Mammals have a double circulation, which means that the right hand side of the heart pumps deoxygenated blood to the lungs in the pulmonary artery to pick up oxygen and release carbon dioxide. The oxygenated blood then returns to the left hand side of the heart in the pulmonary vein.
Ø Blood returns from the vena cave (the body) and the pulmonary vein (the lungs) to the atria.
Ø Atrioventricular valves are open.
Ø Sino Atrial Node sends out an electrical signal which causes both atria to contract.