It is the longest and loudest of the four sounds. S 1 is heard best over the apical region of the heart. S 1 is produced by the oscillation of blood in the ventricular chambers and vibration of the ventricular free wall. At the onset of systole, ventricular pressure begins to increase and forces the blood within the ventricles toward the atria through the mitral and tricuspid valves. As these one way valves close, blood and the ventricular wall vibrates producing the S 1 sound.
Heart now is in the iso-volumetric contraction phase of the cardiac cycle.
The second heart sound is generated the end of the ejection of the blood from the ventricles by the closure of the semilunar valves, i.e. aortic and pulmonic valves. At this point, blood from the ventricle is pumped into the arteries, arterial pressure exceeded the ventricular pressure, hence the one way valves are closing as the pressure is reversed. These sounds have higher frequency components and the aortic valve sound is louder than the pulmonic valve sound. This heart sound is heard in young children and patients with left ventricular failure and believed to be resulted by the deceleration of blood entering the ventricle. In enlarged hearts, ventricular walls are stretched beyond normal ranges and are under tension allowing vibrations.