Question 4

College Answer

It was expected that answers include sections on the blood supply, the nephron (including the difference between the cortical and juxta-medullary nephrons) and innervation. A number of candidates failed to quantify renal blood flow and to define autoregulation. The concept that it’s the flow that’s regulated was not described by some. Tubuloglomerular feedback was generally described correctly but a reasonable number had the blood flow increasing when an increased sodium was sensed at the macula densa.

Discussion

There's not much you can say in 4 minutes about the functional anatomy of the kidneys, and after many drafts this is the best one could come up with. Though it still looks empty of meaning, it still should hit the right notes, judging by the examiner comments.

Functional anatomy:

• Paired abdominal (retroperitoneal) organ
• Cortex: cortical labyrinth and medullary rays

• Outer medulla: inner stripe and outer stripe
• Innervation:
  • efferent is strictly sympathetic, from T9-T13
  • afferent (pain) via the least splanchnic nerve (T12)
• Nephrons:
  •  Cortical nephrons: short-looped
  • Juxtaglomerular nephrons: have long loops of Henle, contribute the most to the process of producing concentrated urine. 

• Renal blood flow
  • Total blood flow: 20-25% of cardiac output, or 1000ml/min, or 400ml/100g/min
    • 95% goes to the cortex, 5% goes to the medulla
    • Medullary blood flow must remain low to maintain the urea concentration gradient, to facilitate the concentration of urine
  • Total renal blood is high for reasons of filtration rather than metabolism
  • Total renal oxygen extraction is low (10-15%)
  • Renal oxygen extraction remains stable as renal blood flow changes, because renal metabolic rate depends on glomerular filtration rate and tubular sodium delivery
• Autoregulation 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

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.

Just, Armin. "Mechanisms of renal blood flow autoregulation: dynamics and contributions." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 292.1 (2007): R1-R17.

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.

Braam B., Yip S., Cupples W.A. (2014) Anatomy, Physiology, and Pathophysiology of Renal Circulation. In: Lanzer P. (eds) PanVascular Medicine. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-37393-0_146-1