Blue blood returns from the body through the venae cavae into the right atrium, passes the tricuspid valve into the right ventricle and is pushed through the pulmonary valve to the lungs. It comes back red through the four pulmonary veins into the left atrium, crosses the mitral valve, and the thick left ventricle drives it through the aortic valve into the aorta. The valves open and shut only when the pressures on either side cross; their closing is the heartbeat's “lub” (S1) and “dub” (S2). Every motion here is computed from a time-varying-elastance model of one beat: change the heart rate and diastole shortens while systole barely changes.