565
CHAPTER FIFTEEN
Cardiovascular System
such structures are in the heart—in the atrial walls and in
the ventricular walls. Portions of the heart’s F
brous skeleton
separate these masses of cardiac muscle F
bers, except for a
small area in the right atrial fl
oor. In this region, F
bers of the
cardiac conduction system connect the
atrial syncytium
and
the
ventricular syncytium.
PRACTICE
17
Describe the pressure changes in the atria and ventricles during a
cardiac cycle.
18
What causes heart sounds?
19
What is a functional syncytium?
20
Where are the functional syncytia of the heart?
Cardiac Conduction System
Throughout the heart are clumps and strands of specialized
cardiac muscle tissue whose F
bers contain only a few myoF
-
brils. Instead of contracting, these areas initiate and distrib-
ute impulses (cardiac impulses) throughout the myocardium.
They comprise the
cardiac conduction system,
which coor-
dinates the events of the cardiac cycle.
A key portion of this conduction system is the
SA node
(sinoatrial node
or
sinuatrial node)
, a small, elongated mass
of specialized cardiac muscle tissue just beneath the epicar-
dium. It is in the right atrium near the opening of the superior
erode the edges of the valvular cusps. As a result, the cusps
may not close completely, and some blood may leak back
through the valve, producing an abnormal sound called
a
murmur.
The seriousness of a murmur depends on the
degree of valvular damage. Many heart murmurs are harm-
less. It is often possible to repair damaged valves or to
replace them. ±rom Science to Technology 15.1 describes
treatments for a failing heart.
Using a stethoscope, it is possible to hear sounds associ-
ated with the aortic and pulmonary valves by listening from
the second intercostal space on either side of the sternum.
The
aortic sound
comes from the right, and the
pulmonic
sound
from the left. The sound associated with the mitral
valve can be heard from the fifth intercostal space at the
nipple line on the left. The sound of the tricuspid valve can
be heard at the F fth intercostal space just to the right of the
sternum
(f g. 15.17)
.
Cardiac Muscle Fibers
Recall that cardiac muscle F
bers function like those of skel-
etal muscles, but the F
bers connect in branching networks
(chapter 9, p. 301). Stimulation to any part of the network
sends impulses throughout the heart, which contracts as a
unit.
A mass of merging cells that act as a unit is called a
functional syncytium
(funk
shun-al sin-sish
e-um). Two
Aortic area
Tricuspid
area
Pulmonary area
Mitral area
FIGURE 15.17
Thoracic regions where the sounds of each heart valve are most easily heard.
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