The heart is a very specialized muscle consisting of four chambers: a right and a left upper chamber (atrium) and a right and a left lower chamber (ventricle). The atria are thin-walled reservoirs for blood and empty directly into the thick walled, muscular lower chambers, which contract and pump the blood out of the heart. The heart is really "two hearts that beat as one." The left atrium and ventricle receive red, oxygenated blood from the lungs and pump it to the body. The right atrium and ventricle receive blue, de-oxygenated blood from the body and pump it to the lungs, where the blue blood is oxygenated.
The arteries are blood vessels that carry blood away from the heart, whereas veins carry blood back towards the heart. The heart muscle itself receives blood supply from two arteries: the left coronary and right coronary artery. "Coronary" means "crown" and the coronary arteries get their name by the fact that they encircle the top portion of the heart much like a crown. Specialized nerve cells in the heart generate and conduct electrical impulses that initiate the heart beat and coordinate the timing of contraction of the various chambers and allow the heart to beat in an organized fashion. A heart beat cycle consists of systole, when the heart is contracting and ejecting blood, and diastole, when the heart is relaxing and filling with blood for the next contraction.
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