Which of the following has the greatest effect on blood flow?

Which of the following has the greatest effect on blood flow?

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

Answer: (A)

The main idea is that blood flow is driven by how easily a vessel can be traversed, and the radius of the vessel is the most powerful lever. Flow through a vessel follows Poiseuille’s law, which shows flow is proportional to the radius to the fourth power (r^4) and inversely proportional to viscosity and to the vessel’s length. Because the radius is raised to the fourth power, even small changes in radius produce very large changes in flow, making radius the strongest determinant of how much blood gets to a tissue. This is why vasodilation can dramatically increase perfusion and vasoconstriction can sharply reduce it. The length of a vessel affects flow, but its impact scales linearly, so it’s far less influential on a moment-to-moment basis. Viscosity also matters—thicker blood raises resistance—but physiological changes in viscosity tend to be slower and smaller than radius changes, so they don’t regulate flow as dramatically in real time. Red blood cell count affects viscosity indirectly, but its immediate effect on flow in a single vessel is not as large as the radii change. In short, radius is the greatest driver of changes in blood flow.

The main idea is that blood flow is driven by how easily a vessel can be traversed, and the radius of the vessel is the most powerful lever. Flow through a vessel follows Poiseuille’s law, which shows flow is proportional to the radius to the fourth power (r^4) and inversely proportional to viscosity and to the vessel’s length. Because the radius is raised to the fourth power, even small changes in radius produce very large changes in flow, making radius the strongest determinant of how much blood gets to a tissue. This is why vasodilation can dramatically increase perfusion and vasoconstriction can sharply reduce it. The length of a vessel affects flow, but its impact scales linearly, so it’s far less influential on a moment-to-moment basis. Viscosity also matters—thicker blood raises resistance—but physiological changes in viscosity tend to be slower and smaller than radius changes, so they don’t regulate flow as dramatically in real time. Red blood cell count affects viscosity indirectly, but its immediate effect on flow in a single vessel is not as large as the radii change. In short, radius is the greatest driver of changes in blood flow.