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

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.

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.