Now that we've been through each chamber individually, have a look at the heart overall. The image shows that the left and right sides work at the same time.
The atria are contracting simultaneously to send blood into the right and left ventricles, which then contract at the same time to send it out to the lungs and the body.
Check out a CGI animation of a beating heart.
Or if you're really interested, see the real thing bop around in an open chest!
