This cause the AV valves to shut and creates what is referred to as the 1 st heart sound that is heard through a stethoscope. As the ventricles continue to contract and with the valves closed, the tension in ventricular muscle and pressure in the ventricular lumen rise rapidly. Since blood neither enters nor leaves the ventricles, this period is the isometric phase of ventricular contraction. The slight rise in atrial pressure is due to the AV valves bulging back into the atria. Eventually the ventricular pressure exceeds both aorta and pulmonary arterial pressures and causes these valves to open.
Blood is rapidly ejected into the arteries and ventricular volume falls.
The arterial pressure closely follows the ventricular pressure and rises from a diastole norm of 80 mmHg to systole norm of 120 mmHg. The atrial pressure falls and becomes negative because the contraction of the ventricles actually lengthens the atria and increases their capacity. In mid-systole the T wave of ventricular repolarization occurs, which means the ventricular muscles begin to relax and the pressure drops. While the ventricular pressure initially drops, blood continues to be ejected from the momentum imported by the initial acceleration in systole. In the aorta the pressure does not drop as rapidly as in the ventricles due to the resistance to blood flow and only about half the stroke volume has propelled through the aorta with the remainder in the elastic distension of the aorta.