Describe the autoregulation of renal blood flow.
To achieve a pass in this question, candidates needed to briefly define autoregulation and
state the range of Mean Arterial Pressure over which this occurs, and why it occurs, then
provide a more detailed discussion about the mechanisms thought to be responsible for
The main site of autoregulation in the kidney is the afferent arteriole. There are two main
factors that affect vascular tone in the afferent arteriole, these are stretch-activated
constriction of vessels (myogenic mechanism) and tubulo-glomerular feedback (TGF).
Both of the above mechanisms are important to maintenance of near-constant blood flow.
Stretch results in membrane depolarisation, increased intra-cellular concentrations of
calcium ions, and ultimately, vasoconstriction.
In tubulo-glomerular feedback, complex signals pass from the macula densa to the
afferent arteriole, regulating its tone. The fundamental theme of TGF is that increased
delivery of fluid and/or NaCl to the distal tubule causes vasoconstriction, thus limiting the
flow (negative feedback).
The major weakness in answers was again the failure to include sufficient information to
achieve a pass mark