An increase in the end diastolic volume results in a corresponding increase in ________.

• A. Stroke volume
• B. Heart rate
• C. Cardiac contractility
• D. Afterload

Answer: (C)

An increase in end-diastolic volume tests the preload-stretch relationship of the heart. When the ventricle fills more, the muscle fibers are stretched to a more optimal length for overlap between actin and myosin, so the force of contraction increases and the heart ejects more blood with each beat. This is the Frank-Starling mechanism: stroke volume rises as preload (end-diastolic volume) increases.

Contractility, the intrinsic strength of the heart muscle at a given filling level, is mainly about calcium handling and sympathetic influence, not about how much the ventricle fills. So preload doesn’t directly boost contractility. Heart rate is set by autonomic input and pacemaker activity, not by how much the ventricle fills, and afterload is the pressure the ventricle must overcome to eject blood, which isn’t determined by preload. Therefore, the change that best corresponds to a higher end-diastolic volume is an increase in stroke volume.