"The first heart sound
or "lub" results from closure of the tricuspid
valves. It is a rather low-pitched and a relatively long sound
which, as indicated
in, represents the beginning
of ventricular systole.
, or "dub," marks
of ventricular diastole. It is produced by closure of the aortic and pulmonary (pulmonic) semilunar vanes when the intraventricular pressure begins to fall. This "dub" sound
is typically heard as a sharp snap because
the semilunar valves tend to close much more rapidly than
the AV valves. Because
diastole occupies more time than
systole, a brief pause occurs
after the second
when the heart is beating at a normal rate. Therefore, the pattern that
one hears is one of: "lub-dub" pause, "lub-dub" pause, and so on.
Sometimes, especially in young normal individuals, a third heart sound
can be heard. This sound
is produced by the very
rapid influx of blood into
the partially filled ventricle. It is typically very
faint and as such
difficult to hear"