The document summarizes the histology of the circulatory system. It describes the blood vascular system, which transports oxygen and nutrients to tissues, and the lymphatic vascular system, which returns fluid from tissues to the blood. It details the structure of blood vessels including the three layers - tunica intima, media, and adventitia. It also discusses capillaries, veins, arteries, heart structure, and the conducting system involved in heart rhythm regulation.

1. Histology of Circulatory System
By: Fikre Bayu
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2. Introduction
➢comprises both the blood & lymphatic
vascular systems
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3. Blood vascular system
• The blood vascular system consists of the heart, arteries, veins, and capillaries
• This system transports oxygen and nutrients to tissues, carries carbon dioxide and waste
products from tissues, and circulates hormones from the site of synthesis to their target cells
3

4. Lymphatic vascular system
• Begins in the lymphatic
capillaries, closed-ended tubules
that anastomose to form vessels
of steadily increasing size
• These vessels terminate in the
blood vascular system emptying
into the large veins near the heart
• One of the functions of the
lymphatic system is to return the
fluid of the tissue spaces to the
blood
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5. Endothelium
• Selective permeability layer
• Non-thrombogenic barrier
• Modulates blood flow and vascular
resistance
• Works with immune cells
• Synthesizes chemical messengers
• Oxidizes lipoproteins
• Affects relaxation and contraction of
smooth muscle cells in the tunica
media
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6. Cont.…
• The circulatory system is considered to consist of the
macrovasculature, vessels that are more than 0.1
mm in diameter (large arterioles, muscular and
elastic arteries, and muscular veins), and the
microvasculature (metarterioles, capillaries, and
postcapillary venules) visible only with a microscope
• The microvasculature is particularly important
functionally, being the site of interchanges between
blood and the surrounding tissues both under normal
conditions and during inflammatory processes
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7. Histology of blood vessels
• The wall is composed of
three layers/tunics
• Tunica intima
• Tunica media
• Tunica adventitia
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8. Tunica intima/interna
• inner layer of a vessel
• consists of
• simple squamous epithelium [endothelial cells] present in all vessels supported by
• subendothelial layer of loose connective tissue contains occasional smooth muscle
cells (highly variable depending on vessel)
• Both CT fibers & smooth muscle cells, when present, tend to be arranged
longitudinally
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9. Tunica media
• in most arteries and veins it is the thickest of the three tunics
• consists primarily of concentric layers of helically arranged smooth muscle cells
• autonomic control can alter the diameter of the vessel and affect the blood pressure
• are the cellular source of this extracellular matrix
• in contrast to cardiac and skeletal muscles, have secretory capabilities
• The media of arteries is generally thicker than the media of veins of comparable diameter
• In capillaries & postcapillary venules, the media is replaced by pericytes
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10. Tunica adventitia/externa
• outermost layer
• attaches the vessel to the neighboring tissue
• is very dense fibrous CT with varying quantities of elastic fibers and longitudinally oriented
collagen fibers (type I)
• The tunica adventitia tends to be much larger in veins than arteries
• It becomes loose CT near the outer portion of the vessel
• contains a few cells, macrophages, mast cells, and fibroblasts
• In large vessels, it contains:
• vasa vasorum → blood vessels of the blood vessels, which supply the vessel wall
• nervi vascularis → nerves of the blood vessels, which supply the vessel wall 10

11. Cont.…
•Vasa Vasorum
• Large vessels usually have vasa vasorum ("vessels of the
vessel"), which are arterioles, capillaries, and venules
that branch profusely in the adventitia and the outer part
of the media
• Vasa vasorum provide metabolites to the adventitia and the
media, since in larger vessels the layers are too thick to be
nourished solely by diffusion from the blood in the lumen
• Vasa vasorum are more frequent in veins than in arteries
• In arteries of intermediate and large diameter, the intima
and the most internal region of the media are devoid of
vasa vasorum
• These layers receive oxygen and nutrition by diffusion from
the blood that circulates into the lumen of the vessel
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12. Elastic arteries
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13. Muscular arteries
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➢ Internal elastic lamina and external elastic lamina more prominent in muscular arteries
➢ Having pores/fenestration for passageways of molecules

14. Arterial sensory structures
• Carotid sinuses are slight dilatations of the internal carotid
arteries which contain baroreceptors detecting increases in
blood pressure
• The tunica media of each carotid sinus is thinner, allowing
greater distension when blood pressure rises, and the
intima and adventitia are rich in sensory nerve endings
from cranial nerve IX, the glossopharyngeal nerve
• The afferent nerve impulses are processed in the brain to
trigger adjustments in vasoconstriction that return pressure
to normal
• Similar baroreceptors occur in aortic arches and other large
arteries
14

15. Cont.…
• The carotid bodies are small, ganglia-like structures
(paraganglia) near the bifurcation of the common carotid
arteries that contain chemoreceptors sensitive to blood CO2
and O2 concentrations
• A network of sinusoidal capillaries is intermixed with
glomus (type I) cells containing numerous dense-core
vesicles with dopamine, serotonin, and adrenaline
• Dendritic fibers of cranial nerve IX, the glossopharyngeal
nerve, synapse with the glomus cells
• The sensory nerve is activated by neurotransmitter release
from glomus cells in response to changes in the sinusoidal
blood: increased CO2, decreased O2, or increased H+ levels
• Aortic bodies located on the arch of the aorta are similar in
structure and function to carotid bodies
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16. Small arteries and arterioles
are referred to as peripheral resistance vessels
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17. Capillaries
• Capillaries permit different levels of metabolic exchange between blood and surrounding
tissues
• They are composed of a single layer of endothelial cells rolled up in the form of a tube
• The average diameter of capillaries varies from 5 to 10 um and their individual length is usually
not more than 50 um
• Altogether capillaries comprise over 90% of all blood vessels in the body, with a total length of
nearly 96,000 km (60,000 miles)
• The total diameter of the capillaries is approximately 800 times larger than that of the aorta
• The velocity of blood in the aorta averages 320 mm/s, but in capillaries blood flows only about
0.3 mm/s
• Because of their thin walls and slow blood flow, capillaries are a favorable place for the
exchange of water, solutes, and macromolecules between blood and tissues
17

18. Cont.…
❑ Pericytes (Perivascular cells)
• It may surround a capillary or a venule➔ satellite cells
• is mesenchymal in origin and enclosed in their own basal lamina
• The cell plays a contractile function since it contains contractile proteins (Myosin,
Actin and Tropomyosin)
• After tissue injury it proliferates and differentiates to form new blood vessels and
connective tissue cells➔ repair process
• have great potential for transformation into other cells ➯ reserve cells
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19. Cont.…
Types of blood capillaries
1. Continuous (somatic) capillaries
• characterized by the absence of fenestrae in their wall
• Many pinocytotic vesicles are present
• are present in:
• Muscular tissues
• Connective tissues
• Exocrine glands
• Nervous tissues
• Lung
• The central nervous system capillaries is free from
the pinocytic vesicles
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20. Cont.…
2. Fenestrated capillaries
• are characterized by the presence of large fenestrae in their endothelial
wall with diameter about 60-80 nm
• There is a continuous basal lamina
• The fenestrae may be closed by diaphragm or not closed
• They are present in the:
• Kidney
• Pancreas
• Intestine
• Endocrine glands
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21. Cont.…
3. Sinusoidal capillaries
• They are tortuous dilated capillaries (about
30-40 um in diameter) which lead to
slowing of the blood circulation
• The endothelial wall shows multiple
fenestrae without diaphragm
• The basal lamina is discontinuous
• Precapillary sphincters can control the
flow of blood in the capillaries
• Present in the spleen, liver, lymph nodes
and the bone marrow
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22. Venules (Postcapillary & Collecting)
• Venules have very thin wall
• The media contains only contractile PERICYTE cells and/or few smooth
muscle cells
22

23. Medium sized veins
1. The intima is thin with no internal elastic
lamina
• Valves are common structure in the lumen of veins
• They are intimal luminal foldings
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 well developed in the
wall of medium sized veins, it contains bundles
of collagenous fibers
23

24. Cont.…
24

25. Large veins (SVC & IVC)
1. Tunica intima is well developed
2. Tunica media is thin and may contain few layers of
smooth muscle fibers
3. Tunica adventitia is the thickest layer, it contains
longitudinal bundles of smooth muscle fibers
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26. Special vessels
• Other variations in the structure of blood vessels occur in certain organs in response to
special functional and anatomical conditions. Some examples of special vessels include:
1. Cerebral arteries and veins
• These arteries are thin-walled for their caliber, with a well-developed internal elastica and
virtually no elastic fibers in the rest of the vascular wall
• The veins have a thin wall devoid of smooth muscle cells
2. Pulmonary arteries and veins
• These arteries have thin walls as a result of a significant reduction in both muscular and
elastic elements, while the veins have a well-developed media of smooth muscle cells
3. Umbilical vessels
• These arteries have two layers of smooth muscle cells without a prominent internal elastica
or adventitia
• The vein has a thick muscular wall with two to three muscle layers
4. Portal systems
• A portal system is one in which two capillary networks are connected in series by an
arteriole or venule
• In the kidney, an arterial portal system is present at the level of the glomeruli. A venous
portal system exists in the liver 26

27. Lymphatic vascular system
• Consists of lymphatic capillaries, lymphatic vessels ➔ trunks (of gradually increasing size)
and lymphatic ducts
• Collects excess tissue fluid (lymph) and returns it to the venous system (unlike blood, lymph
circulates in only one direction ➯ towards the heart)
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28. Lymphatic capillaries
• are closed at one end and located in the spaces between cells
• single layer of attenuated endothelial cells that lack fenestrae, interrupted basal lamina
and fasciae occludens
• have greater permeability than blood capillaries
• thus can absorb large molecules such as proteins and lipids
• are also slightly larger in diameter than blood capillaries
• have a unique structure that permits interstitial fluid to flow into them but not out
• can penetrate the media of veins but are present in the arteries only in the adventitia
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29. Cont.…
• The ends of endothelial cells that make up the wall of a lymphatic capillary overlap
• Anchoring filaments: extend out from the lymphatic capillary, attaching lymphatic endothelial
cells to surrounding tissues
• Unite to form larger lymphatic vessels which resemble small veins in structure but have
thinner walls and more valves
• Tissues that lack lymphatic capillaries include avascular tissues (such as cartilage, the teeth,
the epidermis, and the cornea of the eye), CNS, portions of the spleen, and red bone marrow
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30. Cont.…
30

31. Lymphatic vessels (afferent & Efferent)
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32. Cont.…
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33. Lymphatic trunks & ducts
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34. Cont.…
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35. Heart
• The heart is a four-chambered pump composed of
two atria and two ventricles and is surrounded by
a fibro-serous sac called the pericardium
• The heart produces atrial natriuretic peptide
(ANP), a hormone that increases secretion of
sodium and water by the kidneys, inhibits renin
release, and decreases blood pressure
• It receives sympathetic and parasympathetic
nerve fibers, which modulate the rate of the
heartbeat but do not initiate it
35

36. Cont.…
❑ Pulmonary circulation
• Right atrium
• Right ventricle
❑ Systemic circulation
• Left atrium
• Left ventricle
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37. Heart wall layers
37

38. Heart wall layers
• Endocardium lines the lumen of the
heart and is composed of simple
squamous epithelium (endothelium)
and a thin layer of loose connective
tissue & some SM ➔ subendothelium
• It is underlain by subendocardium,
a connective tissue layer that
contains veins, nerves, and
Purkinje fibers
38

39. Cont.…
Atrium Ventricle
39

40. Cont.…
❑ Myocardium consists of layers of cardiac
muscle cells arranged in a spiral fashion
about the heart's chambers and inserted
into the fibrous skeleton
• The myocardium contracts to propel
blood into arteries for distribution to the
body
• Specialized cardiac muscle cells in the
atria produce several peptides including
atrial naturietic polypeptide, atriopeptin,
cardiodilatin, and cardionatrin,
hormones that maintain fluid and
electrolyte balance and decreases blood
pressure 40

41. Cont.…
Trabeculation is most marked in ventricles
Elastic fibers are scarce in myocardium of
ventricles, plentiful in atrial myocardium Papillary muscles are the sites of attachment
of chordae tendineae
41

42. There are differences in cardiac and skeletal muscle observable at the light
microscope and ultrastructural level
• Cardiac muscles fibers are of smaller diameter (about 15 micrometers) than most
skeletal muscle fibers (10-100 micrometers)
• Cardiac muscle fibers are formed by individual muscle cells with one or two centrally
placed nuclei, while skeletal muscle fibers are multinucleated protoplasmic units in
which the nuclei are peripherally located
• Cardiac muscle fibers have branch and anastomose, skeletal muscle fibers do not
• The junction between two cardiac muscle cells, called an intercalated disk, is is made
up of three types of cell junctions:
• fascia adherens,
• desmosomes and
• gap junctions 42

43. Cont.…
• At the ultrastructural level, the arrangement of T-tubules is more regularly organized
in skeletal muscle, and they are found at the A-I junction, in contrast to the Z-line in
cardiac muscle
• T-tubules are usually associated with two terminal cisternae (triad) in skeletal muscle,
versus one (diad) in cardiac muscle
• The cisternae of skeletal muscle cell are much larger than those of cardiac muscle
• Cardiac muscle is more vascularized and has more abundant mitochondria than
does skeletal muscle, it also contains glycogen granules between the myofibrils
• Cardiac muscle is intrinsically rhythmic (contracts without outside stimulation)
although it is regulated through nervous and hormonal mechanisms
• The rate of cardiac muscle contraction is set by the sinoatrial node, whose intrinsic
rhythm is the most rapid
43

44. Cont.…
❑ Epicardium is the outermost layer of the heart and constitutes the visceral
layer of the pericardium
• It is composed of simple squamous epithelium (mesothelium) on the
external surface, fibroelastic
connective tissue containing nerves and the coronary vessels, and adipose
tissue
44

45. Cont.…
45

46. Fibrous skeleton (Cardiac skeleton)
▪ Serves as the base of valves & site of origin and insertion of cardiac muscle cells
▪ Composed of dense CT with thick collagen fibers oriented in various directions to which
the cardiac muscles are attached
46

47. Cont.…
47

48. Heart valve
•Consist of central core of dense, fibrous CT: collagen fibers & elastic fibers
•Lined on both sides by endothelial layers 48

49. Conducting system
Nodal cells are specialized myocytes that don't have striations and intercalated discs
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50. Purkinje fibers
50

51. Cont.…
51

52. Sequence of impulse excitation
▪ The SA node sends out a stimulus, which
cause the atria to contract
▪ When this stimulus reaches the AV node, it
signals the ventricles to contract
▪ Impulses pass down the two branches of
the atrioventricular bundle to the Purkinje
fibers, and thereafter the ventricles
contract
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