668
UNIT 4
Maintenance of the Body
18
An
incompetent
, or
insufficient
,
valve
forces the heart to re-
pump the same blood over and over because the valve does not
close properly and blood backflows. In valvular
stenosis
(“nar-
rowing”), the valve flaps become stiff (typically due to calcium
salt deposits or scar tissue that forms following endocarditis) and
constrict the opening. Tis stiffness compels the heart to con-
tract more forcibly than normal. Both conditions increase the
heart’s workload and may weaken the heart severely over time.
Te faulty valve (most oFen the mitral valve) can be replaced
with a mechanical valve, a pig or cow heart valve chemically
treated to prevent rejection, or cryopreserved valves from human
cadavers. Heart valves tissue-engineered from a patient’s own cells
grown on a biodegradable scaffold are being developed.
Check Your Understanding
4.
What is the function of the papillary muscles and chordae
tendineae?
For answers, see Appendix H.
Pathway of Blood Through the Heart
Trace the pathway of blood through the heart.
As ventricles
contract and
intraventricular
pressure rises,
blood is pushed
up against
semilunar valves,
forcing them
open.
As ventricles
relax and
intraventricular
pressure falls,
blood flows
back from
arteries, filling
the cusps of
semilunar valves
and forcing
them to close.
(a) Semilunar valves open
(b) Semilunar valves closed
Aorta
Pulmonary
trunk
Figure 18.8
The semilunar (SL) valves.
Having covered the basic anatomy of the heart, we can now
follow the path that blood takes through the heart and its
associated circuits.
Focus on Blood Flow Trough the Heart
(Figure 18.9)
follows a single “spurt” of blood as it passes
through all four chambers of the heart and both blood cir-
cuits in its ever-repeating journey.
As you work your way through this figure, keep in mind that
the leF side of the heart is the
systemic circuit pump
and the
right side of the heart is the
pulmonary circuit pump
. Notice
how unique the pulmonary circuit is. Elsewhere in the body,
veins carry relatively oxygen-poor blood to the heart, and ar-
teries transport oxygen-rich blood from the heart. Exactly the
opposite oxygenation conditions exist in veins and arteries of
the pulmonary circuit.
Equal volumes of blood are pumped to the pulmonary and
systemic circuits at any moment, but the two ventricles have
very unequal workloads. Te pulmonary circuit, served by the
right ventricle, is a short, low-pressure circulation. In contrast,
the systemic circuit, associated with the leF ventricle, takes a
long pathway through the entire body and encounters about five
times as much friction, or resistance to blood flow.
Tis functional difference is revealed in the anatomy of the
two ventricles (±igure 18.5e and
Figure 18.10
). Te walls of
the leF ventricle are three times thicker than those of the right
ventricle, and its cavity is nearly circular. Te right ventricular
cavity is flattened into a crescent shape that partially encloses
the leF ventricle, much the way a hand might loosely grasp
a clenched fist. Consequently, the leF ventricle can generate
much more pressure than the right and is a far more powerful
pump.
Coronary Circulation
Name the major branches and describe the distribution of
the coronary arteries.
Although the heart is more or less continuously filled with
blood, this blood provides little nourishment to heart tissue.
(Te myocardium is too thick to make diffusion a practical
means of delivering nutrients.) How, then, does the heart get
nourishment? Te
coronary circulation
, the functional blood
supply of the heart, is the shortest circulation in the body.
Coronary Arteries
Te
lef
and
right coronary arteries
both arise from the base
of the aorta and encircle the heart in the coronary sulcus.
Tey provide the arterial supply of the coronary circulation
(Figure 18.11a)
.
Te
lef coronary artery
runs toward the leF side of the heart
and then divides into two major branches:
Te
anterior interventricular artery
(also known clinically
as the
lef anterior descending artery
) follows the anterior in-
terventricular sulcus and supplies blood to the interventricu-
lar septum and anterior walls of both ventricles.
Te
circumflex artery
supplies the leF atrium and the poste-
rior walls of the leF ventricle.
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