Your Heart.
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An adult's is roughly the size of a clenched fist.
Your heart lies in the centre of your chest, surrounded by a protective membrane called the pericardium. In an average day, the heart pumps nearly 5000 gallons of blood.
Your heart is a muscular organ, divided into left and right sides.
The right side of your heart receives deoxygenated blood though the veins. It pumps this blood to your lungs where it picks up more oxygen. This oxygenated blood then returns to the left side of your heart, which pumps it out to the rest of your body through the arteries. The left side is slightly larger because it has more work to do, pumping blood around your body.
Each side of your heart is divided into an upper chamber called an atrium and a larger, lower chamber, called a ventricle. Blood flows from the atria to the ventricles through one-way valves. Your heart works as a pump, with its muscular walls contracting to force the movement of blood.
This article was originally based on an entry from a public domain edition of Gray's Anatomy.