The first heart sound (S1) represents closure of the atrioventricular (mitral and tricuspid) valves as the ventricular pressures exceed atrial pressures at the beginning of systole (point a). S1 is normally a single sound because mitral and tricuspid valve closure occurs almost simultaneously.
Its photosynthesis because Plants can do two important things: Use energy from the sun to turn CO2<span> (</span>carbon dioxide) and H2<span>O (water) </span>
