Blood flows into the heart through vena carva and pulmonary vein and then flows into right and left atrium
Pulmonary veins blood ( left ) just came from lungs - oxygenated
Blood from vena carva ( right ) just came from body so deoxygenated
Two atrium contract together and push blood into ventricles and then they contract to push blood out to pulmonary artery ( going to lungs + on right side ) and the aorta ( to body and on the left side )
Atrium will fill up with blood again and cycle repeats around 70 times a minute