Deoxygenated blood enters the heart from the superior and inferior vena cava. It enters the first of four chambers, called the right artium. The top of the first chamber is home of the heart’s natural pacemaker, the sinoatrial (SA) node. Blood then passes through first of two atrioventricular valves called the valve into the (chamber) during a momentary relaxed period of the cardiac cycle called ___. From there, it passes through the __ valve and enters the __ artery. After becoming oxygenated in the __, blood returns via the __ veins and enters the __. Blood is squeezed downward through the second atrioventricular valve called the __ valve where it enters the __. It is pumped during __ out through the ___ valve into the ascending aorta___. for medical terms.