Cardiac output may decrease if the heart beats too rapidly because:

Cardiac output may decrease if the heart beats too rapidly because:

• A. The heart cannot generate enough force
• B. The ventricles stop contracting
• C. There is not enough time in between contractions for the heart to refill completely
• D. Blood goes to the lungs instead of the body

Answer: (C)

The key idea is that cardiac output depends on how fast the heart beats and how much blood it ejects with each beat. If the heart beats too rapidly, there isn’t enough time between contractions for the ventricles to fill completely during diastole. With less filling, preload drops, which reduces stroke volume. Since cardiac output is the product of heart rate and stroke volume, a rapid heart rate can actually lower overall output because the decrease in stroke volume outweighs the increase in rate. So, the rapid rhythm shortens filling time, leading to a smaller amount of blood pumped with each beat, and that lowers the total blood pumped per minute. The other options don’t capture this filling-time issue: the ventricles aren’t stopping pumping, though their fill is insufficient; the problem isn’t that blood is preferentially going to the lungs; and the heart can still contract but with less blood to push out due to reduced filling.

The key idea is that cardiac output depends on how fast the heart beats and how much blood it ejects with each beat. If the heart beats too rapidly, there isn’t enough time between contractions for the ventricles to fill completely during diastole. With less filling, preload drops, which reduces stroke volume. Since cardiac output is the product of heart rate and stroke volume, a rapid heart rate can actually lower overall output because the decrease in stroke volume outweighs the increase in rate.

So, the rapid rhythm shortens filling time, leading to a smaller amount of blood pumped with each beat, and that lowers the total blood pumped per minute. The other options don’t capture this filling-time issue: the ventricles aren’t stopping pumping, though their fill is insufficient; the problem isn’t that blood is preferentially going to the lungs; and the heart can still contract but with less blood to push out due to reduced filling.