In humans, blood passes from the post caval to the diastolic right atrium of heart due to

Due to the pressure difference between the post caval and atrium, the blood passes from the post caval to the diastolic right atrium. Because the action of heart includes contractions and relaxations of the atria and ventricles. The dynamics of blood flow in blood vessels is no exception and blood flows through the blood vessels along a pressure gradient, always moving from higher to lower pressure areas.