the loop of Henle and vasa recta together with the
movement of various solutes and water and answer the question from it.
Unfortunately mistakes in these diagrams only confused their answers further.
Syllabus: Section D1, 2c
Recommended sources: Principles of Physiology for the Anaesthetist, Power and
Kam, page 234

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

References

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.