Chapter 18
The Cardiovascular System: The Heart
679
18
turbulent and generates abnormal heart sounds, called
heart
murmurs
, 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
insufficient
or
incompetent
valve fails to close completely. Tere
is a swishing sound as blood backflows or regurgitates through
the partially open valve
afer
the valve has (supposedly) closed.
A
stenotic
valve fails to open completely and its narrow open-
ing restricts blood flow
through
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
sys-
tole
(sis
9
to-le) refers to these periods of contraction, and
dias-
tole
(di-as
9
to-le) refers to those of relaxation. Te
cardiac cycle
includes
all
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
events always
Follow
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
thorax
(Figure 18.20)
. 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.
Homeostatic Imbalance
18.7
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.
Figure 18.19
Normal and abnormal ECG tracings.
Tricuspid valve
sounds
typically heard in right
sternal margin of 5th
intercostal space
Aortic valve
sounds
heard in 2nd intercostal
space at right sternal
margin
Pulmonary valve
sounds heard in 2nd
intercostal space at left
sternal margin
Mitral valve
sounds
heard over heart apex
(in 5th intercostal space)
in line with middle of
clavicle
Figure 18.20
Areas of the thoracic surface where the sounds
of individual valves can best be detected.
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