Processes of the heart and lungs

I was initially struggling with grasping this process until I started viewing the heart as two separate parts in one unit – the deoxygenated side and the oxygenated side, or the interatrial and interventricular, respectively (Fremgen et al 134).  As this is an ongoing, circular process it has no beginning, nor end, so I will begin describing the process from the point where the deoxygenated blood arrives at the heart from the body.