Which statement best contrasts preload and afterload in terms of their effects on stroke volume?

• A. Preload decreases stretch; afterload increases ejection and SV.
• B. Preload has no effect on SV.
• C. Preload increases stretch and tends to increase SV via Frank-Starling; afterload impedes ejection and tends to reduce SV.
• D. Preload reduces SV; afterload increases SV.

Answer: (C)

Preload raises the degree of stretch in the ventricular fibers at the end of filling, and by the Frank-Starling mechanism this greater stretch leads to a stronger contraction and a higher stroke volume (within physiological limits). Afterload is the pressure the heart must overcome to eject blood; when afterload is high, ejection is harder, so less blood is ejected and stroke volume falls. The statement that preload increases stretch and tends to raise stroke volume via Frank-Starling, while afterload impedes ejection and tends to reduce stroke volume, best contrasts these effects.