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Updated: Mar 13 2018

Resistance, Pressure, and Flow

  • 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 the viscosity
      • hyperproteinemic states (e.g., multiple myeloma)
      • hereditary spherocytosis
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