156
UNIT 3
Organ Systems
The Capillary System
Figure 9-13A,B
Continuous Capillaries
Figure 9-14A,B
Fenestrated Capillaries
Figure 9-15A,B
Discontinuous (Sinusoidal) Capillaries
The Venous System
Figure 9-16A–C
Venules and Small Veins
Figure 9-17A–C
Medium Veins
Figure 9-18A–C
Large Veins
Table 9-1
Blood Vessels
The Lymphatic Vascular System
Figure 9-19A,B
Lymphatic Vessels
Figure 9-19C
Clinical Correlation: Lymphangioma
Introduction and Key Concepts
for the Circulatory System
The
circulatory system
includes the
cardiovascular system
and the
lymphatic vascular system
. The
cardiovascular system
includes the
heart
and the
arterial
,
capillary
, and
venous sys-
tems
. Blood is transported from the heart through the arterial
system to the capillaries, where exchange of gases, nutrients,
and other substances takes place. Blood is carried back to the
heart by the venous system. Blood fl
ows through two routes:
(1) The
systemic circulation system
transports oxygenated
blood from the heart to the capillaries in the tissues and
organs of the body and then collects and carries the blood
back to the heart (Fig. 9-1). (2) The
pulmonary circulation
system
transports deoxygenated blood from the heart to the
capillaries of the lungs. After gas exchange, blood is carried
back to the heart (see Fig. 9-1). The
lymphatic vascular sys-
tem
consists of
lymphatic capillaries
,
lymphatic vessels
, and
lymphatic ducts
. This system collects
lymph
(excess tissue
uid) from the tissues of all organs (except the nervous sys-
tem, bone marrow, and hard tissues) by lymphatic capillar-
ies and then transports it through lymphatic vessels to the
lymphatic ducts, which eventually empty the lymph into the
venous system. The collected lymph passes through lymph
organs, where it is ±
ltered, and lymphocytes are exposed to
antigens. Lymphopoiesis and the immune response occur here
(Fig. 9-19A,B).
The Cardiovascular System
The Heart
The
heart
contains four chambers: the left and right atria and
the left and right ventricles. The
atria
receive blood fl
ow dis-
charged from the venous system, whereas the
ventricles
pump
blood into the arterial system (Fig. 9-1). The wall of the heart
is composed of three layers:
endocardium
(innermost layer),
myocardium
(middle layer), and
epicardium
(outermost layer).
(1)
Endocardium
consists of
endothelium
,
subendothelial
connective tissue
, and
subendocardium
(Purkinje ± bers, small
coronary blood vessels, and nerve ±
bers). (2)
Myocardium
, the
thickest layer of the heart, contains an abundance of
cardiac
muscle cells
(Fig. 9-3A,B). Cardiac muscle contracts
producing
heart beats, which are generated and regulated by the heart
conductive
system including the
sinoatrial (SA) node
, the
atrioventricular (AV) node
, the
AV bundle
, and
Purkinje ±
bers
(Fig. 9-2). (3)
Epicardium
is covered by
mesothelium
and con-
tains ±
brous connective tissue, nerves, coronary vessels, and
adipose tissue (Fig. 9-3C).
Types of Blood Vessels
THE ARTERIAL SYSTEM
is composed of
large
(
conducting
)
arteries,
medium
(
distributing
) arteries,
small arteries
, and
arte-
rioles
. The arterial system conducts blood (under higher pres-
sure than veins) from the ventricles to the capillary networks.
The walls of arteries can be
generally divided into three layers:
tunica intima
,
tunica media
, and
tunica adventitia
(Figs. 9-5
and 9-6).
Large arteries
are also called
elastic arteries
because of the
large quantity of elastic material in their walls (Fig. 9-7A–C).
They have a thick tunica media with numerous elastic mem-
branes. The
internal
and
external elastic laminae
are hard to
distinguish from the nearby elastic membranes. Large arter-
ies conduct blood from the ventricles into the medium arter-
ies. Rich elastic materials in large arteries enable the vessels to
recoil to accommodate pressure changes and maintain a con-
tinuous fl
ow of blood during ventricular diastole (relaxation).
Medium arteries
are also called
muscular arteries
because
of their thick tunica media, which contains circularly arranged
multiple layers of
smooth muscle cells
in a distinct sheath
(Figs. 9-8 to 9-9A). Internal and external elastic laminae are
easy to distinguish from nearby tissues.
Small arteries
and
arterioles
are smaller diameter vessels.
The walls of small arteries contain two to six layers of smooth
muscle cells (Figs. 9-9B and 9-10A,B).
Arterioles
are the small-
est components of the arterial system, with only one or two
layers of smooth muscle cells (Figs. 9-10A,C, and 9-11). They
control the blood fl
ow into the capillaries.
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