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Vessel Pressure
  • Overview  
    •  a change in pressure in a vessel is equal to flow times resistance:  ΔP = Q x R
      • this is similar to Ohm's law where a change in voltage is equal to current times resistance:  ΔV = IR
    • a pressure gradient drives flow from high pressure to low
Resistance
  • Formula 
    • resistance = (driving pressure ΔP) / (flow Q) = (8η)(viscosity)(length) / (πr4)
    • is directly proportional to viscosity and inversely proportional to the radius of a vessel raised to the 4th power
  • Peripheral resistance
    • arterioles account for most of the total peripheral resistance in the cardiovascular system
    • regulates capillary flow
  • Total resistance
    • the total resistance of vessels in series is determined by summing the resistance of each vessel:  RTOTAL  = R1 + R2 + R3 + ...
    • the total resistance of vessels in parallel is determined by taking the inverse of the sum of the inverses of the resistance of each vessel:  1/RTOTAL= 1/R1 + 1/R2 + 1/R3 + ...
    • these calculations are done similarly to adding electrical resistors
Viscosity
  • Viscosity depends mostly on hematocrit
  • Viscosity is increased in several disease states:
    • polycythemia
      • conversely, anemia decreases viscosity
    • hyperproteinemic states (e.g. multiple myeloma)
    • hereditary spherocytosis
 

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