What factors determine cardiac output?

• A. Cardiac output = heart rate × stroke volume; influenced by preload, afterload, and contractility
• B. Cardiac output is independent of heart rate
• C. Cardiac output = stroke volume ÷ heart rate; influenced by afterload only
• D. Cardiac output = heart rate + stroke volume

Answer: (A)

Cardiac output is determined by heart rate and stroke volume, and it changes when either component changes. The fundamental relation is CO = heart rate × stroke volume. The forces that shape these components are preload, afterload, and contractility. Preload is the stretch of the cardiac muscle at the end of diastole due to venous return; more preload generally increases stroke volume via the Frank-Starling mechanism. Afterload is the pressure the ventricles must overcome to eject blood; higher afterload tends to reduce stroke volume because it makes ejection more difficult. Contractility is the intrinsic strength of the heart’s contraction; increased contractility raises stroke volume for a given preload. Together, these factors modulate the heart’s pumping by changing how much blood is ejected each beat and how often the heart beats, thereby shaping cardiac output. The other statements don’t fit because CO is not independent of heart rate, not determined by division or addition of the terms, and not controlled by afterload alone.