Approximately what percentage of ventricular filling is contributed by atrial contraction just before ventricular systole?

1. Most blood flows passively into the ventricle while both the atria and ventricles are relaxed. 2. Toward the end of the ventricular relaxation period, the left atrium contracts and actively pumps additional blood into the ventricle. 3. This active atrial contraction accounts for approximately 20 percent of ventricular filling. 4. 50 percent would overstate the atrial contraction's real share by a wide margin. 5. 80 percent is actually the passive share that fills the ventricle before atrial contraction, not atrial contraction's own share. 6. Nearly 100 percent would mean the ventricle barely fills passively at all, which contradicts the mostly-passive filling described. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 19 "The Cardiovascular System: The Heart", section 19.1, p. 836_