Heart has 4 chambers- right atrium, right ventricle, left atrium. left ventricle.
1) Blood flows into 2 atria from vena cava and pulmonary vein.
2) Atria contract, pushing blood into ventricles.
3) Ventricles contract, forcing blood into pulmonary artery and aorta, and out of heart.
4) Blood flows to organs through arteries, returns through veins.
5) Atria fill again- cycle repeats.