Heart:
The heart is the organ which pumps blood throughout the circulatory system.  All vertebrates have a heart, as do many invertebrates. The human heart is made up of cardiac muscle, which must keep working throughout your lifetime. The average human heart (based on a 78 year life span) beats about 3 billion times! But for all that work it does, the heart usually weighs less than a pound.

The human heart has four chambers, through which blood passes-
Deoxygenated blood enters the chamber called the right atrium,
it then moves to the next chamber called the right ventricle.
From the right ventricle it passes through the pulmonary arteries to the lungs where it receives oxygen.
From the lungs, blood returns to the heart's left atrium and left ventricle.

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