The initial stretching of the cardiac muscle fibers prior to contraction is called?

• A. Preload
• B. Afterload
• C. Contractility
• D. End-diastolic volume

Answer: (A)

Preload is the amount of stretch the ventricular muscle fibers experience just before they contract. It reflects how much the ventricle has filled during diastole, with more filling causing greater sarcomere stretch. This stretching up to a point increases the force of the subsequent contraction via the Frank-Starling mechanism, helping to balance the output with venous return. End-diastolic volume is the actual blood volume in the ventricle at the end of diastole and is a measure that relates to preload, but the term that describes the stretch itself is preload. Afterload is the pressure the ventricle must overcome to eject blood, and contractility is the intrinsic strength of the myocardium independent of preload.