Question 3

Describe the physiological mechanisms by which the kidney is able to concentrate urine.

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College Answer

This question required the identification of the components of the kidneys function that work to concentrate urine and a detailed description of how each of these components contribute to this. These components included the role of the loop of henle and vasa recta in the establishment and maintenance of the medullary osmotic gradient, the contribution of urea and urea cycling to this gradient and the subsequent contribution of antidiuretic hormone. A focus on not how urine is formed but how it is concentrated was expected

Discussion

This question is very similar to Question 7 from the second paper of 2022, "describe the physiological mechanisms by which the kidney is able to maximally concentrate urine." That one was done unexpectedly badly (35% pass rate), and the removal of the "maximally"  seems to have helped.

  • Countercurent exchange in the vasa recta maintains a high urea concentration in the renal medulla
    • 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
    • In this way, the inner medulla achieves an osmolality of 1200 mOsm/kg
  • Vasopressin increases water reabsorption along the medullary concentration gradient
    • Vasopressin increases the luminal expression of aquaporin channels
    • Water is reabsorbed through aquaporin channels (driven mainly by  the osmotic gradient of the hypertonic medullary interstitium)
    • With maximal vasopressin stimulus, maximally concentrated urine can be produced (~1200 mOsm/kg)
  • 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.

References

Schafer, James A. "Renal water reabsorption: a physiologic retrospective in a molecular era." Kidney International 66 (2004): S20-S27.

McDonald, Keith M., et al. "Hormonal control of renal water excretion." Kidney international 10.1 (1976): 38-45.

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.