Define stroke volume and identify its three determinants.

• A. Stroke volume is the amount of blood pumped per beat; determinants are preload, contractility, and afterload.
• B. Stroke volume measures heart rate.
• C. Stroke volume is the total blood volume in the circulatory system.
• D. Stroke volume is the amount of blood left in the ventricle after contraction.

Answer: (A)

Stroke volume is the amount of blood pumped by a ventricle with each heartbeat. The three determinants are preload, contractility, and afterload. Preload is the filling of the ventricle at the end of diastole, which stretches the myocardial fibers; more filling generally increases stroke volume through the Frank-Starling mechanism. Contractility is the intrinsic ability of the heart muscle to contract; greater contractile force pushes more blood out per beat, raising stroke volume. Afterload is the pressure the ventricle must overcome to open the semilunar valves and eject blood; higher afterload makes ejection harder and lowers stroke volume. In other words, stroke volume ≈ end-diastolic volume minus end-systolic volume. The other statements mix up what stroke volume represents: heart rate measures how often the heart beats, total blood volume is the entire circulatory volume, and the amount left in the ventricle after contraction is end-systolic volume, not stroke volume.