Polycythemia results in _______.

Polycythemia results in _______.

• A. decreased resistance to flow
• B. no change
• C. variable
• D. increased resistance to flow

Answer: (D)

Polycythemia thickens the blood by increasing the hematocrit, which raises blood viscosity. Potent effects on flow come from how viscosity contributes to resistance: the more viscous the blood, the harder it is to push it through vessels. According to Poiseuille’s relation, resistance increases with viscosity and decreases strongly with vessel radius, so higher viscosity raises overall vascular resistance. In practice, this means flow is reduced for a given pressure gradient, especially in small vessels where the impact of viscosity is most felt. Therefore polycythemia leads to increased resistance to flow.

Polycythemia thickens the blood by increasing the hematocrit, which raises blood viscosity. Potent effects on flow come from how viscosity contributes to resistance: the more viscous the blood, the harder it is to push it through vessels. According to Poiseuille’s relation, resistance increases with viscosity and decreases strongly with vessel radius, so higher viscosity raises overall vascular resistance. In practice, this means flow is reduced for a given pressure gradient, especially in small vessels where the impact of viscosity is most felt. Therefore polycythemia leads to increased resistance to flow.