Complete description of microscopic anatomy of blood vessels.

1. Blood Vessels

2. Objectives
• Introduction
• Components of circulatory system
• Blood vessels
– Basic structure
• Arteries: Elastic & Muscular
• Arterioles, Meta-arteriole
• Capillaries: Continuous, Fenestrated, Sinusoidal
• Venules, Veins
• Clinical Correlation

3. The transport system of the body that permit blood
and lymph circulation to transport
• Nutrients, oxygen, hormones etc, to the place
where they are utilized (cells) and
• The metabolites(waste products) and carbon
dioxide are conveyed to appropriate places from
where they are expelled.
Circulatory System

4. Circulatory System
Two sub systems:
1. Cardiovascular system:
Heart, Arteries, Veins and
capillaries.
2. The Lymphatic system:
Lymphatic capillaries, lymphatic
vessels, lymphoid organs and
lymphatic ducts.

5. Cardio-vascular System
• Central pumping organ - Heart
• Conveying channels – Blood
vessels
• Conveying medium – Blood

6. Cardio-vascular System
• Components of CVS
 Heart
 Arteries
 Veins
 Capillaries
 Sinusoids &
Cavernous tissues

7. Blood Vessels

8. • Intricate network of hollow tubes that transport
blood throughout the entire body
• Two systems:
• Arterial System: Transport blood away from heart
• Arteries
• Arterioles
• Capillaries
• Venous System: Transport blood towards the heart
• Venules
• Veins
Blood Vessels

9. Basic Structure of Blood Vessels
• All blood vessels except capillaries have
3 concentrically arranged basic Tunics or Coats

10. Tunica
Intima
Tunica
Media
Tunica
Adventitia

11. Tunica
Intima
Tunica
Media
Tunica
Adventitia

12. Tunica Intima
Endothelium
Basal Lamina
Sub-endothelial CT
Internal Elastic Lamina
Fenestrated sheet of elastic fibers
Fenestration provide
passage for nutrients
Simple Squamous epithelium
Separates Tunica Intima from Media

13. Tunica Media
Concentric layers of
Smooth Muscle &
Elastic Fibers
Few collagen & reticular
fibers & proteoglycans
External Elastic Lamina
Fenestrated sheet of elastic fibers
Thinner than internal lamina
Separates Tunica Media from Externa

14. Tunica Media
Thicker in arteries than
in veins of similar size
Has relatively more
smooth muscle and
elastic fibres in arteries
than in veins of same
size

15. Tunica Adventitia
Prevent excessive stretching
Connective Tissue
Numerous collagen fibers
& few elastic fibers
Continuous & fused with
stroma of organs

16. Fibers
Runs Longitudinally
Mostly Elastic fibers
Mostly collagen fibers
• In Tunica intima &
Tunica adventitia
• In Tunica media
Runs Circularly

17. Muscular Artery with Thickening of Tunica Intima
Vessel tunics
A. Tunica intima
•Endothelium
•Subendothelium
Connective tissue
Smooth muscle
(Longitudinal fibers)
•Internal elastic lamina
B. Tunica media
•Vascular smooth
muscle
•External elastic lamina
(Circular fibers)
C. Tunica adventitia
•Connective tissue
(Longitudinal)

18. Nourishment of Blood Vessels
• Small & Medium sized Vessels:
• All layers receive O2 & nutrition directly from
blood in the lumen
• Large Vessels:
• Diffusion of O2 & nutrients possible up to tunica
intima & inner part of tunica media only
• Outer part of Tunica media & Adventitia:
• Received nourishment from arterioles,
capillaries & venules present in this layers
“Vasa Vasorum” = Vessels of Vessel

20. Endothelium
• Lines inner surface of heart &
blood vessels
• Simple squamous epithelium
• Endothelial cells held
together by cell junctions
• Surrounded by basal lamina
• Cells: polygonal & elongated
• Cytoplasm: sparse & contains
ER, mitochondria, MF & IF
• Numerous pinocytotic vesicles -
help in transport

21. Endothelium
• Secrete substances involved in
• Diapedesis,
• Blood coagulation
• Maintenance of tone of
smooth muscles of vessels
• Nitric oxide: vasodilation
• Prostacyclin: vasodilation and
inhibits platelet adhesion.
• Thromboplastin and von
Willebrand factor: involved in
blood coagulation

22. Arterial System

23. Arteries
• Thick walled blood vessels that
carry blood away from the heart
• Divide repeatedly like
branch of a tree
– Gradually become smaller in
size
– Decrease in the rate of
blood flow – facilitate
exchange through
capillaries

24. Arteries
The arterial system consists of:
1. Elastic arteries:
– Lager arteries
– More elastic fibres and less
smooth muscles in TM
2. Muscular arteries:
– Medium-sized arteries
(0.5–10 mm in diameters)
– More smooth muscles and
less elastic fibres in TM

25. Arteries
The arterial system consists of:
3. Arterioles:
– 1 or 2 layers of smooth
muscle cells in TM
4. Capillaries:
– Only tunica Intima &
Underlying basal lamina

26. Elastic Artery

27. Elastic Arteries
• Lager arteries
• Conducts blood from heart
to smaller arteries
– Conducting Arteries
• Thickness of the wall is
1/10 the diameter of lumen

28. Tunica Intima - Elastic Arteries
Endothelium with basal lamina
Sub-endothelial CT
Internal Elastic Lamina
Not clearly distinguished
• Longitudinally arranged collagen
& elastic fibers
• Elastic fibers are more
as compared to muscular artery
Same structure of tunica media

29. Tunica Media - Elastic Arteries
Smooth Muscle & loose CT
between elastic membranes
Large amount of
Elastic tissue in the form of
40-70 Fenestrated elastic
membranes
External Elastic Lamina not
seen clearly
Thinner than internal lamina

30. Tunica Adventitia - Elastic Arteries
Very thin
Contain elastic & collagen
fibers running longitudinally
Vasa Vasorum are
present

31. Important Features of Elastic Artery
• Large amount of
elastic fibers in
Tunica Media
• Smooth Muscle &
loose CT between
elastic membranes
• Elastic fibres in all
layers the are often in
the form of 40-70
Fenestrated Sheets
(Holes in them)
• Internal & External
Elastic Lamina are not
prominent because of
the presence of large
amount of elastin
within tunica media
• In Adventitia, small
blood vessels may be
found, termed Vasa
Vasorum. They provide
nourishment to the
outer part of the vessel.

32. Elastic Artery
• Presence of elastic fibres in
the wall allows expansion
during contraction (systole)
and recoil during relaxation
(diastole).
• Maintains necessary Blood
Pressure and permits the
blood to flow more evenly
through other arterial
channels

33. Elastic Arteries - Examples
• Aorta and its branches
– Brachiocephalic
– Carotid
– Subclavian
– Common iliac
– Pulmonary
– Vertebral

34. Large or Elastic Arteries
A = tunica intima (interna) B = tunica media C = tunica adventitia (externa)
Aorta–section – H&E – 4x objective

35. Elastic Artery (Aorta), Trichrome (10x Obj.)
TI
TM
TA
elastic fibers
NOT vasa
vasorum

36. Elastic Artery (Aorta), (40x obj.)TI
TM
TA
Elastic fibers

37. Elastic Artery (Aorta), Trichrome (40x Obj.)
TM
TA
Elastic fibers

38. Muscular Artery

39. Muscular Arteries
• Medium-sized arteries
• Arise from the large elastic
arteries and distribute blood
to various tissues and organs
via arterioles & capillaries
– Distributing Arteries
• Maintain steady blood flow &
blood pressure
• Wall thickness: 1/4th of the
luminal diameter

40. Tunica Intima - Muscular Arteries
Endothelium with basal lamina
Very thin Sub-endothelial CT
Internal Elastic Lamina
Well distinguished
Appear as bright refractive
membrane thrown into wavy folds
due to contraction of the elastic
membrane during the fixation process
• Longitudinally arranged
collagen(more) & elastic fibers

41. Tunica Media - Muscular Arteries
Few collagen and elastic
fibres intermixed with
smooth muscle
40 layers of smooth muscle
cells arranged circularly
External Elastic Lamina is
clearly seen

42. Tunica Adventitia - Muscular Arteries
Thicker than elastic artery
Contain collagen & elastic
fibers running longitudinally
Vasa Vasorum
Sometimes may be equal to
the thickness of the media

43. Muscular Arteries - Examples
• Branches of external
carotid artery
• Radial & ulnar arteries
• Femoral
• Popliteal etc…

44. Muscular Artery

45. Vessel Tunics
A. Tunica Intima
•endothelium
•subendothelium
connective tissue
smooth muscle
(longitudinal fibers)
•internal elastic lamina
B. Tunica Media
•vascular smooth muscle
•external elastic lamina
(circular fibers)
C. Tunica Adventitia
•connective tissue
(longitudinal)
Muscular artery (40x obj.)

46. muscular artery w/ intimal thickening (40x obj.)
Vessel Tunics
A. Tunica Intima
•endothelium
•subendothelium
connective tissue
smooth muscle
(longitudinal fibers)
•internal elastic lamina
B. Tunica Media
•vascular smooth muscle
•external elastic lamina
(circular fibers)
C. Tunica Adventitia
•connective tissue
(longitudinal)

47. Muscular artery, trichrome (4x obj.)

48. Muscular artery, trichrome (40x obj.)
tunica media
internal
elastic
lamina
elastic fibers
tunica
adventitia
external
elastic
lamina

49. Difference between Elastic & Muscular artery
Elastic Artery
T Intima: is much thicker,
sometimes 20% of the total
wall thickness.
• Subendothelial C T has
more elastic fibres
• IEL is not distinct
T Media: Elastic fibres
++
T adventitia: Thin
Muscular Artery
T Intima: Thinner
• Subendothelial C T has less
elastic fibres
• IEL is well demarcated.
T Media: Smooth M +
• EEL is also very
prominent.
T adventitia: Thickness is
more and sometimes may be
equal to the thickness of the
media.

51. Elastic and Muscular artery

52. Arteriole

53. Arteriole
• Small arteries with diameter -
less than 100 μm
– Larger Arterioles: 50-100 μm
– Terminal Arterioles: < 50 μm
• Arise from the muscular arteries
& deliver blood to capillaries
• Wall is relatively thicker than the
lumen
• Specialized for controlling blood
flow – Regulate PVR
• Major determinant of systemic
blood pressure

54. Arteriole
Tunica Intima - Thin
No Sub-endothelial CT
Internal Elastic Lamina
– Absent or Very thin
Tunica Media - Thin
1 – 3 layers of concentrically
arranged smooth muscles
Tunica Adventitia- Thin
& poorly developed
No external elastic lamina

55. Arteriole
Large
Small

56. Terminal Arteriole
• < 50 μm in diameter
– Smallest :– 12 μm
• Have only a thin layer of muscle
in their walls
• Give off lateral branches k.a. :
– Meta-arterioles
• Initial segment of each lateral
branch is surrounded by a few
smooth muscle cells k.a.-
– Pre-capillary sphincter

57. Metarteriole
• Smallest arterioles.
• Have discontinuous layers
of smooth muscle
• Terminate into capillaries.
• Terminal portion o the
metarterioles, just before the
beginning of the capillaries,
surrounded by a few smooth
muscle cells k.a.-
– Pre-capillary sphincter
– Regulate the flow o blood
to the capillaries

58. Capillaries

59. Capillaries
• Smallest blood vessels
• Thin-walled & form plexus
which spread throughout the
tissue & continuous with
– Smallest arteriole at one end
– Smallest venules at other end
• Site of exchange of gases,
nutrients and metabolic
wastes
• Abundant in tissue with high
metabolic rate like
– Kidney, liver and cardiac muscle
• Average diameter – 8 μm

60. Structure of Capillaries
The capillary wall is formed by Single layer of
endothelial cells
Glycoprotein layer of
Basal lamina
Out side the basal lamina-
Contractile cells wrapped
around the capillaries
Pericytes
Lacks T Media and therefore no smooth muscle cells
Has only Tunica Intima

61. Types of Capillaries
• According to the
appearance under the
electron microscope, there
are 3 types of capillaries:
1. Continuous
2. Fenestrated
3. Sinusoidal
– 6-10 μm D
– 30-40 μm D

62. Pericyte
Endothelial
cell
Continuous
Basement
membrane
Tight junction
Pinocytotic
vesicles
Least permeable, and most common
E.G., skin, muscle, nervous tissues, etc…
Continuous Capillaries

63. Pinocytotic
vesicles
Fenestrations
(pores)
Intercellular
cleft
Endothelial
cell
Continuous
Basement membrane
Large fenestrations (pores) increase permeability
Occurs in areas of active absorption or filtration
E.G., kidney, small intestine, endocrine glands etc…
Fenestrated Capillaries

64. Endothelial
cell
Large
intercellular
cleft
Discontinuous
basement
membrane
Most permeable. Occurs in special locations
E.G., liver, bone marrow, spleen
Sinusoidal Capillaries
Fenestrations (pores)

65. Vascular shunt
Precapillary sphincters
Metarteriole Thoroughfare
channel
Terminal arteriole
True
capillaries
Postcapillary venule
Sphincters open—blood flows through true capillaries
Terminal arteriole Postcapillary venule
Sphincters closed—blood flows through metarteriole – thoroughfare
channel and bypasses true capillaries.
Active
Rest

66. Capillaries
Arrowheads = capillaries
Striated muscle – cross section – H&E – 40x objective

67. Capillaries
Arrowhead = capillaries
Capillaries have very small diameters and walls that
consist of only a single layer of endothelium.
Adipose tissue – cross section – H&E – 40x objective

68. Arteriole & Capillary within subcutaneous
connective tissue
Arteriole
Capillary

69. Venous System

70. Venous System
• Veins are thin walled
• Carry blood from capillaries to
heart
• Large veins are formed by the
union of smaller vein like
tributary of a river
• Often provided with valves
– Prevent reflux of blood
– Maintain unidirectional flow
of blood

71. Types of Veins
• 3 types:
– Large sized Veins
– Medium sized veins
– Venules
• Same 3 layered
organisation as arterial
system
– Tunica intima
– Tunica media
– Tunica adventitia

72. Difference in Structure of Veins from the Arteries
• Wall thickness to lumen
diameter ratio is less
– Wall of a vein is distinctly
thinner than that of same
sized artery
• Tunica Media is thin and
poorly developed
– Contain more collagen &
– Less elastic tissue
– Veins are usually
collapsed after death

73. Difference in Structure of Veins from the Arteries
• Tunica Intima & adventitia
are more prominent
– Adventitia of veins is
thicker than media
• Internal and external
elastic lamina are difficult
to distinguish (Mainly in
small veins)
– No clear distinction
between the tunica intima,
media and adventitia

74. Artery Vein
Wall Thickness to Lumen Diameter Ratio

75. Muscular artery Vein
Intima
Media
Adventitia
Vasa vasorum +++
Structure

76. Large Veins
Tunica Intima -
Well developed
Endothelium & Sub-endothelial CT
Tunica Media - Thin
Less smooth muscle & elastic
fibers & more connective tissue
Tunica Adventitia
Both internal & external elastic lamina are poorly defined
≥1 cm in diameter
Well developed &
thicker than the media
Examples:
SVC and IVC

77. Large Veins
Tunica Adventitia
≥1 cm in diameter
Longitudinal bundles of
smooth muscle
Facilitate shortening &
elongation of vena cava
with respiration

78. Large Vein (Inferior Vena Cava)
Tunica intima
Tunica media
Tunica adventitia

79. Vein

80. Medium - Sized Veins
Tunica Intima
Very thin
Endothelium & Thin
Sub-endothelial CT
Tunica Media
Less muscular
Few smooth
muscle fibers
Tunica Adventitia
1–10 mm in diameter
Well developed &
thicker than the
media
All layers are thinner than large vein Femoral vein &
Superior
mesenteric vein

81. Medium vein, H&E (4x obj.)

82. Medium vein, H&E (10x obj.)
Tunica media
• 3-15 layers of VSMCs
• no external elastic lamina
• Tunica adventitia
• thicker than tunica media
• larger veins have
longitudinal smooth
muscle in adventitia
• Tunica intima
• endothelium
• subendothelium
• IEL generally not evident

83. Medium vein, trichrome (4x obj.)

84. Medium vein, trichrome (10x obj.)
Tunica adventitia
Tunica media
Elastic fibers

85. Differences between Medium sized Vein
and a Medium sized Artery
Medium sized vein
• A collapsed larger lumen
• Thin wall
• No IEL
• Tunica media has large
quantity of collagen (few
smooth muscles and less elastic
fibers)
• Tunica adventitia is thicker
than Tunica media
• Presence of valves to prevent
back flow
Medium sized artery
• Lumen is patent & small
• Thick wall
• Internal Elastic Lamina
is present
• Tunica media is thicker
than adventitia &
has large quantity of
smooth muscles &
elastic fibers

86. Artery and Vein

88. small vein & artery, H&E
(40x obj.)
vein
artery

89. small vein & artery, trichrome
(40x obj.)
vein
artery

90. Venules
• Smallest Veins
• Capillaries drain into
post-capillary venules
(10–40 μm in diameter)
• Post-capillary venules drain
into large muscular venules
(40–100 μm in diameter)
• Important sites or exchange of
metabolites

91. • Wall is thin
• Large collapsed lumen
• T intima: endothelium
• T media: 1-2 layers of
smooth muscle fibers
• T adventitia: thick and
composed of connective
tissue rich in collagen
fibres
Venules

92. Arterioles, venules and capillaries
within adipose tissue(200X)

93. Valves of Veins
• Valve of a vein is composed of 2 leaflets
• Each leaflet has a thin fold of the Tunica Intima
• Components: Endothelium + Core of C.T.
• Maintain unidirectional flow of blood towards the heart

94. Valves of Veins
Valves are absent in
• Very small veins;
• Veins within the cranial cavity & vertebral canal
• Venae cavae; SVC & IVC
• In some other veins

95. Arterioles and venules, trichrome (40x obj.)
arteriole
venule

96. Arteriole and venule (?), trichrome (40x obj.)
arteriole?
venule?
Remember, the change in vessel size is gradual, so you’ll find intermediate sizes. To
make classification easy, focus on vessels that are clearly one type or another.

97. Arteriole and venule (?), trichrome (40x obj.)
arteriole?
(sectioned
obliquely)
venule?

98. Changes due to age in Artery
• Thickening of Tunica Intima due to migration and
proliferation of smooth muscle cells from T Media.
• Formation of fibrofatty plaques by the deposition of
fat and collagen in Tunica Intima (Atheroma)
• Atheroma leads to narrowing of the arterial lumen and
consequently reduced blood flow.
• Calcification of Tunica Media (Arteriosclerosis)
• Atherosclerosis (atheroma+arteriosclerosis)

99. Changes due to age in Artery
• Damage to the endothelium can cause coagulation
of blood forming a thrombus which can completely
obstruct the artery leading to death of the tissue it
supplies
• If this happens with Myocardium: Coronary
thrombosis leading to Myocardial Infarction (may
manifest as heart attack)
• In brain: Cerebral thrombosis leading to stroke
and paralysis

100. Arteriosclerotic changes in a medium-sized artery
Internal
Elastic membrane:
fragmented
T. media
T. adventitia
External
Elastic membrane
Loss of elastic
fibers apparent
Lumen
T. intima

101. Coronary artery arteriosclerosis & thrombosis
T. Adventitia
T. media
T. intima
thickened
Thrombus
Narrowed lumen
of artery

102. Any Questions ?

103. Thank you