Download tunica adventitia

The Cardiovascular
System IV
Heart & Blood Vessels
Histology
By Dr. Nabil, Khouri
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The Heart Wall
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The myocardium.
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Note the endocardial layer, which consists of endothelium
supported by a rather thick layer of sub-endocardial connective
tissue (green). The ventricular lumen is indicated.
Lumen
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Cardiac muscle orientation
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The tissues that comprise the
myocardium, as well as the
adjacent tissues of the
endocardium and
pericardium, are continuous,
which means that the cardiac
muscle is one single tissue
that wraps around itself to
form the heart.
The myocardial tissue in
normal heart spirals up from
the base to the apex, causing
a series of clear intersections
of cardiac muscle tissue.
The double spiral formation
of the myocardial tissue
allows a 60% increase in
ejection fraction with a fiber
shortening of 15%.
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Tricuspid &
bicuspid
Valves
Different
Cardiac
muscle layers
& Orientations
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Cardiac Muscle Longitudinal Section

Cardiac muscle consists
of Branched muscle
cells with one centrally
placed nucleus.

The Nuclei are oval,
rather pale and located
centrally in the muscle
cell which is 10 - 15 µm
wide.

Cardiac muscle exhibits
cross-striations.

Cardiac muscle is for
these reasons also
called involuntary
striated muscle.
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cell nucleus
One cell
Intercalated Discs
X40 Magnification
Cardiac Muscle
N
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Cardiac Muscle Cross section &
Purkinje fibers
X20 Magnification
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X40 Magnification

This medium power view of the inner wall of the left ventricle shows
a cluster of Purkinje fibers, that are partof the conduction system of
the heart. The fibers run within the subendocardial connective
tissue.
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
At high power, the structure of individual Purkinje fibers can
be seen.
Much larger than normal cardiac muscle fibers, these cells still
contain centrally-placed nuclei and, although they do still
contain myofibrillar elements, their cytoplasm appears
somewhat vacuolated
Purkinje fibers
Sub-endocardial CT
Cardiac Cells
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
Note the coronary vessels surrounded by epicardial fat.
Can you determine which are arteries and which are
veins
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Blood Vessels
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Blood is carried in a closed system of vessels that begins
and ends at the heart
The three major types of vessels are arteries, capillaries,
and veins
Arteries carry blood away from the heart, veins carry
blood toward the heart
Capillaries contact tissue cells and directly serve cellular
needs
www.lab.anhb.uwa.edu.au/.../Vascular.htm
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Generalized Structure of Blood Vessels
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Arteries and veins are
composed of three tunics –
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tunica externa
tunica media
tunica interna
Capillaries are composed of
endothelium with sparse
basal lamina
Lumen – central bloodcontaining space
surrounded by tunics
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Tunics
Tunica interne (tunica
intimae)
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Endothelial layer that
lines the lumen of all
vessels

In vessels larger than
1 mm, a sub-endothelial
connective tissue basement
membrane is present

Tunica media
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Smooth muscle and
elastic fiber layer,
regulated by sympathetic
nervous system
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Controls
vasoconstriction/
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vasodilatation of vessels
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Tunica externa (tunica
adventitia)
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Collagen fibers that
protect and reinforce
vessels

Larger vessels contain
vasorum
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Education, Inc. publishing as Benjamin Cummings

Histological Structure of Blood Vessels
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Large Elastic (Conducting) Arteries

Thick-walled arteries near the heart; the aorta and its major
branches
 Large lumen allow low-resistance conduction of blood
 Contain internal elastic lamina and fibers in all three
tunics
 Withstand and smooth out large blood pressure
fluctuations
 Allow blood to flow fairly continuously through the body
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Elastic (Conducting) Arteries
Tunica intima:
Large arteries often have a large subendothelial layer. The border of the
intima is delineated by the internal elastic membrane. The internal elastic
membrane may not be conspicuous because of the abundance of elastic material
in the tunica media.
Tunica media:
Both connective tissue and smooth muscle are present in the intima. It is
the thickest of the three layers. The smooth muscle cells are arranged in a spiral
around the long axis of the vessel. They secrete elastin in the form of sheets, or
lamellae, which are fenestrated to facilitate diffusion. These lamellae, and the large
size of the media, are the most striking histological feature of elastic arteries. In
addition to elastin, the smooth muscle cells of the media secrete reticular and fine
collagen fibers and proteoglycans (all not identifiable). No fibroblasts are present.
Tunica adventitia:
This is a relatively thin connective tissue layer. Fibroblasts arethe
predominant cell type, and many macrophages are also present. Collagen fibres
predominate and elastic fibres (not lamellae) are also present. Blood vessels
supplying the adventitia and outer media are also present, these are called vasa
vasorum ("vessels of the vessels").
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Adventitia
{
{
Intima Media
{
ELASTIC ARTERY
Endothelium
Collagen
Smooth muscle cells
Vasa vasorum
Elastic laminae
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Elastic artery Aorta
el = elastic lamellae
end = endothelial cell nuclei
n = smooth muscle cell nuclei
TA = tunica adventitia
TI = tunica intima
TM = tunica media
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vasa vasorum
Elastic fibers
Lumen
Endothelium
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Muscular Arteries and Arterioles

Muscular arteries – distal to elastic arteries; deliver blood
to body organs
 Have thick tunica media with more smooth muscle
and less elastic tissue
Active in vasoconstriction
Arterioles – smallest arteries; lead to capillary beds
 Control flow into capillary beds via vasodilation and
constriction
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Medium Arteries:
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Tunica intima:
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The tunica intima is thinner than in large arteries, there are fewer smooth
muscle cells and less elastic tissue.
 The outermost part of the intima is defined by a very prominent internal
elastic membrane (not obscured by elastic lamellae as in large arteries).
 The basement membrane of the endothelium may rest directly on the
internal elastic membrane, or be separated by a sub-endothelial layer of
CT.
Tunica media:
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Smooth muscle cells predominate in the tunica media, and little elastic
material is present. As in large arteries, no fibroblasts are present.
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lastic fibres (few), collagen, and ground substance are produced by the
smooth muscle cells.
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In tissue preparation, the internal elastic membrane of the intima appears
wavy due to the contraction of the smooth muscle of the media.
Tunica adventitia:
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The main constituent of the adventitia is collagen fibres, secreted by
fibroblasts. Elastic fibres are also present, a concentration of such fibres at
the inner boundary of the adventitia is called the external elastic membrane.

The external elastic membrane is not as prominent as the internal, and as
arteries get smaller (see small arteries, below) disappears much earlier.

The tunica adventita is relatively larger than in elastic arteries,
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VESSEL-WALL LAYERING
Endothelium
}
Lamina
propria
Internal elastic
lamina
Tunica
INTIMA
External elastic lamina
Tunica
MEDIA
Tunica
ADVENTITIA
This layering scheme works well for muscular arteries, but
becomes strained when applied to other vessels, e.g., in regions of
the vena cava there may be no T media; in arterioles, the intima has
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a Copyright
convincing
identity only in TEM
Muscular Artery and Vein
V
A
artery
tunica externa
thickest layer of
vein
vein
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Artery and vein
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Smallest ARTERIOLE
Endothelial
cell
For fast flow &
non-stick, until
clotting is
needed
Controls passage
through the wall
Helps control
blood flow
Smooth
muscle cell
SMC/ VSMC
Contraction
regulates flow by need
Reticular
fibers
Mechanical support
Smallest arteriole, is a capillary with smooth muscle cells wrapped around it,
with modifications to the endothelial cells - less transport, more interaction
CopyrightSMCs.
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with
Arteriole
arteriole (center) and an accompanying venule (right).
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arteriole and an accompanying venule.
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Capillaries
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Capillaries are the smallest blood vessels
 Walls consisting of a thin tunica interna, one cell
thick
Allow only a single RBC to pass at a time
 Pericytes on the outer surface stabilize their walls
There are three structural types of capillaries:
continuous, fenestrated, and sinusoids
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Capillary Structure
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Figure 21.4
Capillary
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Continuous Capillaries

Continuous capillaries are abundant in the skin and
muscles, and have:
 Endothelial cells that provide an uninterrupted lining
Adjacent cells that are held together with tight
junctions
 Intercellular clefts of unjoined membranes that allow
the passage of fluids
Continuous capillaries of the brain:
 Have tight junctions completely around the
endothelium
 Constitute the blood-brain barrier
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
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Continuous Capillaries
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Fenestrated Capillaries
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Found wherever active capillary absorption or filtrate
formation occurs (e.g., small intestines, endocrine
glands, and kidneys)
Characterized by:
 An endothelium riddled with pores (fenestrations)

Greater permeability to solutes and fluids than
other capillaries
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Fenestrated Capillaries
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Sinusoids
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Highly modified, leaky, fenestrated capillaries
with large lumens
Found in the liver, bone marrow, lymphoid
tissue, and in some endocrine organs
Allow large molecules (proteins and blood
cells) to pass between the blood and
surrounding tissues
Blood flows sluggishly, allowing for
modification in various ways
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Sinusoids
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Capillary Beds
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An interconnected network of vessels
consisting of
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Collateral arteries feeding an arteriole
Metarterioles
Arteriovenous anastomoses
Capillaries
Venules
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Capillary Beds

A microcirculation of interwoven networks of
capillaries, consisting of:
 Vascular shunts – metarteriole–
thoroughfare channel connecting an
arteriole directly with a postcapillary
venule
 True capillaries – 10 to 100 per capillary
bed, capillaries branch off the metarteriole
and return to the thoroughfare channel at
the distal end of the bed
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Capillary Beds
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The Organization of a Capillary Bed
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Capillary
Capillary
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Veins
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Collect blood from all tissues and organs and
return it to the heart
Are classified according to size
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Venules
Medium-sized veins
Large veins
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Vein
tunica externa
tunica media
tunica interna
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LARGE VEIN Details
Adventitia
{
{
Intima
Occasional circular SMC
Numerous elastic fibers
Bundles of longitudinal smooth muscle
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Large vien
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Muscular vein
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Medium size veins
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The tunica Externa
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The tunica media
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consists of the endothelium and a thin subendothelial layer with smooth
muscle cells among the connective tissue elements. A thin internal elastic
membrane may or may not be present. (If present, it is not nearly as
prominent as in arteries).
is much thinner relative to that of an artery, and consists mostly of circularly
arranged smooth muscle but also contains collagen fibres.
The tunicas intima and media therefore tend to be less distinct from one
another than is the case in arteries.
The tunica adventitia is usually thicker than the media and is made up
mostly of collagen fibres.

It may contain longitudinally oriented smooth muscle bundles. (Remember
gradations between the vessels of different sizes are continuous.)
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Medium size vein

Medium-sized vein with a much less compact muscle layer than you
saw in the preceding arteries. The tunica media is indicated by bar
"a". Bar "b" = adventitia, which is at least as wide as the media, and
often even wider. There is no evident inner elastic membrane. (Blood
in the lumen stains red here.) To the right, compare sizes and walls of
one small artery (d) and two very small veins (c) and (e
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The Function of Valves in the
Venous System
Venules and medium-sized
veins contain valves
Prevent backflow of blood
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A Comparison of a Typical Artery and a
Typical Vein
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