Outline the functions of the kidney.

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College Answer

Outline type questions require a comprehensive list and a brief explanation of each major 
function of the kidney. This should have included water balance, electrolyte balance, 
endocrine function, filtration, metabolism, acid-base balance, excretion and blood pressure 
control.
Adequate breadth was lacking in many answers. Marks were awarded for discussion of 
water reabsorption at specific points along the length of the nephron and the control 
mechanism through Aquaporins and ADH. It was expected answers would include calcium, 
magnesium, glucose and amino acid handling by the kidney (as well as sodium and 
potassium). Better answers discussed the elimination of fixed as well as volatile acids and 
outlined the role of ammonia. Discussion of the endocrine functions would include EPO, 
1,25 dihydroxy-cholecalciferol, prostaglandin E, renin, angiotensin II and kallekrein.
Superficial responses such as, “the kidney is important in regulating volume” (or “waste” or 
“water” or “sodium”) were not sufficient.

Discussion

This is a ridiculous "briefly outline a massive complex thing" sort of question which mainly challenges the candidates' ability to quickly write coherent-looking motherhood statements without really testing their knowledge.

Functions of the kidney include:

  • Water balance
    • Reabsorption of water by means of a countercurrent concentration multiplier mechanism, which is carefully regulated to maintain the water balance
  • Electrolyte balance
    • Reabsorption of filtered electrolytes, which mainly occurs in the proximal tubule (sodium, potassium, phosphate, calcium, magnesium)
    • Reabsorption of nutrients (glucose) by SGLT2 transporters
  • Elimination of wastes
    • Active secretion and filtration of creatinine
    • Active secretion and filtration of urea 
    • Active secretion of organic acids and drugs (proximal tubule)
    • Elimination of ammonia
  • Acid-base homeostasis
    • Reabsorption of controlled amounts of bicarbonate
    • Elimination of non-volatile acids eg. phosphate, sulfate, organic acids
  • Blood pressure control
    • Control of sodium reabsorption (regulated by aldosterone)
    • Control of water reabsorption (regulated by vasopressin)
    • Release of renin from macula densa cells (thus, regulation of angiotensin II synthesis)
    • Thus, control of extracellular fluid volume (influencing preload) and peripheral vascular resistance (influencing afterload)
  • Non-filtration functions
    • Secretion of hormones:
      • Erythropoietin
      • Urodilatin
      • Thrombopoietin
      • Renin
    • Biotransformation:
      • Calcitriol (active form of Vitamin D)
      • Insulin (90% cleared by the kidney (proximal tubule)