Chronic endurance training increases plasma volume and preload, resulting in a larger stroke volume. Which statement best describes this effect?

Chronic endurance training increases plasma volume and preload, resulting in a larger stroke volume. Which statement best describes this effect?

• A. Decreases plasma volume, reducing preload
• B. Increases plasma volume, improving venous return and preload
• C. No change in plasma volume
• D. Increases plasma viscosity and afterload

Answer: (B)

A key idea here is how plasma volume affects preload and, in turn, stroke volume. When plasma volume expands with chronic endurance training, more blood returns to the heart during filling. This boosts the end-diastolic volume, so the heart fibers are stretched more (greater preload). According to the Frank-Starling mechanism, a greater preload leads to a stronger contraction and a larger stroke volume. So the description that plasma volume increases and improves venous return and preload best captures how endurance training enhances stroke volume.

The other statements don’t fit the adaptive pattern. Decreasing plasma volume would reduce preload, which contradicts the training effect. No change in plasma volume ignores the well-documented plasma-volume expansion from endurance training. Increasing plasma viscosity and afterload would oppose the rise in stroke volume rather than promote it, whereas endurance training tends to support greater preload and cardiac output.