Which of the following is NOT a major determinant of stroke volume?

The major determinants of stroke volume are preload, afterload, and contractility, which collectively influence the amount of blood ejected by the heart with each contraction.

Preload refers to the degree of stretch of the cardiac muscle fibers at the end of diastole, which directly influences stroke volume according to the Frank-Starling mechanism. Increased preload typically results in greater stroke volume due to enhanced myocardial stretch and subsequent force of contraction.

Afterload denotes the resistance the heart must overcome to eject blood during systole. Higher afterload can reduce stroke volume because the heart is working against greater resistance, requiring more effort to pump the same amount of blood.

Contractility is the intrinsic ability of the cardiac muscle fibers to contract independently of preload and afterload. Increased contractility enhances stroke volume because it allows more forceful contractions and more efficient ejection of blood during systole.

On the other hand, heart rate, while it influences cardiac output (which is the product of stroke volume and heart rate), does not directly determine stroke volume itself. Heart rate affects how many times the heart beats but does not change the volume of blood the heart pumps with each beat. Therefore, it is not considered a major determinant of stroke volume.