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

• Pushing open of the venous valves

• suction pull

• Stimulation of the sino-auricular node

• the pressure difference between the caval and atrium

D.

the pressure difference between the caval and atrium

the action of the heart includes contractions and relaxations of the atria and ventricles. Contraction of the heart is called systole and relaxation is called diastole. The dynamics of blood flows through the blood vessels is no exception and blood flow through the blood vessels along a pressure gradient, always moving from higher to lower pressure areas. Fundamentally, the pumping action of heart generated blood flow. The contraction of atria is initiated and activated by the sinoatrial node (SA node- Pacemaker), which spreads waves of contraction across the wall of atria via muscle fibres at regular intervals.