662
UNIT 4
Maintenance of the Body
18
Check Your Understanding
1.
The heart is in the mediastinum. Just what is the
mediastinum?
2.
From inside to outside, list the layers of the heart wall and
the coverings of the heart.
3.
What is the purpose of the serous fluid inside the pericardial
cavity?
For answers, see Appendix H.
Chambers and Associated Great Vessels
Describe the structure and functions of the four heart
chambers. Name each chamber and provide the name and
general route of its associated great vessel(s).
Te heart has four chambers
(Figure 18.5e)
—two superior
atria
(a
9
tre-ah) and two inferior
ventricles
(ven
9
trĭ-klz). Te
internal partition that divides the heart longitudinally is called
the
interatrial septum
where it separates the atria, and the
in-
terventricular septum
where it separates the ventricles. Te
right ventricle forms most of the anterior surface of the heart.
Te leF ventricle dominates the inferoposterior aspect of the
heart and forms the heart apex.
±wo grooves visible on the heart surface indicate the bound-
aries of its four chambers and carry the blood vessels supply-
ing the myocardium. Te
coronary sulcus
(²igure 18.5b, d), or
atrioventricular groove
, encircles the junction of the atria and
ventricles like a crown (
corona
5
crown). Te
anterior inter-
ventricular sulcus
, cradling the anterior interventricular artery,
marks the anterior position of the septum separating the right
and leF ventricles. It continues as the
posterior interventricu-
lar sulcus
, which provides a similar landmark on the heart’s
posteroinferior surface.
Atria: The Receiving Chambers
Except for small, wrinkled, protruding appendages called
au-
ricles
(or
9
ĭ-klz;
auricle
5
little ear), which increase the atrial
volume somewhat, the right and leF atria are remarkably free of
distinguishing surface features. Internally, the right atrium has
two basic parts (²igure 18.5c): a smooth-walled posterior part
and an anterior portion in which bundles of muscle tissue form
ridges in the walls. Tese muscle bundles are called
pectinate
muscles
because they look like the teeth of a comb (
pectin
5
comb). Te posterior and anterior regions of the right atrium
are separated by a C-shaped ridge called the
crista terminalis
(“terminal crest”).
In contrast, the leF atrium is mostly smooth and pectinate
muscles are found only in the auricle. Te interatrial septum
bears a shallow depression, the
fossa ovalis
(o-vă
9
lis), that
marks the spot where an opening, the
foramen ovale
, existed in
the fetal heart (²igure 18.5c, e).
²unctionally, the atria are receiving chambers for blood re-
turning to the heart from the circulation (
atrium
5
entryway).
Te atria are relatively small, thin-walled chambers because they
need to contract only minimally to push blood “downstairs”
into the ventricles. As a rule, they contribute little to the propul-
sive pumping activity of the heart.
Blood enters the
right atrium
via three veins (²igure 18.5c–e):
Te
superior vena cava
returns blood from body regions
superior to the diaphragm.
Te
inferior vena cava
returns blood from body areas below
the diaphragm.
Te
coronary sinus
collects blood draining from the myo-
cardium.
²our
pulmonary veins
enter the
leF atrium
, which makes
up most of the heart’s base. Tese veins, which transport blood
from the lungs back to the heart, are best seen in a posterior
view (²igure 18.5d).
Ventricles: The Discharging Chambers
±ogether the ventricles (
ventr
5
underside) make up most of the
volume of the heart. As already mentioned, the right ventricle forms
most of the heart’s anterior surface and the leF ventricle dominates
its posteroinferior surface. Irregular ridges of muscle called
trabec-
ulae carneae
(trah-bek
9
u-le kar
9
ne-e; “crossbars of flesh”) mark
the internal walls of the ventricular chambers. Still other muscle
bundles, the conelike
papillary muscles
, which play a role in valve
function, project into the ventricular cavity (²igure 18.5e).
Te ventricles are the discharging chambers, the actual
pumps of the heart. Teir walls are much more massive than
the atrial walls, reflecting the difference in function between the
atria and ventricles (²igure 18.5e and f). When the ventricles
contract, they propel blood out of the heart into the circulation.
Te right ventricle pumps blood into the
pulmonary trunk
,
which routes the blood to the lungs where gas exchange occurs.
Te leF ventricle ejects blood into the
aorta
(a-or
9
tah), the larg-
est artery in the body.
Heart Valves
Name the heart valves and describe their location, function,
and mechanism of operation.
Blood flows through the heart in one direction: from atria to ven-
tricles and out the great arteries leaving the superior aspect of the
heart. ²our valves enforce this one-way traffic (²igure 18.5e and
Figure 18.6
). Tey open and close in response to differences in
blood pressure on their two sides.
Atrioventricular (AV) Valves
Te two
atrioventricular (AV) valves
, one located at each
atrial-ventricular junction, prevent backflow into the atria when
the ventricles contract.
Te right AV valve, the
tricuspid valve
(tri-kus
9
pid), has
three flexible cusps (flaps of endocardium reinforced by con-
nective tissue cores).
Te leF AV valve, with two cusps, is called the
mitral valve
(mi
9
tral) because it resembles the two-sided bishop’s miter or
hat. It is sometimes called the
bicuspid valve
.
(Text continues on p. 667.)
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