Knowledge Check

Sort the clinical manifestation (top) according to whether it is a general findings in all cases of polycythemia or specific to certain causes:

Clubbing of nails
Heart murmur
Renal artery bruit
Engorged retinal veins
Low SaO2
General finding
Specific finding

Poiseuille’s law

Poiseuille’s law states that flow of a fluid (Q), or cardiac output, correlated directly with the pressure gradient and inversely with the total peripheral resistance:

Cardiac output = ∆P/TPR

Where TPR = πr4/8ηL

  • The hematocrit (Hct) is the primary determinant of blood viscosity.
  • Blood viscosity is a determinant of the total peripheral resistance.
  • As the Hct increases, so too does blood viscosity and total peripheral resistance.
  • To maintain cardiac output in the face of an increasing total peripheral resistance:
    • ∆P increases, leading to hypertension.
    • The diameter (thus the radius) of blood vessels increases, leading to plethora.

Note how you get a big bang for you buck with vasodilation (cardiac output increases to the power of 4)!

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