At R, the closing valves produce the heart sound. Between point R and S ventricular pressure increases greatly since the semilunar valves are still closed and there is no blood flow. The semilunar valves open at S when the ventricular pressure equals the aortic pressure. The ventricular contraction forces blood into the aorta and there is an increase in both aortic and ventricular pressure. The blood is pumped from the ventricles and is carried away in the aorta.
Ventricular pressure drops.
When the pressure drops below aortic pressure, the semilunar valves shuts. After the T-wave the ventricular pressure falls below atrial pressure and the mitral and bicuspid valves open up. Normal Sinus Rhythm P Waves represent depolarization of the atrial myocardium. The QRS complex represent depolarization of the ventricular myocardium. On EKG paper, one large square represents 0.20 second, a small one 0.04 seconds.