Deoxygenatedblood enters the heart from the superior and inferior . It enters the first of four chambers,

called the . The top of the first chamber is home

of the heart’s natural pacemaker, the 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.