Vessel Diameter

• Vessel Diameter:

  • When we are talking about total peripheral resistance and the effect vessel calibre has on total peripheral resistance we are talking exclusively about arterioles
  • Resistance is inversely proportional to the 4th power of the radius of the arteriole
  • This means changing vessel diameter can significantly alter resistance and therefore total peripheral resistance
• Vessel Diameter

  • if an arteriole vasoconstricts its lumen will be reduced this means that in any vasoconstriction state a larger proportion of cells and platelets will come into direct contact with the vessel wall, which will increase the amount of friction (resistance) produced
  • The more vasodilated an arteriole, the wider the vessel lumen will be and conversely a smaller percentage of blood will interact with the endothelial wall – so resistance to flow is lower in vasodilated arterioles
• Vessel Diameter:

  • 4th power of the radius shows that very small changes in arterial lumen sizes translate to significant increases or decreases in resistance
  • Small amounts of arteriole vasoconstriction can significantly increase resistance and therefore total peripheral resistance & blood pressure
  • Conversely small degrees of arteriole vasodilation can significantly decrease resistance, total peripheral resistance & blood pressure
• What makes arterioles vasodilate or vasoconstrict?

  • Vasodilation
  • reduced excitation from sympathetic nerves and sympathetic hormones e.g. nor-adrenaline (nor-epinephrine)
  • increased exposure to lack of O2 and increased metabolic waste products
  • Vasoconstriction
  • increased excitation from sympathetic nerves and sympathetic hormones e.g. nor-adrenaline (nor-epinephrine)