THE HEART

The heart is a double pump.

Right atrium
The right atrium takes in deoxygenated blood from the body.
Left atrium
The left atrium takes in oxygenated blood from the lungs
Right ventricle
The right ventricle sends this blood to the lungs.
Left ventricle
The left ventricle sends this blood to the body.

The atria contract first, sending blood into the ventricles.
The ventricles then contract, sending the blood out of the heart.