What is cardiac cycle and it is important

   Propagation of excitation through the ventricular myocardium triggers the ventricular systole. The increasing pressure forces some blood into the atrium, this closes the AV valves, the ventricle builds up pressure (tension time; the vibrations that occur produce the 1st heart sound). The rate of pressure increase (dp / dtMax) is a contractility index. Once the pressure has overcome that behind the arterial valve, it is opened, blood flows out of the ventricle, ~ 75% of the stroke volume in the first half of the expulsion time. At the end of this, <5% of the stroke volume flows back, the arterial valve closes (2nd heart sound, incision in the aortic pulse). During the relaxation time, the remaining volume (~ 50 ml) is in the ventricle. At the beginning of the filling phase, the open AV valve slides over part of the blood temporarily stored in the atrium (valve level mechanism); the atrial contraction completes the filling time. The end-diastolic filling volume in healthy adults is ~ 130 ml (stroke volume ~ 80 ml)
   Laplace's law describes the relationship between wall thickness, wall tension and internal pressure: The wall tension necessary to maintain the pressure is proportional to the internal pressure and the internal radius, inversely proportional to the wall thickness: the larger the internal diameter, the more force the myocardium must develop less, the thicker the muscle wall (cardiomyocytes are challenged to different degrees depending on their position: subendocardials are less stressed than subepicardial ones). If the afterload increases, the wall tension in the ventricle also increases. The afterload depends not only on the pressure, but also on the geometry of the ventricle (the narrower, the lighter); as the expulsion increases, the ventricle narrows and the afterload decreases. In a healthy heart the Frank-Starling effect predominates, with ventricular dilation the Laplace effect, the ventricle becomes insufficient. Reactive hypertrophy (greater wall thickness) reduces wall tension and facilitates cardiac action
   The pressure-volume diagram