There are valves between each atrium and ventricle -and between each ventricle and the blood vessel that leaves it. As the blood swishes through, the valves slam shut, producing what we hear on TV as a heartbeat. (Doctors listening through the stethoscope are trying to hear how your heart valves are functioning; the "crispness of the sound" describes how well the valves are coming together; timing differences between the valves' sounds can indicate electrical differences between the two sides of the heart, and murmurs can indicate the size of the valve openings or any leakage through the valves.)
The valves are crucial to the heart's functioning because a little leakage within the heart is a dangerous thing. If the valves are inefficient or malformed, if they waver rather than slam open and closed, blood can pool in the atrium, just like with atrial fibrillation . And when blood pools for any amount of time, it is programmed to do one thing: clot. Inside the heart, a clot leads to disaster.