Download tunica media

Histology and Functions
of Blood Vessels
WS O
Department of Anatomy, HKU
Objectives:
• Describe the basic histological structures and
functions of endocardium, myocardium,
epicardium and the conducting system of the
heart.
• Define the basic structure and function of blood
vessels.
• Distinguish the major differences between
various types of vessels: elastic and muscular
arteries, arterioles, venules, veins.
• Describe the various types of capillaries and
define their functions.
Functions of the Circulatory System
1.
2.
3.
4.
5.
Carry blood
Transport of hormones,
components of the immune
system, molecules required for
coagulation, enzymes, etc.
Exchange nutrients, waste
products, and gases
Regulate blood pressure
Directs blood flow
21-3
Organization of Heart & Blood Vessels
• Heart wall can be viewed as a
three-layered structure
 Inner layer = endocardium
 Middle Layer = myocardium
 Outer layer = epicardium (part
of the pericardium)
• Blood and lymphatic vessel
walls (except for capillaries)
can also be viewed as threelayered structures
 Inner layer = tunica intima
 Middle layer = tunica media
 Outer layer = tunica adventitia
Epicardium
Myocardium
Endocardium
Tunica adventitia
Tunica media
Tunica intima
Heart Wall has Three Layers
Cardiac Muscle
Intercalated Disc
Conducting system of the heart
• Consists of modified cardiac muscle
and conducting fibers that are
specialized for initiating impulses &
conducting them rapidly through the
heart
Purkinje fibers
Cardiac muscle
• Made up of three components:
- sinoatrial (SA) node serve as the
pacemaker
- atrioventricular (AV) node receive
the wave of excitation from the
cardiac muscle of the atria and pass
the excitation on to the bundle of His
- Purkinje fibers are organized into
a branched bundle (Bundle of His)
which extends from the atrioventricular (AV) node, through the
interventricular septum down to the
apex of the ventricles
Blood Vessel Wall has three layers
1. Endothelial cell lining
Tunica intima 2. Subendothelial layer
3. Internal elastic membrane
1. Smooth muscle cells,
collagen fibers
Tunica media 2. Elastic fenestrated
lamellae
3. External elastic lamina
1. Mostly collagen fibers
2. Elastic fibers (not
lamellae)
Tunica adventitia
3. Fibroblasts and
macrophages
4. Vasa vasorum
Vessel Wall
 Tunica intima
– lines the blood vessel and is
exposed to blood
– endothelium – simple squamous
epithelium overlying a basement
membrane and a sparse layer of
loose connective tissue
• acts as a selectively permeable
barrier
• secrete chemicals that stimulate
dilation or constriction of the vessel
• normally repels blood cells and
platelets that may adhere to it and
form a clot
• when tissue around vessel is
inflamed, the endothelial cells
produce cell-adhesion molecules
that induce leukocytes to adhere to
the surface
– causes leukocytes to congregate in
tissues where their defensive
actions are needed
20-9
Tunica intima
Vessel Wall
 Tunica media
– middle layer
– consists of smooth
muscle, collagen, and
elastic tissue
– strengthens vessel and
prevents blood pressure
from rupturing them
– vasomotion – changes in
diameter of the blood
vessel brought about by
smooth muscle
20-10
Tunica media
Vessel Wall
• tunica externa (tunica
adventitia)
– outermost layer
– consists of loose connective
tissue that often merges with
that of neighboring blood
vessels, nerves, or other
organs
– anchors the vessel and
provides passage for small
nerves, lymphatic vessels
– vasa vasorum – small vessels
that supply blood to at least
the outer half of the larger
vessels
• blood from the lumen is
thought to nourish the inner
half of the vessel by diffusion
20-11
Tunica adventitia
Differences of Vessel Wall
Thin-walled vessel
Thick-walled vessel
Tunica intima
Tunica intima
Tunica media
Tunica media
Large artery
Tunica externa
Tunica externa
Medium artery
Medium vein
Comparison of Blood vessels
Ascending aorta
Pulmonary trunk (artery)
Superior
vena cava
Veins
Arteries
Large vein
Elastic artery
Inferior
vena cava
Tunica intima
Valve
Tunica intima
Tunica media
Tunica media
Tunica externa
Tunica externa
Descending
aorta
Small to mediumsized vein
Muscular artery
Tunica intima
Valve
Tunica media
Tunica externa
Tunica intima
Internal elastic lamina
Tunica media
External elastic lamina
Tunica externa
Venule
Arteriole
Tunica intima
Tunica media
Tunica externa
Tunica intima
Tunica media
Tunica externa
Basement membrane
Endothelium
Capillary
Blood Pressure Changes With Distance
120
Systolic pressure
100
80
60
Diastolic
pressure
Distance from left ventricle
Vena cava
0
Large veins
Small veins
Venules
Capillaries
Arterioles
Small arteries
20
Large arteries
40
Aorta
Systemic blood pressure (mm Hg)
• Due to increase in overall cross-sectional area of vessels in branching
Artery
Artery
Tunica intima
Tunica media
Tunica externa
Structural Features of Blood Vessels
• Arteries
– Elastic
– Muscular
– Arterioles
• Capillaries: site of exchange
with tissues
• Veins: thinner walls than
arteries, contain less elastic
tissue and fewer smooth
muscle cells
– Venules
– Small veins
– Medium or large veins
21-16
Elastic Artery
• Elastic or conducting arteries
– Largest diameters, pressure high and fluctuates between
systolic and diastolic. More elastic tissue than muscle.
– Relatively thin tunica intima & tunica adventitia, thick
tunica media (40-60 distinct concentrically arranged elastic
laminae)
– Include aorta, pulmonary arteries, common carotid arteries,
common iliac arteries
Elastic arteries. Elastic laminae in
the Tunica media gives these vessels
elastic properties. They expand as
the heart contracts (to modulate BP
and store energy) and recoil during
ventricular relaxation (to maintain
more even pressure in large
arteries)
21-17
Elastic Artery
Muscular Artery
• Muscular or medium or distributing arteries
– Smooth muscle allows vessels to regulate blood supply by constricting
or dilating
– Thick tunica media due to 25-40 layers of smooth muscle. Thin intima
– Also called distributing arteries because smooth muscle allows vessels
to partially regulate blood supply to different regions of the body.
– Mostly named arteries, such as brachial & femoral arteries but also
include some smaller unnamed arteries
• Smaller muscular arteries
– Adapted for vasodilation and vasoconstriction.
Prominent in
muscular artery
 Muscular arteries. The thick
muscular layer in the tunica
media resists damage due to
relatively high BP in these vessels.
They regulate blood flow to
different regions of the body.
External elastic
membrane
Internal elastic
membrane
Elastic artery
(tunica media – elastic fibers)
Muscular artery
(tunica media – smooth muscles)
Arterioles
• Transport blood from small
arteries to capillaries
• Smallest arteries where the three
tunics can be differentiated
• Tunica media-one or more layers
of smooth muscle cells
• Like small arteries, capable of
vasoconstriction and dilation
- metarterioles are short vessels that
link arterioles to capillaries
- contain small muscle cells form
precapillary sphincters
~ control blood flow to capillary bed
~ alter BP by altering peripheral
resistance to blood flow –
“peripheral resistance vessels”
21-22
Endothelial cell
Tunica media
Tunica adventitia
Arteriole
Arteriole H&E
Capillaries
• Thin walled single endothelial cells (simple squamous epithelium),
basement membrane and a delicate layer of loose C.T.
• Specialized for exchange of substances -Many pinocytic vesicles
• Pericytes:
– have their own basement membrane
– may develop into smooth muscle cells (capillary → arteriole)
• Presence of tight-junction between endothelial cells
– Blood-brain Barrier
• Tight junction is complete in brain capillaries
• Very few or absence of pinocytic vesicles
Pericyte
21-24
Three Types of Capillaries
Sinusoids or Sinusoidal capillary
Continuous capillaries
• Have relatively think cytoplasm
and the capillary wall continuous
• Presence of tight junctions
between adjacent cells
• Less permeable to large
molecules than other capillary
types
• Cytoplasm contains pinocytotic
vesicles
• Found in muscle, nervous tissue
Pericyte
Basal
lamina
Intercellular
cleft
Pinocytotic
vesicle
Endothelial
cell
Erythrocyte
Tight
junction
Continuous capillary
Fenestrated Capillary
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Endothelial
cells
• Have thin cytoplasm
and
Nonfenestrated
Erythrocyte
capillary wall is area
perforated
at intervals by pores
Filtration pores
(fenestrations)
(fenestrations)
• Materials can cross the cell
Basal
lamina
through the fenestrations
• Found in kidney &
endocrine glands
Intercellular
cleft
20-28
Fenestrated Capillary
Sinusoid in Liver
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Macrophage
Endothelial
cells
Erythrocytes
in sinusoid
Liver cell
(hepatocyte)
Microvilli
Sinusoid
20-30
• Larger in diameter than the
other types
• have wide spaces between
edges of adjacent cells
• Materials can move freely
in and out
• Found in liver, spleen, &
bone marrow
Aging of the Arteries
• Arteriosclerosis:
general term for
degeneration changes
in arteries making
them less elastic
• Atherosclerosis:
deposition of plaque
on walls
21-31
Vein
Vein
Tunica intima
Tunica media
Tunica externa
Venules and Small Veins
• Venules drain capillary network. Endothelial
cells and basement membrane with a few
smooth muscle cells. As diameter of venules
increases, amount of smooth muscle
increases.
• Small veins. Tunica intima is thin; tunica
media is thin with circular smooth muscle.
Smooth muscle cells form a continuous layer.
Tunica adventitia is well developed and is
made of longitudinally arranged collagenous
elastic fibers
21-33
Medium vein
Medium and Large Veins
• Medium veins. Go-between between small veins and large
veins.
• Large veins. Tunica intima is thin: endothelial cells, relatively
thin layer of C.T and a few scattered elastic fibers. Tunica media
is thin and poorly developed. Adventitia is thick and a
predominant layer with spirally arranged collagen fibers, elastic
lamellae, longitudinal smooth muscle
21-35
Large Vein
Veins
Comparison of Major Features of Artery & Veins
 Arteries – carrying blood at high pressure
• Smooth muscle and/or elastic
fibers/lamellae predominate over
collagen fibers in tunica media
• Elastic fibers/lamellae are highly
developed in the tunica media
• Smooth muscles and other components
are arranged circularly in tunica media –
allows change in diameter to regulate
blood pressure and blood flow
• No valves
 Veins - carrying blood at very low pressure
• Collagen fibers are relatively abundant among
the smooth muscles in the tunica media
• Collagen fibers and othe components arranged
longitudinally - prevent excess stretching of
vessel wall
• Valves present; to prevent backflow
Lymphatic Vessels
• Larger than blood capillaries
• Very irregular shaped
• Basal lamina is almost absence
• Endothelial cells do not form tight
junctions (allow entry of flluid)
• Larger lymph vessels contain valves
A villus of the jejunum containin a lymph
capillary. A lacteal is a lymphatic capillary
that absorbs dietary fats in the villi of
small intestien.
References
1. Hole’s Human Anatomy & Physiology, 13th Edition,
David Shier, McGraw-Hill
2. Seeley’s Principles of Anatomy & Physiology, 2nd Edition,
Philip Tate, McGraw-Hill
3. Anatomy & Physiology with Integrated Study Guide, 5th
Edition, Stanley E Gunstream, McGraw-Hill
4. Histology, A Text and Atlas, 6th Edition, Michael H Ross &
Wojciech Pawlina, Wolters Kluwer/Lippincott Williams & Wilkins