Final answer:
During the cardiac cycle, about 70-80 percent of ventricular filling happens in diastole, with the remaining portion completed by atrial contraction. This phase is essential for adequate cardiac function as it ensures the ventricles have sufficient blood volume before they contract and pump blood to the body.
Step-by-step explanation:
Approximately 70-80 percent of ventricular filling occurs during diastole in the cardiac cycle. This phase is crucial as it ensures that a significant volume of blood enters the ventricles prior to contraction. When the heart is in diastole, both the atria and ventricles are relaxed, allowing blood to flow freely into the ventricles. This is followed by atrial systole, which provides the remaining 20-30 percent of the blood volume that enters the ventricles, often referred to as the "atrial kick."
Ventricular systole occurs after the atria have contracted, signified by the QRS complex on an ECG, and this is when the ventricles contract to pump blood out of the heart. At the end of ventricular systole, the ventricles will have pushed out most of their blood volume, termed the stroke volume, but some blood, the end systolic volume (ESV), remains in the ventricles.