The Cardiovascular System: The Heart
turbulent and generates abnormal heart sounds, called
, that can be heard with a stethoscope. Heart mur-
murs are fairly common in young children (and some elderly
people) with perfectly healthy hearts, probably because their
heart walls are relatively thin and vibrate with rushing blood.
Most oFen, however, murmurs indicate valve problems. An
valve fails to close completely. Tere
is a swishing sound as blood backﬂows or regurgitates through
the partially open valve
the valve has (supposedly) closed.
valve fails to open completely and its narrow open-
ing restricts blood ﬂow
the valve. In a stenotic aortic
valve, for instance, a high-pitched sound or click can be de-
tected when the valve should be wide open during ventricular
contraction, but is not.
Mechanical Events: The Cardiac Cycle
Describe the timing and events of the cardiac cycle.
Te heart undergoes some dramatic writhing movements as it
alternately contracts, forcing blood out of its chambers, and then
relaxes, allowing its chambers to reﬁll with blood. Te term
to-le) refers to these periods of contraction, and
to-le) refers to those of relaxation. Te
events associated with the blood ﬂow through the
heart during one complete heartbeat—atrial systole and diastole
followed by ventricular systole and diastole. Tese mechanical
the electrical events seen in the ECG.
Te cardiac cycle is marked by a succession of pressure and
blood volume changes in the heart. Because blood circulates
endlessly, we must choose an arbitrary starting point for one
Because the mitral valve closes slightly before the tricuspid
valve does, and the aortic SL valve generally snaps shut just be-
fore the pulmonary valve, it is possible to distinguish the indi-
vidual valve sounds by auscultating four speciﬁc regions of the
. Notice that these four points, while not
directly superﬁcial to the valves (because the sounds take oblique
paths to reach the chest wall), do handily deﬁne the four corners
of the normal heart. Knowing normal heart size and location is
essential for recognizing an enlarged (and oFen diseased) heart.
Blood ﬂows silently as long as its ﬂow is smooth and uninter-
rupted. If blood strikes obstructions, however, its ﬂow becomes
(a) Normal sinus rhythm.
(c) Second-degree heart block.
Some P waves are not conducted
through the AV node; hence more P than QRS waves are seen. In
this tracing, the ratio of P waves to QRS waves is mostly 2:1.
(d) Ventricular fibrillation.
These chaotic, grossly irregular ECG
deflections are seen in acute heart attack and electrical shock.
(b) Junctional rhythm.
The SA node is nonfunctional, P waves are
absent, and the AV node paces the heart at 40–60 beats/min.
Normal and abnormal ECG tracings.
typically heard in right
sternal margin of 5th
heard in 2nd intercostal
space at right sternal
sounds heard in 2nd
intercostal space at left
heard over heart apex
(in 5th intercostal space)
in line with middle of
Areas of the thoracic surface where the sounds
of individual valves can best be detected.