Which of the following is a determinant of stroke volume?

• A. Preload
• B. Heart Rate
• C. Systemic Vascular Resistance
• D. Oxygen Saturation

Answer: (A)

Stroke volume is largely determined by how much the ventricle fills during diastole. This filling, called preload, stretches the cardiac muscle fibers so that the next contraction can generate a stronger squeeze, increasing the amount of blood ejected with each beat (the Frank-Starling mechanism). Within normal ranges, greater preload leads to a higher stroke volume because the heart contracts more forcefully when it is more stretched.

Heart rate influences cardiac output (the product of stroke volume and heart rate) but does not set the volume ejected per beat itself; it mainly changes how often the heart beats rather than how much it puts out each time. Systemic vascular resistance, or afterload, does affect stroke volume: higher afterload makes it harder for the ventricle to eject blood, typically reducing the stroke volume. Oxygen saturation reflects how much oxygen is carried in the blood, not the mechanics of how much blood the heart pumps per beat. Hence preload is the best-determined factor among the options for stroke volume.