The document provides details about the cardiovascular system histology. It begins with an introduction to the circulatory system and cardiovascular system. It then describes the layers of the heart - endocardium, myocardium and epicardium. Next, it discusses the types of blood vessels - arteries, which are divided into elastic arteries, muscular arteries and arterioles, and veins. It provides information on the layers of blood vessel walls - tunica intima, tunica media and tunica externa/adventitia. Finally, it describes the different types of capillaries - continuous, fenestrated and sinusoidal capillaries.

1. CARDIO VASCULAR
HISTOLOGY
• By Don Siyum A.
5/26/2022
By Don Siyum A.
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2. INTRODU
CTION
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The circulatory system
includes CVS and lymphatic
systems.
The Cardio Vascular System
The blood vascular system
consists of the heart,
major arteries, arterioles,
capillaries, venules, and
veins.
The main function of this
system is to deliver
oxygenated blood to cells
and tissues and to return
venous blood to the lungs
for gaseous exchange.

3. HEAR
T
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The heart is a muscular organ
that contracts rhythmically,
pumping the blood through the
circulatory system
The right and left ventricles
pump blood to the lungs and the
rest of the body respectively
The right and left atria receive
blood from the body and the
pulmonary veins respectively
The walls of all four heart
chambers consist of three major
layers or tunics:
The internal endocardium
The middle myocardium
The external epicardium

4. ENDOCAR
DIUM
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Consists of a simple
squamous endothelium and a
thin subendothelial loose
connective tissue containing
elastic and collagen fibers as
well as some smooth muscle
cells
Deeper to the endocardium
Subendocardial layer
containing veins, nerves, and
branches of the impulse-
conducting system of the
heart niv
The endocardium lines the
cavities of the atrium and
ventricle.
Endocardium (En); subendocardial layer (SEn); Purkinje fibers
(P) running separately within the subendocardial layer

5. Valve leaflet and fibrous skeleton: atrioventricular valve
(arrows); dense connective tissue (C); endocardium (En);
atrium (A); ventricle (V); chordae tendinae (CT);
myocardium (M)
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6.  The Purkinje fibers are located under
the endocardium, which represents
the endothelium of the heart cavities.
 The Purkinje fibers are different
from typical cardiac muscle fibers .
 The heart musculature has a rich
blood supply.
 Are a capillary , arteriole , and
venule.
 The cardiac muscle fibers are
connected to each other via the
prominent intercalated disks.
 The intercalated disks are not
observed in the Purkinje fibers.
 The Purkinje fibers are connected to
each other via desmosomes and gap
junctions, and eventually merge
with cardiac muscle fibers
CARDIAC MUSCLE FIBERS AND
PURKINJE FIBERS
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The Myocardium
 Is the thickest layer and
consists of cardiac muscle
fibers.
 Contains contractile cells
and impulse generating-
conducting cells
 The myocardium in both the
atrium and ventricle
consists of cardiac muscle
fibers.

9. THE
EPICARDIU
M
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 Consists of a simple
squamous mesothelium
 an underlying
subepicardial layer of
connective tissue.
 contains coronary blood
vessels, nerves, and
adipose tissue.
 The outer epicardium of
the atrium and ventricle
is continuous and covers
the heart externally with
mesothelium.

10. Atrium:
Myocardium (M) and Epicardium (Ep).
The epicardium consists of loose connective tissue (CT) containing
both autonomic nerves (N) and fat (F).
The epicardium is covered by the simple squamous-to-cuboidal
epithelium (arrows)
Much thicker in the ventricles than in the atria
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11. WALLS OF BLOOD VESSEL
• The walls of all blood
vessels, except the very
smallest, are composed of
three distinct layers, or
tunics.
 The tunica intima
 Tunica media
 Tunica externa
That surround the central
blood-filled space, the
lumen.
common to arteries as well
as veins.
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12. Tissues of the Vascular Wall
• Walls of larger blood vessels
contain three basic structural
components:
1. Endothelium: a simple
squamous epithelium
2. Smooth muscle
3. Connective tissue: elastic
and collagen fibers
Endothelium
 Special type of epithelium that acts
as a semipermeable barrier
between two internal
compartments: the blood plasma
and the interstitial tissue fluid
 Highly differentiated to mediate
and actively monitor the
bidirectional exchange of small
molecules and restrict the
transport of some macromolecules
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13. Smooth muscle
 Occur in the walls of all vessels
larger than capillaries and are
arranged helically in layers
 In arterioles and small arteries
the smooth muscle cells are
frequently connected by
communicating gap junctions
Connective tissue
 Collagen fibers are found
throughout the wall:
 In the subendothelial layer,
between muscle layers, and in
the outer layers
 Elastic material provides the
resiliency for the vascular wall
expanded under pressure
 Elastin predominates in large
arteries
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14. I The tunica intima
• The innermost wall which is in
“intimate”. This tunic contains
1.The endothelium
It is a layer of simple squamous
endothelial cells which lines the
inner surface of all the blood
vessels.
The flat endothelial cells form a
smooth surface that minimizes
the friction of blood moving
across them.
Its smooth surface which
prevents the intravascular
clotting.
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15. 2.Subendothelial layer:
It is formed of loose layer of
connective tissue and few
smooth muscle fibers
arranged longitudinally.
3.Internal elastic
lamina: It is only
present in the arteries
The intima is separated
from the media by an
internal elastic lamina
Composed of elastin, has
holes (fenestrae) that allow
the diffusion of substances
to nourish cells deep in the
vessel wall
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16. II THE TUNICA MEDIA
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It is a concentric layer of helically
arranged smooth muscle cells in
between the elastic and the
collagen fibers type ІІІ.
contribute elasticity and strength
for resisting the blood pressure
that each heartbeat exerts on the
vessel wall.
In the large arteries thin
External elastic lamina is
located between tunica media
and adventitia.
Tunica media is replaced in the
capillaries by Pericytes.
The tunica media is thicker in
arteries than in veins.

17. IN ARTERIES, WHICH FUNCTION
TO MAINTAIN BLOOD
PRESSURE, THE TUNICA MEDIA
IS THE THICKEST LAYER.
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among the smooth muscle cells are
variable amounts of elastic fibers
and lamellae, reticular fibers of
collagen type III and glycoproteins
Contraction of the smooth muscle
cells decreases the diameter of the
vessel, a process called
vasoconstriction.
Relaxation of the smooth muscle
cells, a process called vasodilation,
increases the vessel’s diameter.
Both activities are regulated by
vasomotor nerve fibers of the
sympathetic division of the
autonomic nervous system.
Vena cava

18. It is formed of collagen type І and
elastic fibers which are arranged
longitudinally.
In large vessels it may contain
blood vessels called vasa
vasorum for the nourishment of
the vessel wall.
Functionally, the tunica externa
protects the vessel, further
strengthens its wall, and anchors
the vessel to surrounding
structures.
III THE TUNICA EXTERNA OR TUNICA
ADVENTITIA
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19. TYPES OF
ARTERIES
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 There are three types of
arteries in the body:
 Elastic arteries,
 Muscular arteries, and
 Arterioles.
 Arteries that leave the
heart to distribute the
oxygenated blood exhibit
progressive branching.
 With each branching, the
luminal diameters of the
arteries gradually
decrease, until the
smallest vessel, the
capillary, is formed.

20. ELASTIC
ARTERY
 Are the largest blood vessels in the
body and include the pulmonary
trunk and aorta with their major
branches, the brachiocephalic,
common carotid, subclavian,
vertebral, pulmonary, and common
iliac arteries.
 The walls of these vessels are primarily
composed of elastic connective tissue
fibers.
 These fibers provide great resilience
and flexibility during blood flow.
 The wall has three layers
1. Tunica intima:- single layer of
endothelial cells
2.Tunica media:- has more elastic
fibers
3.Tunica externa:- has vasa vasorum &
nerves
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22. LARGE ELASTIC
ARTERIES
1 Tunica intima:
The intima is thicker than the
corresponding tunic of a muscular
artery
Thick subendothelial layer with
longitudinally arranged fiber The
internal elastic lamina is
evident but cannot be differentiated
from the next layer
2 Tunica media:
Formed of concentrically arranged
perforated elastic laminae.
The media consists of elastic fibers
and a series of concentrically
arranged, perforated elastic laminae
The fibers are separated from each
other by smooth muscle fibers,
reticular fibers.
.
Aorta: Simple squamous endothelial
cells (arrows); tunica intima (I);
internal elastic lamina (IEL); media (M);
By Don Siyum A..- .e06laEs.Ct)icfibers (EF); tunica adventiti2a2 (A);
Vasa vasorum (V)
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24. 3- Tunica adventitia:
There are no external elastic laminae but
only elastic fibers and collagen fibers.
It is formed of loose connective tissue
containing vasa vasorum.
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25. MUSCULAR ARTERY
Include: radial, femoral,
coronary, cerebral & other
medium sized arteries
Have less elastic fibers &
more smooth muscles
Has two lamina
1. Internal elastic lamina:-
 B/n T
. intima & media
2. External elastic lamina:-
 B/n T
. media & externa
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26. MUSCULAR
ARTERIES
 The best example is the femoral artery.
1 Tunica intima:
Endothelium
It contains smooth muscle fibers
In contrast to the walls of elastic
arteries, those of muscular arteries
contain greater amounts of smooth
muscle fibers.
There is a prominent internal elastic
lamina
2 Tunica media:
It is thick layer of smooth muscle
fibers
Elastic and collagen fibers are located
in between the muscle fibers
In large medium sized arteries, there
is an external elastic lamina,
It is located between the media and 26
th1
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a/
2
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entitia.
Muscular artery: multiple layers
of smooth muscle in the media
(M); The smooth muscle layers
are more prominent than the
elastic lamellae and fibers; Vasa
vasorum are seen in the tunica
By Don Siyum A.
adventitia
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27. 3- Tunica adventitia:
It consists of connective tissue
elastic fibers, collagen, few
fibroblasts, Lymphatic
capillaries, vasa vasorum,
and nerves.
Usually the thickness of the
adventitia in medium sized
arteries is less than the
media
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29. ARTERIOL
ES
 Arterioles are the smallest
branches of the arterial system.
 Arterioles deliver blood to the
smallest blood vessels, the
capillaries.
 Capillaries connect arterioles with the
smallest veins or venules.
 Arterioles are generally less than
<0.5 mm in diameter
 The lumen is narrow, very thin
subendothelial layer and no
internal and external elastic
lamina
 The tunica media is about 2-3 layers
of smooth muscle fibers and no
external elastic lamina.
.
Arterioles: tunica intima (I)
consists only of the endothelium
(E); tunica media (M) with only one
or two layers of smooth muscle;
By Don Siyum A..- .0t6hEi.Cn),inconspicuous adventitia2(9Ad)
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30. 30
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31. CAPILLARI
ES
 Capillaries are the smallest
blood vessels.
 Their average diameter is
about 8 μm, which is about the
size of an erythrocyte (red
blood cell).
 Outer part is made up of a
basal lamina.
 Interconnect the arteriole to
the venule
 They anastomose freely and
form rich networks.
 Capillaries permit different
levels of metabolic exchange
between blood and
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nding tissues 31
By Don Siyum A.
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Types of capillaries
 There are three types of capillaries:
 They can be grouped into three types,
depending on the continuity of the
endothelial cells and the external lamina
1. Continuous capillaries,
2. Fenestrated /Perforated capillaries
3. Sinusoids
 These structural variations in capillaries
allow for different types of metabolic
exchange between blood and the
surrounding tissues.

33. Continuous Capillary
lack of pores in its walls.
The most common type
Is found in all kinds of
muscle tissue, connective
tissue, skin, respiratory
organs, exocrine glands,
and nervous tissue.
In some places, but not in
the nervous system,
numerous pinocytotic
vesicles are present on
both endothelial cell
surfaces
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34. Characterized by the
presence of small circular
pores or fenestra through
the very thin squamous
endothelial cells
Allows more extensive
molecular exchange across
the endothelium
Found in tissues where
rapid interchange of
substances occurs between
the tissues and the blood
– kidney, intestine, choroid
plexus and endocrine
glands
FENESTRATED/PERFORATED
CAPILLARY
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35. SINUSOIDAL
CAPILLARY
They are mainly found in the
liver an hematopoetic organs.
Has the following characteristics
A tortuous path and is greatly
enlarged (30-40 micrometers)
Absence of a continuous basal
lamina and Lack of continuous
wall of endothelial cells
Presence of multiple
fenestrations
Presence of phagocytic activity
Found in liver, spleen and bone
marrow
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36. STRUCTURAL
PLAN OF VEINS
 The venous blood initially flows
into smaller post capillary
venules and then into veins of
increasing size.
 The veins are arbitrarily
classified as small, medium, and
large.
 Compared with arteries, veins
typically are more numerous
and have thinner walls, larger
diameters, and greater
structural variation.
 Small-sized and medium-sized
veins, particularly in the
extremities, have valves.
36
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Because of the low blood pressure
in the veins, blood flow to the heart
in the veins is slow and can even
back up. The presence of valves in
veins assists venous blood flow by
preventing back flow.
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37. VENULE
S
 The transition from
capillaries to venules
occurs gradually
1 Venules have very thin wall
2 The media contains only
contractile Pericyte cell and
few smooth muscle fibers
3 Relatively thick adventitia
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38. Small vein (V): relatively large lumen compared to the small muscular
artery (A) with its thick media (M) and adventitia (Ad). The wall of a
small vein is very thin, containing only two or three layers of smooth
mu
1s
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39. They have a diameter of 1-9 mm.
The intima is thin with a thin
subendothelial layer with no
internal elastic lamina
2 The media
is formed of thin layer,
which contains small bundles of
smooth muscle fibers.
Some elastic and reticular fibers
are included in the wall of the
tunica media.
3 Tunica adventitia
is rich with collagen fibers.
Contain semilunar shaped
valves.
MEDIUM SIZED
VEINS
1- THE INTIMA
Medium vein (MV): thicker wall,
but still less prominent than that
of the accompanying muscular
artery (MA). Both the media and
adventitia are better developed,
but the wall is often folded around
the relatively large lumen.
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40. STRUCTURE
OF VEINS
A- Large veins (Inferior vena cava)
1- Tunica intima
 Is part of the endothelium and is supported by a small amount
of subendothelial connective tissue.
 large veins may also exhibit an internal elastic lamina that is
not as well developed as in the arteries.
2- Tunica media
is thin and may contain few layers of smooth muscle fibers
3- Tunica adventitia
 Is the thickest layer,
 It contains bundles of smooth muscle fibers
 Most veins have valves, but these are most prominent in large
veins
 The valves, which are especially numerous in veins of the legs,
help keep the flow of venous blood directed toward the heart.
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42. LYMPHATIC VASCULAR SYSTEM
FUNCTION IS TO RETURN TO
THE BLOOD THE FLUID OF
THE TISSUE SPACES.
Also to circulate the lymph which contains
lymphocytes and other immunologic
factors
Unlike the blood, lymph is circulated in one
direction, towards the heart
Parts of the Lymphatic Vascular System
Lymphatic capillaries
 Blind-ended vessels.
 Consists of a single layer of endothelium.
 Do not contain fenestrations but have very little basal lamina.
They all drain into 2 large trunks,
Thoracic duct and
Right lymphatic duct.
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43. LYMPHATIC
VESSELS
 Similar structure to veins.
 Contains longitudinal fibers.
 Adventitia is
• underdeveloped.
 Contain vasa vasorum.
 Similar to veins but have
thinner walls and clear cut
divisions of the layers.
 They contain numerous
internal valves which
prevents backflow.
Lymphatic ducts Lymphatic Vessel
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44.  The atrioventricular (mitral)
valve are formed by a double
membrane of the
endocardium and a dense
connective tissue core that is
continuous with the annulus
fibrosus.
 On the ventral surface of each
cusp are the insertions of the
connective tissue cords, the
chorda tendineae, which
extend from the cusps of the
valve and attach to the
papillary muscles, which
project from the ventricle
wall.
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45. f
 The inner surface of the
ventricle also contains
prominent muscular
(myocardial) ridges called
trabeculae carneae that
give rise to the papillary
muscles.
 The papillary muscles via
the chorda tendineae hold
and stabilize the cusps in
the atrioventricular valves o
the right and left ventricles
during ventricular
contractions.
 The Purkinje fibers, or
impulse-conducting fibers,
are located in the
subendocardial connective
tissue.
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46. POST CAPILLARY VENULES
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Similar structurally to
capillaries, with pericytes, but
range in diameter from 15 to
20 µm
Participate in the exchanges
between the blood and the
tissues
The primary site at which white
blood cells leave the circulation
at sites of infection or tissue
damage
Collecting venules
Post capillary venules converge
into larger collecting venules
which have more contractile
cells