Describe the concept of autoregulation as it relates to the renal circulation.

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College Answer

This question was concerned with a very important physiological principle and was generally
well answered. A good answer included a definition of autoregulation. In relation to the renal
circulation, the kidneys extract only 10% of the available O2 supply and therefore the renal
blood flow is high for the purpose of filtration and not metabolic demand, renal blood flow is
autoregulated to remain constant against arterial blood pressures from 75 – 160 mmHg (an
illustration helps explain this concept), Tubuloglomerular Feedback (including a description
of the mechanism). A discussion about other mechanisms thought to play a role is important
– eg intrinsic contractile response of smooth muscle to stretch (myogenic theory of
autoregulation). Vasodilator substances tend to accumulate in active tissues, and these
"metabolites" (decreases in O2 tension, increased CO2 tension and decreased pH) also
contribute to autoregulation (metabolic theory of autoregulation). The sympathetic nerves
innervate afferent and efferent arterioles. Renal autoregulation usually overrides mild to
moderate degrees of sympathetic stimulation. Strong sympathetic stimulation however will
constrict renal arterioles reducing flow to 10% of normal. GFR falls to a lesser extent than renal blood flow owing to a differential effect of sympathetic stimulation constricting the efferent arteriole to a greater degree than the afferent arteriole.


Syllabus: C1f 2a, d D2i
Reference Text: Guyton Chp 26

Discussion

  • 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 blood flow:
    • This is a property of regional circulatory systems which allows them to maintain stable blood flow over a range of changing pressure conditions
  • 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
  • Sympathetic regulation of renal blood flow
    • Sympathetic tone regulates the range fo 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

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