How does the heart primarily alter stroke volume?

How does the heart primarily alter stroke volume?

• A. By changing heart rate
• B. By changing afterload
• C. By increasing venous return
• D. By changing contractility

Answer: (D)

The main factor that changes stroke volume on a beat-to-beat basis is how strongly the heart contracts, i.e., its contractility. Stroke volume depends on the difference between how much blood fills the ventricle (end-diastolic volume) and how much remains after contraction (end-systolic volume): SV = EDV − ESV. Contractility directly tunes ESV: stronger contraction pulls more blood out, lowering ESV and increasing SV; weaker contraction leaves more blood in the ventricle, raising ESV and reducing SV. Venous return sets EDV via preload, and afterload opposes ejection by increasing the pressure the ventricle must overcome, both of which also influence SV, but the most immediate lever for changing the amount ejected per beat is contractility. For example, sympathetic stimulation increases contractility, making the heart eject more of the filled blood and raising stroke volume and cardiac output, even if preload is unchanged. Heart rate affects cardiac output by changing the number of beats per minute, not the amount ejected per beat, so it’s not the primary way to alter stroke volume.

The main factor that changes stroke volume on a beat-to-beat basis is how strongly the heart contracts, i.e., its contractility. Stroke volume depends on the difference between how much blood fills the ventricle (end-diastolic volume) and how much remains after contraction (end-systolic volume): SV = EDV − ESV. Contractility directly tunes ESV: stronger contraction pulls more blood out, lowering ESV and increasing SV; weaker contraction leaves more blood in the ventricle, raising ESV and reducing SV. Venous return sets EDV via preload, and afterload opposes ejection by increasing the pressure the ventricle must overcome, both of which also influence SV, but the most immediate lever for changing the amount ejected per beat is contractility. For example, sympathetic stimulation increases contractility, making the heart eject more of the filled blood and raising stroke volume and cardiac output, even if preload is unchanged. Heart rate affects cardiac output by changing the number of beats per minute, not the amount ejected per beat, so it’s not the primary way to alter stroke volume.