Outline the functional anatomy of the kidneys (40% of marks). Outline the regulation 
of renal blood flow (60% of marks).

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College Answer

Candidates who scored well weighted their answers according to the marks allocation outlined in the question and adopted a good structure. A number of candidates confused the roles of tubuloglomerular feedback and the renin angiotensin aldosterone pathway.

Discussion

Functional anatomy of the kidneys in under 4 minutes? Sure:

Gross anatomy

  • Paired abdominal (retroperitoneal) organ
  • Blood supply: single renal artery
  • Venous drainage: single renal vein 

Structural organisation, from the outside in:

  • Cortex: cortical labyrinth and medullary rays
  • Outer medulla: inner stripe and outer stripe
  • Inner medulla: forms the tip (papilla) of the renal pyramids
  • Minor calyces (tips of renal papillae) joint to form major calyces
  • Major calyces join to form the renal pelvis.

Structure of the nephron

  • The renal corpuscle: glomerulus and Bowman's capsule
  • The juxtaglomerular apparatus,  containing the macula densa
  • The proximal tubule (convoluted and straight segments)
  • The distal tubule (Loop of Henle: descending thin limb, ascending thin limb and ascending thick limb), and distal convoluted tubule
  • The collecting duct system

Regulation of renal blood flow

  • Renal blood flow remains constant over a MAP range of 75-160 mmHg
  • This regulation is produced by:
    • Myogenic response (50% of the total autoregulatory response)
    • Tubuloglomerular feedback (35%)
    • Other mechanisms involving angiotensin-II and NO (<15%)
  • Intrinsic myogenic mechanisms:
    • Vasoconstriction in response to wall stretch
    • This is a stereotyped vascular smooth muscle response, not unique to the kidney
  • Tubuloglomerular feedback
    • This is a negative feedback loop which decreases renal blood in response to increased sodium delivery to the tubule
    • The mechanism is mediated by ATP and adenosine secreted by macula densa cells, which cause afferent arterolar vasoconstriction
  • Sympathetic regulation of renal blood flow
    • Sympathetic tone regulates the range of renal blood flow autoregulation
    • Autoregulation typically maintains stable renal blood flow over a wide range of systemic sympathetic conditions
    • Massive sympathetic stimulus (eg. shock) overrides autoregulation and markedly decreases renal blood flow
    • Glomerula filtration rate is less affected (out of porportion to blood flow) because the efferent arterioles vasoconstrict more than the afferent in response to a sympathetic stimulus.

References

Kaissling, B., and J. Dørup. "Functional anatomy of the kidney." Diuretics. Springer, Berlin, Heidelberg, 1995. 1-66.

Tisher, C. Craig. "Functional anatomy of the kidney." Hospital practice 13.5 (1978): 53-65.

Sands, JEFF M., and J. W. Verlander. "Functional anatomy of the kidney." (2010): 1-26.

Stein, Jay H. "Regulation of the renal circulation." Kidney international 38.4 (1990): 571-576.

Bertram, John F. "Structure of the renal circulation." Advances in Organ Biology Volume 9, 2000, Pages 1-16 (2000)

Kriz, Wilhelm, and Brigitte Kaissling. "Structural organization of the mammalian kidney." The kidney: physiology and pathophysiology 3 (1992): 587-654.