YOUR HEART is about the size of your clenched fist.
It has thick muscular walls and is divided into two pumps.
Each pump has two chambers.
The upper, smaller, thin-walled atrium receives blood coming in from the veins.
The blood flows through a one-way valve, which makes sure it always moves in the correct direction,
into the larger, lower chamber called the ventricle. It has thick strong walls that
contract to squeeze blood through another valve, out into the arteries.
