What percentage of blood coming into the ventricles flows passively through the aorta?

The correct answer indicates that approximately 70% of blood that fills the ventricles does so through passive flow. This passive filling primarily occurs during diastole, when the heart muscles relax and the ventricles are at rest. As the atria contract, they push blood into the ventricles, but because of the pressure gradients created during this phase of the cardiac cycle, a significant volume of blood enters the ventricles without the need for muscular contraction.

This passive filling is crucial to the efficiency of the heart's pumping action. By allowing most of the ventricular volume to fill without active contraction, the heart can conserve energy and ensure that sufficient blood is ready for ejection during systole. The remaining volume, which is actively pushed into the ventricles by the contraction of the atria—often referred to as the "atrial kick"—completes the filling process.

Understanding that a large percentage of blood flows passively helps to clarify the dynamics of the cardiac cycle and emphasizes the importance of pressure gradients in cardiovascular function. This concept is particularly relevant for EKG technicians, who need to comprehend the relationship between electrical activity, mechanical function, and blood flow dynamics in the heart.