Which factor is the primary determinant of vascular resistance in the systemic circulation?

Which factor is the primary determinant of vascular resistance in the systemic circulation?

• A. Blood vessel diameter
• B. Blood viscosity
• C. Vessel length
• D. Blood pressure

Answer: (A)

The effect of radius on flow is the dominant idea here. Vascular resistance in the systemic circulation is governed by how narrow the vessels are, because resistance scales very steeply with radius: it is inversely proportional to the fourth power of the radius. In other words, small changes in diameter cause large changes in resistance. This is why the arterioles, which can actively constrict or dilate, are the main controllers of systemic vascular resistance and thus play a central role in regulating blood pressure and flow to tissues. Blood viscosity and the total length of the vessels do contribute to resistance, but their impact is much smaller in comparison to radius. Blood pressure reflects the result of flow against that resistance, not the factor that determines resistance itself. So the primary determinant of vascular resistance is the diameter (radius) of the vessels.

The effect of radius on flow is the dominant idea here. Vascular resistance in the systemic circulation is governed by how narrow the vessels are, because resistance scales very steeply with radius: it is inversely proportional to the fourth power of the radius. In other words, small changes in diameter cause large changes in resistance. This is why the arterioles, which can actively constrict or dilate, are the main controllers of systemic vascular resistance and thus play a central role in regulating blood pressure and flow to tissues.

Blood viscosity and the total length of the vessels do contribute to resistance, but their impact is much smaller in comparison to radius. Blood pressure reflects the result of flow against that resistance, not the factor that determines resistance itself. So the primary determinant of vascular resistance is the diameter (radius) of the vessels.