Describe the control of cerebral blood flow.

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College Answer

Good answers included an equation and then explored the various components of the
equation. Main points for a pass included pressure and metabolic autoregulation and the
various factors that affect cerebral vascular resistance. Graphs were a useful way to answer
this question but were generally underutilised. Several candidates wrote about the Monroe-
Kellie doctrine which was not directly relevant to the question.
Syllabus C1d2a
Reference: Power and Kam 1st edition p 42-43
Guyton and Hall 11th edition p 761-3

Discussion

The most economical way to answer this question would probably look something like this:

  • Cerebral blood flow is supplied by the carotid (70% and vertebral (30% arteries)
  • It is usually 50ml/100g/min, or 14% on normal cardiac output
  • It is described by the Ohm equation,  Q = (Pa- Pv) / R, where
    • (Pa- Pv is the cerebral perfusion pressure (CPP)
    • is the cerbral vascular resistance 
  • Cerebral perfusion pressure = MAP - (ICP or CVP, whichever is higher)
    • The higher the ICP (or CVP), the lower the CPP, if the MAP remains stable
  • Cerebral resistance (R)  = (8 η) / πr4, where
    • = length of the vessel
    • η = viscosity of the blood
    • r = radius of the cerebral vessels, which is the main variable susceptible to regulation
  • Cerebral autoregulation is a homeostatic process that regulates and maintains cerebral blood flow (CBF) constant and matched to cerebral metabolic demand across a range of blood pressures.
  • It is affected by:
    • PaCO2increased PaCO2 leads to increased CBF 
    • PaO2 PaO2 falling below 50 mmHg leads to exponentially increased CBF
    • MAP: CBF is stable over a range of MAP between 50 and 150 mmHg

References

Paulson, O. B., S. Strandgaard, and L. Edvinsson. "Cerebral autoregulation." Cerebrovascular and brain metabolism reviews 2.2 (1989): 161-192.

Busija, David W., and Donald D. Heistad. Factors involved in the physiological regulation of the cerebral circulationSpringer Berlin Heidelberg, 1984.

Mchedlishvili, George. "Physiological mechanisms controlling cerebral blood flow." Stroke 11.3 (1980): 240-248.