Final answer:
Bradycardia increases the duration of ventricular diastole and ventricular filling time, which increases preload. This is due to a longer diastolic period allowing a higher end diastolic volume before the heart contracts.
Step-by-step explanation:
In bradycardia, the heart rate is slower than normal, which increases the duration of ventricular diastole and ventricular filling time, subsequently increasing preload. Because bradycardia extends the heart's resting phase between beats, the ventricles have more time to fill with blood. Given that preload refers to the end diastolic volume (EDV), a longer diastolic period allows for a greater volume of blood to fill the ventricles, thus enhancing preload. In normal physiological circumstances, the ventricles fill to about 70-80% of their capacity during diastole, before the atrial contraction tops up the remaining volume through the "atrial kick" which contributes the last 20-30 percent of ventricular filling. Consequently, any increase in duration of diastole augments preload by maximizing the EDV before ventricular contraction.
Bradycardia can be viewed as a condition with a compensatory mechanism, ensuring adequate ventricular filling and maintenance of cardiac output despite a lower heart rate. Nevertheless, excessive bradycardia can eventually lead to inadequate cardiac output if the heart rate becomes too slow to meet the metabolic demands of the body, despite an increased preload.