The right atrium receives deoxygenated blood from all parts of the body through the vena cava and the left atrium receives oxygenated blood from the lungs through the pulmonary veins. This blood is then passed into the ventricles. This is a mechanical event of circulation. On the other hand, the sino-atrial (SA) node generates the action potential that stimulates contraction of the atria. The impulse then passes to the atrioventricular (AV) node, from where it is conducted to the ventricles through the bundle of His and Purkinje fibres, bringing about ventricular contraction. This is an electrical event of the cardiac cycle. Since the flow of blood into the atria and ventricles is dependent on the passage of electrical impulse from SA node to AV node, the two statements are dependent on each other.