What is the sequence of events that occur from the beginning of one heart beat to the beginning of the next heart beat?
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The human heart has four chambers - two atria on top and two ventricles below. The heart is equipped with one-way valves separating the chambers and the major arteries. These valves prevent backflow of blood.
During one cardiac cycle, or the sequence of events in a single heartbeat, the blood flow through the heart. We see that it first enters the atria and then passively flows into the ventricles. When the ventricles are almost full, the atria contract and they push as much blood as possible into the ventricles. Now, the ventricles are almost full, and they're under very high pressure. At this time, they contract and push blood out of the heart and into the arteries.

This is sequence of events in a single heartbeat. Since the average heart beats about 75 beats per minute, the length of each cardiac cycle is less than one second. 
 There are two phases of the cardiac cycle. In the diastole phase, the heart ventricles are relaxed and the heart fills with blood. In the systole phase, the ventricles contract and pump blood out of the heart and to arteries. One cardiac cycle is completed when the heart chambers fill with blood and blood is then pumped out of the heart.

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