Now, these two individual pumps are placed in serious with respect to one another. And what that means is they work together in a simultaneous way to create a movement of blood that is unidirectional and continuous. And this is exactly what we're going to focus on in this lecture. We're going to discuss the way that our blood actually moves within the four chambers of the heart. So let's begin with stage number one. Now, in stage number one, stage number two, and stage number three, we're taking a cross section of the heart so that we expose the four different chambers.