The heart is the pump that powers everything you do. Whether you're sleeping, or pushing life to its limits, it's the constant beat of your heart that keeps you alive.
About the size of your fist, the heart is actually a muscle. Its one and only function is to pump blood.
It's the contraction of the microscopic muscle cells of the heart that squeezes blood at enough pressure to move it round the whole body.
Unlike your arm and leg muscles, the heart works without you thinking about it and it never tires.
Over a lifetime, the heart works harder than any other muscle.
Your heart beats 100,000 times a day
More than 3 billion beats in a lifetime
The heart has two sides, each with two hollow chambers, the atrium and ventricle.
The blood enters the heart into the atria, and is pumped out from the ventricles.
The job of the right side of the heart is to receive blood from the body, and pump it to the lungs... where it picks up oxygen.
The left side receives this oxygen-rich blood from the lungs, and pumps it from the left ventricle out to the body.
A pig's heart
Because it must work harder and produce more force to do this, the left side of your heart has thicker walls, and is larger than the right.
As the blood moves out of the heart, it enters a system of tubes that act like a transport network, directing the blood to the different parts of the body, where it delivers vital oxygen and fuel powering everything from your brain to your muscles.
Blood is kept flowing in one direction by flaps of tissue in the heart, called heart valves.
There are two pairs of valves, and they act like gates that can only be pushed open in one direction, stopping the blood moving backwards.
It's the closing of the valves that make the characteristic sound of a heartbeat.