Final answer:
About 70-80% of the blood from the atria passes into the ventricles passively before they contract, with the remaining 20-30% added by the contraction of the atria, known as the 'atrial kick.'
Step-by-step explanation:
About 70-80 percent of the blood from the atria passes into the ventricles before they contract. During the cardiac cycle, when both the atria and ventricles are in a state of relaxation, known as diastole, this passive flow of blood occurs unimpeded due to the open atrioventricular valves - the tricuspid and mitral valves. The remainder of the ventricular filling, approximately 20-30 percent, occurs due to contraction of the atria, often referred to as the 'atrial kick.'