Question 18

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.

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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.