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