First, to remind you of basic descriptive points about the heart.

What it is. A hollow, muscular organ, tough and superbly engineered, which beats for a lifetime. It divides into two halves, the right and the left sides of the heart.
Where it is. In the centre of the chest, between the lungs and behind the breastbone.
Its function. To receive “used” blood from all parts of the body, pump it to the lungs, where the carbon dioxide is removed and a fresh supply of oxygen is given, receive it back again, and then pump the “new” (oxygenated) blood back to all tissues of the body.
But there is a lot more to it than that. Examine the sketch
The pathway of blood through the heart
Via lanimg.com
The two halves of the heart must function as receivers and deliverers of blood and a chamber is provided for each of these functions.
The receiving chamber, known as the atrium, is a thin-walled cavity separated by a one-way valve from the thick walled, more powerful delivering chamber, known as the ventricle.
Thus, there is a right atrium (which receives the “used” blood from the body), a right ventricle (which pumps this blood to the lungs), a left atrium (which receives the oxygenated blood from the lungs), and a left ventricle (which pumps this blood into the aorta, the big artery which has branches to all parts of the body).
You see all this in the sketch.
To make sure that the blood circulates in the right direction, valves are situated at the sites where the left and right ventricles open into their respective arteries, and in this way the blood pumped into the arteries is prevented from regurgitating back.
Since the heart muscle encloses the atria and the ventricles, the volume of these chambers is diminished and enlarged by the alternate contracting and relaxing of the beating heart.
This rhythmical squeezing action acts with the valves to make the blood flow in the right direction. Obviously, if the muscle of the ventricles is diseased or weak, the ability of the heart to do its pumping may be impaired.
If, on the other hand, the valves should leak, the blood may oscillate back and forth ineffectually instead of flowing purposefully in the proper direction.
The human heart consists of many millions of muscle cells, each in intimate contact with at least two of its neighbors.
But chaos would result if every cell were to beat at its own rate and rhythm, for the effective action of the heart depends on the co-ordinated, concerted action of the organ as a whole.
Due to chemical processes within it, each cell builds up an electrical charge across the membrane that surrounds it, so that the outside of the cell is positively charged and the inside of the cell is negatively charged.
Immediately before contraction, the cell membrane “leaks,” and the voltage difference between the two sides is momentarily dissipated by a transient “short circuit.” This phenomenon, called depolarisation, is followed by contraction of the cell.
Not only does depolarisation of the cell initiate contraction in that cell, it also initiates depolarisation of the contiguous cell.
In other words, cells use minute currents of electricity to communicate with each other, and a heartbeat begins.
After contraction, the leaky cell membrane is repaired and the voltage across it is restored by repolarization, to await a repetition of the entire cycle-depolarisation, contraction, repolarization.
Clearly there must be some initiating centre that sends out a regular impulse to the heart muscle cells 60 to 80 or more times a minute. Normally this “pacemaker” is the sinus node, in the right atrium.
If the sinus node is damaged by disease, the A-V node (atrioventricular node, a kind of relay station) assumes control, but the heart’s rhythm is affected adversely. If the A-V node is damaged, some other focus in the ventricles takes over, but the condition known as heart block results.
Coronary arteries
The oxygen and chemical foodstuffs needed for the functioning of the heart can reach the heart muscle only by the bloodstream.
And that is why the coronary arteries, which supply the heart muscle with blood, are so important. If disease blocks them, a “heart attack” may result, with the diminished blood supply causing malfunction in the heart muscle.

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