The heart is the pump that keeps blood flowing throughout the circulatory systems. 
It is divided into four chambers.
The top chambers of the heart are the atria.
Below each atrium is a chamber called a ventricle.
Each of the four chambers has a different job.
The right atrium collects blood returning to the heart. 
Blood entering this atrium has little oxygen in it because the oxygen has already been delivered to cells throughout the body.