Which statement best defines preload?

• A. End diastolic volume
• B. Afterload
• C. Ejection fraction
• D. Cardiac Output

Answer: (D)

Preload is the degree of stretch on the heart muscle fibers at the end of filling, just before contraction. It’s set by how much blood returns to the ventricle during diastole, so the best way to represent preload is with the end-diastolic volume. The more blood in the ventricle at the end of filling, the more the fibers are stretched, and through the Frank-Starling mechanism, the stronger the next contraction tends to be (up to a limit).

Afterload is the pressure the ventricle must overcome to eject blood. Ejection fraction is the fraction of the end-diastolic volume that is ejected with each beat. Cardiac output is heart rate times stroke volume, which depends on preload but is not a direct definition of preload itself.