The Cardiovascular System:
Blood Vessels
Chapter 19
Introduction
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The blood vessels of the body form a
closed delivery system that begins and
ends at the heart
Often compared to a plumbing system, it
is a far more dynamic system of
structures that pulse, constrict and relax
and even proliferate to meet changing
body needs
Blood Vessel Structure & Function

The major types of blood vessels are
– Arteries
• The large distributing vessels that bring blood to
the body
– Capillaries
• The tiny vessels that distribute blood to the cells
– Veins
• The large collecting vessels that bring blood back
to the heart
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Intermediate vessels connect
– Arterioles bring blood to the capillaries
– Venules drain blood from the capillaries
Blood Vessel Structure & Function
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The pattern of distribution starts with
arteries to arterioles to capillaries to
venules to veins
The blood vessels in the adult human
body carry blood in a distribution
network that is approximately 60,000
miles in length
Only capillaries come into intimate
contact with tissue cells and serve cellular
needs
Structure of Blood Vessel Walls
Blood Vessel Walls
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The walls of blood vessels are composed of three
distinct layers or tunics
The tunics surround a central opening called a lumen
Blood Vessel Walls
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The innermost tunic
is the tunica intima
This tunic contains
the endothelium, the
simple squamous
endothelium that
lines all vessels
Its flat cells fit closely
together, forming a
slick surface that
minimizes friction as
blood moves through
the vessel lumen
Tunica
adventitia
Blood Vessel Walls

In blood vessels
larger than 1 mm in
diameter, a subendothelial layer of
loose connective
tissue, subendothelial
layer, (basement
membrane) supports
the endothelium
Blood Vessel Walls
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The middle tunic, the
tunica media, is mostly
circularly arranged
smooth muscle cells
and sheets of elastin
The activity of the
smooth muscle is
regulated by
vasomotor nerve fibers
of the sympathetic
division of the
autonomic nervous
system
Tunica media
Blood Vessel Walls
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Depending on the needs of the body, the
vasomotor fibers can cause vasoconstriction or vasodilation
The activities of the tunica media are
critical in regulating circulatory dynamics
Generally, the tunica media is the bulkiest
layer in arteries, which bear the chief
responsibility for maintaining blood
pressure and continuous blood circulation
Blood Vessel Walls
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The outermost layer
of a blood vessel is
the tunica externa
This tunic is
composed largely of
loosely woven
collagen fibers that
protect blood vessels
and anchor it to
surrounding
structures
Tunica
externa
Blood Vessel Walls
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The tunica externa is
infiltrated with nerve
fibers and lymphatic
vessels and, in larger
vessels, a system of
tiny blood vessels
These vessels, the
vasa vasorum
nourish the external
tissues of the blood
vessel wall
Tunica
externa
Arteries
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Arteries are vessels that carry blood
away from the heart
All arteries carry oxygen rich blood with
the exception of those in the pulmonary
circuit
Blood proceeds to the tissues through
– Elastic arteries
– Muscular arteries
– Arterioles
Elastic (Conducting) Arteries
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Elastic arteries are thick walled arteries
near the heart - the aorta and its major
branches
These arteries are the largest in diameter
and the most elastic
A large lumen allows them to serve as low
resistance pathways that conduct blood
from the heart to medium-sized arteries
and thus are called conducting arteries
Elastic (Conducting) Arteries
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The elastic arteries contain more elastin
than any other type of vessel
While present in all three layers, the
tunica media contains the most
The abundant elastin enables these
arteries to withstand and smooth out
large pressure fluctuations by expanding
when the heart forces blood into them and
then recoiling to propel blood onward into
the circulation when the heart relaxes
Elastic (Conducting) Arteries
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Elastic arteries also contain substantial
amounts of smooth muscle, but they are
relatively inactive in vasoconstriction
Because elastic arteries expand and recoil
passively to accommodate changes in
blood volume, the blood is kept under
pressure
Thus, blood flows continuously rather
than starting and stopping with each
heart beat
Muscular (Distributing) Arteries
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The muscular distributing arteries
deliver blood to specific body organs and
account for most of the named arteries
Proportionately, they have the thickest
media of all vessels
Their tunica media contains relatively
more smooth muscle and less elastic
tissue than that of elastic arteries
They are more active in vasoconstriction
and are less distensible
Muscular (Distributing) Arteries
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As in all vessels, concentric sheets of
elastin occur within the tunica media of
muscular arteries although these sheets
are not as thick or abundant as those of
elastic arteries
Muscular (Distributing) Arteries
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A feature unique
to muscular
arteries,
especially thick
sheets of elastin
lie on each side of
the tunica media
An external
elastic lamina lies
between the
tunica media and
tunica externa
Muscular (Distributing) Arteries
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The elastin in muscular arteries, like that
in elastic arteries, helps dampen the
pulsatile pressure produced by the
heartbeat
Arterioles
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Arterioles have a lumen diameter from
0.3 mm to 10 m, and are the smallest of
the arteries
Larger arterioles exhibit all three tunics,
but their tunica media is chiefly smooth
muscle with a few scattered muscle fibers
The smaller arterioles that lead into
capillary beds, are little more than a
single layer of smooth muscle cells
spiraling around the endothelial lining
Arterioles
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The diameter of each arteriole is regulated
in two ways:
– Local factors in the tissues signal the smooth
musculature to contract or relax, thus
regulating the amount of blood sent
downstream to each capillary bed
– Sympathetic nervous system adjusts the
diameter of arterioles throughout the body to
regulate systemic blood pressure
Capillaries
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The microscopic
capillaries are the
smallest blood vessels
In some cases, one
endothelial cell forms
the entire circumference of the
capillary wall
The average length
of a capillary is 1 mm
and the average
diameter is 8-10 m
Capillaries

Capillaries have a
lumen just large
enough for blood
cells to slip through
in single file
Capillaries
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Capillaries are the body’s most important
blood vessels because they renew and
refresh the surrounding tissue fluid
(interstitial fluid) with which all cells in the
body are in contract
Capillaries deliver to interstitial fluid the
oxygen and nutrients that cells need while
removing carbon dioxide and nitrogenous
wastes that cells deposit in the fluid
Capillaries
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Given their location and the thinness of
their walls capillaries are ideally suited
for their role of providing access to
nearly every cell
Along with the universal functions just
described some capillaries also perform
site-specific functions
–
–
–
–
Lungs: gas exchanges
Endocrine glands: pick up hormones
Small intestine: nutrients
Kidneys: removal of nitrogenous wastes
Capillary Beds
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A capillary bed is a network of the body’s
smallest vessels that run throughout
almost all tissues, especially the loose
connective tissue
This flow is also called a microcirculation
Capillary Beds
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In most body regions, a capillary bed consists
of two types of vessel a vascular shunt (metaarteriole) and true capillaries
Capillary Beds
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The terminal arteriole leads into a metarteriole
which is directly continuous with the thoroughfare channel
Capillary Beds
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The thoroughfare channel joins the postcapillary venule that drains the capillary bed
Capillary Beds
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The true capillaries number 10 to 100 per capillary
bed, depending on the organ served
Branch from metarteriole to thoroughfare channel
Capillary Beds
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A cuff of smooth muscle fibers, called a precapillary sphincter surrounds the root of each
capillary at the metarteriole and acts as a valve
to regulate the flow of blood into the capillary
Capillary Beds
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When the precapillary sphincters are relaxed, blood
flows through the true capillaries and takes part in
exchanges with tissue cells
Capillary Beds
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When the precapillary sphincters are contracted,
blood flows through the shunts and bypasses the
tissue cells
Capillary Beds
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Most tissues have a rich supply, but there
are a few exceptions
–
–
–
–
Tendons and ligaments / poorly vascularized
Cartilage / from adjacent connective tissue
Epithelia / from adjacent connective tissue
Cornea / nourished by aqueous humor
Capillary Beds
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The relative amount of blood entering a
capillary bed is regulated by vasomotor
nerve fibers and local chemical conditions
A capillary bed may be flooded with blood
or almost completely bypassed, depending
on conditions in the body or in that specific
organ
Example of shunting blood from digestive
organs to skeletal muscles
Capillary Permeability
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The structure of capillaries is well suited
for their function in the exchange of
nutrients and wastes between the blood
and the tissues through the tissue fluid
A capillary is a tube consisting of thin
endothelial cells surrounded by a basal
lamina
The endothelial cells are held together by
tight junctions and occasional desmosomes
Capillary Permeability
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Tight junctions block the passage of small
molecules, but such junctions do not
surround the whole perimeter of the
endothelial cells
Instead, gaps of unjoined membrane
called intercellular clefts occur through
which small molecules exit and enter the
capillary
Capillary Permeability
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External to the
endothelial cells,
the delicate
capillary is
strengthened and
stabilized by
scattered pericytes
Capillary Permeability
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The pericytes are
spider shaped cells
whose thin
processes form a
network that is
widely spaced so as
to not to interfere
with capillary
permeability
Capillary Permeability
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Structurally there are three types of
capillaries
– Continuous
– Fenestrated
– Sinusoidal
Continuous Capillaries
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Continuous
capillaries are
abundant in the
CNS, skin and
muscles and are
the most common
They are
continuous in the
sense that their
endothelial cells
provide an
uninterrupted
lining
Continuous Capillaries
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Adjacent cells are joined laterally by tight
junctions
However, these are usually incomplete and
leave gaps of unjoined membrane called
intracellular clefts that are just large
enough to allow limited passage of fluids
Fenestrated Capillaries
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Fenestrated
capillaries have
fenestrations
(pores) spanning
the endothelial cells
Fenestrated
capillaries occur
only where there
are exceptionally
high rates of
exchange of small
molecules between
blood and the
surrounding tissue
Fenestrated Capillaries
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The fenestrations are usually covered by a
thin diaphragm but this variety has much
greater permeability to fluids and small
solutes
Fenestrated capillaries are found where
active capillary absorption or filtrate
formation occurs
Fenestrated Capillaries
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Fenestrated
capillaries are found
in the small
intestine to receive
digested nutrients
These capillaries are
also found in the
synovial membranes
of joints to allow
water molecules to
exit the blood to
form synovial fluid
Intercellular
clefts
Routes of Capillary Permeability
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Molecules pass into and out of capillaries
via four routes
– Direct diffusion through endothelial cell
membranes
– Through the intercellular clefts
– Through cytoplasmic vesicles or caveolae
– Through fenestrations in fenestrated
capillaries
Routes of Capillary Permeability
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Most exchange of small molecules is
thought to occur through intercellular clefts
Caveolae apparently transport a few larger
molecules, such as small proteins
Carbon dioxide and oxygen seem to be the
only important molecules that diffuse
directly through endothelial cells because
these uncharged molecules easily diffuse
through lipid containing membranes of
cells
Low Permeability Capillaries
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The blood-brain barrier prevents all but
the most vital molecules(even leukocytes)
from leaving the blood and entering
brain tissue
The blood-brain barrier derives its
structure from the capillaries of the brain
Brain capillaries have complete tight
junctions, so intercellular clefts are
absent
Low Permeability Capillaries
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Brain capillaries are continuous, not
fenestrated and they also lack caveolae
Vital capillaries that must cross brain
capillaries are “ushered through” by
highly selective transport mechanisms in
the plasma membranes of the endothelial
cells
Sinusoidal Capillaries
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Some organs
contain wide,
leaky capillaries
called sinusoids
Each sinusoid
follows a twisted
path and has both
expanded and
narrowed regions
Sinusoidal Capillaries
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Sinusoids are
usually fenestrated
and their
endothelial cells
have fewer cell
junctions than do
ordinary
capillaries
Sinusoidal Capillaries
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In some sinusoids
the intercellular
cleft is wide open
Sinusoids occur
wherever there is
an extensive
exchange of large
materials, such as
proteins or cells,
between the blood
and surrounding
tissue
Sinusoidal Capillaries
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Sinusoids are found in only in bone
marrow and spleen, where many blood
cells move through their walls
The large diameter and twisted course of
sinusoids ensure that blood slows when
flowing through these vessels, allowing
time for the many exchanges that occur
across their walls
Veins
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Veins are the blood vessels that conduct
blood from the capillaries back to the heart
Because blood pressure declines
substantially while passing through the
high-resistance arterioles and capillary
beds, blood pressure in the venous part of
the circulation is much lower than in the
arterial part
Veins
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Because they need not withstand as much
pressure, the walls of veins are thinner
than those of comparable arteries
The venous vessels increase in diameter,
and their walls gradually thicken as they
progress from venules to the larger and
larger veins leading to the heart
Venules
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Venules, ranging from 8 to 100 m in diameter are
formed when capillaries unite
The smallest venules, the postcapillary venules,
consist of endothelium on which lie pericytes
Venules
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Venules join to form veins
With their large lumens and thin walls,
veins can accommodate a fairly large
blood volume
Up to 65%of the body’s total blood
supply is found in the veins at any one
time although the veins are normally only
partially filled with blood
Veins
externa
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Veins have three distinct tunics, but their walls are
always thinner and their lumens larger than those of
corresponding arteries
There is little smooth muscle even in the largest veins
Veins
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The tunica externa is
the heaviest wall layer
and is often several
times thicker than the
tunica media
In the venae cavae,
the largest veins,
which return blood
directly to the heart
the tunica externa is
further thickened by
longitudinal bands of
smooth muscle
Tunica externa
Veins
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Veins have less elastin in their walls than
do arteries, because veins do not dampen
any pulsations (these have been smoothed
out by the arteries)
Because blood pressure within veins is
low, they can be much thinner walled than
arterioles without danger of bursting
Veins
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Low-pressure conditions demand some
special adaptations to help return blood
to the heart at the same rate as it was
pumped into circulation
One structural feature that prevents the
backflow of blood away from the heart is
the presence of valves within veins
Veins
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Venous valves are
formed from folds of
the tunica intima and
they resemble the
semilunar valves of
the heart in structure
and function
Venous valves are
most abundant in the
veins of the limbs,
where the upward
flow of blood is
opposed by gravity
Veins
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A few valves occur in the veins of the
head and neck, but none are located in
veins of the thoracic and abdominal
cavities
A functional mechanism that aids the
return of venous blood to the heart is the
normal movement of our body and limbs
Veins
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Another mechanism of
venous return is called
the skeletal muscular
pump
Here contracting
muscles press against
the thin-walled veins
forcing valves proximal
to the contraction to
open and propelling the
blood toward the heart
Vascular Anastomoses
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Where vessels unite or interconnect, they
form vascular anastomoses
Most organ receive blood from more than
one arterial branch and arteries
supplying the same area often merge,
forming arterial anastomoses
Arterial anastomoses provide alternative
pathways called collateral channels for
blood to reach a given body region
Vascular Anastomoses
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If one arterial branch is blocked arterial
anastomoses provide the region with an
adequate blood supply
Arterial anastomoses are abundant in
abdominal organs and around joints,
where active movement may hinder blood
flow through one channel
Vascular Anastomoses
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Anastomoses are also prevalent in the
abdominal organs, brain, and heart
Because of the many anastomoses among
the smaller branches of the coronary
artery in the heart wall, a coronary
artery can be 90% occluded by
atherosclerosis (plaque) before a
myocardial infarction (heart attack)
occurs
Vascular Anastomoses
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Arteries that do not anastomose, or
which have a poorly developed collateral
circulation (retina, kidneys, spleen) may
be vulnerable if their blood flow is
interrupted
Veins anastomoses much more freely
than arteries and because of abundant
collateral circulation occlusion of a vein
rarely blocks blood flow leading to tissue
death
Vasa Vasorum
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The wall of the blood vessels contain
living cells and therefore require a blood
supply of their own
For this reason the larger arteries and
veins have tiny arteries, capillaries and
veins in their tunica externa
These tiny vessels the vasa vasorum
nourish the outer half of the wall of a
large vessel with the inner half being
nourished by the blood in the lumen
End of Material
Chapter 19