Final answer:
Preload affects both afterload and contractility. Increasing preload increases contractility and stroke volume (SV). Afterload refers to the tension the ventricles must develop to pump blood against resistance, and increasing resistance can decrease SV.
Step-by-step explanation:
Preload, or the stretch on the ventricles prior to contraction, affects both afterload and contractility. Preload is the amount of blood in the ventricles at the end of diastole, which determines the initial length of the muscle fibers. According to Starling's Law of the Heart, within physiological limits, the greater the stretch of the ventricular muscle, the more powerful the contraction is, increasing stroke volume (SV). By increasing preload, both contractility and SV increase. Afterload, on the other hand, refers to the tension the ventricles must develop to pump blood against the resistance in the vessels. Any condition that increases resistance requires a greater afterload, which can decrease SV.