Which factor generally has the greatest effect on vascular resistance to flow?

Which factor generally has the greatest effect on vascular resistance to flow?

• A. Vessel radius
• B. Blood viscosity
• C. Red blood cell count
• D. Vessel length

Answer: (D)

The dominant factor is the radius of the vessel. Resistance to flow follows Poiseuille’s law, which says R is proportional to (length × viscosity) divided by the radius to the fourth power (R ∝ (L η) / r^4). That r^4 term makes radius changes have the biggest impact: even a modest change in radius causes a large change in resistance. For example, doubling the radius cuts resistance by 16-fold, while halving it raises resistance 16-fold. Vessel length does contribute to resistance, but physiological changes in length are relatively small compared to the effect of diameter changes. Viscosity also affects resistance, but typical physiological variations in viscosity are not as dramatic as those produced by radius changes, except in extreme conditions. So, the radius of the vessel exerts the greatest influence on vascular resistance.

The dominant factor is the radius of the vessel. Resistance to flow follows Poiseuille’s law, which says R is proportional to (length × viscosity) divided by the radius to the fourth power (R ∝ (L η) / r^4). That r^4 term makes radius changes have the biggest impact: even a modest change in radius causes a large change in resistance. For example, doubling the radius cuts resistance by 16-fold, while halving it raises resistance 16-fold. Vessel length does contribute to resistance, but physiological changes in length are relatively small compared to the effect of diameter changes. Viscosity also affects resistance, but typical physiological variations in viscosity are not as dramatic as those produced by radius changes, except in extreme conditions. So, the radius of the vessel exerts the greatest influence on vascular resistance.