Endurance training increases plasma volume. How does this affect hematocrit and blood viscosity?

• A. Increases hematocrit and increases viscosity.
• B. Increases plasma volume, reducing hematocrit and viscosity.
• C. Decreases plasma volume and increases viscosity.
• D. Has no effect on hematocrit or viscosity.

Answer: (B)

When plasma volume expands, the total volume of blood increases but the number of red blood cells doesn’t rise immediately. Hematocrit is the proportion of blood that is red blood cells, so if you dilute the blood with more plasma, hematocrit falls. Viscosity, or thickness of the blood, largely depends on how concentrated the red blood cells are; with fewer cells per unit volume, the blood becomes less viscous. So endurance training can dilute the blood by increasing plasma volume, which lowers both hematocrit and viscosity. The other possibilities don’t fit because increasing plasma volume without a rise in red cells wouldn’t raise hematocrit or viscosity, and decreasing plasma volume would actually raise viscosity.