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Transcript Skeletal System
The Cardiovascular System:
Blood Vessels
Chapter 20
Introduction
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
Intermediate vessels connect
– Arterioles bring blood to the capillaries
– Venules drain blood from the capillaries
Blood Vessel Structure & Function
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
General Structure of Vessels
Blood Vessel Walls
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
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 (a basement
membrane) supports
the endothelium
Blood Vessel Walls
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
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
The outermost layer
of a blood vessel is
the tunica adventitia
This tunic is
composed largely of
loosely woven
collagen fibers that
protect blood vessels
and anchor it to
surrounding
structures
Tunica
adventitia
Blood Vessel Walls
The tunica adventitia
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
adventitia
Elastic (Conducting) Arteries
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
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
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
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
Arterioles
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
Capillaries
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
Most tissues have a
rich supply, but there
are a few exceptions
– Tendons and
ligaments
– Cartilage
– Epithelia
– Cornea
Capillaries
Given their location and the thinness of
their walls capillaries are ideally suited
for their role of providing access to
nearly every cell
– Exchange of materials
• Gases
• Nutrients
• Hormones
Types of Capillaries
Structurally there are three types of
capillaries
– Continuous
– Fenestrated
– Sinusoidal
Continuous Capillaries
Continuous capillaries abundant in the
skin and muscles are the most common
They are continuous in the sense that their
endothelial cells provide an uninterrupted
lining
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
Fenestrated
capillaries are
similar to the
continuous variety
except that some of
their endothelial
cells are riddled
with oval pores or
fenestrations
Intercellular
clefts
Fenestrated Capillaries
The fenestrations is 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 of digestive tract
– Receive digested food nutrients
Fenestrated capillaries perpetually open
– Kidneys for rapid filtration of blood plasma
Sinusoidal Capillaries
Sinusoids are highly modified, leaky
capillaries found in only in certain organs
(liver, bone marrow, lymphoid tissues, and
some endocrine organs)
Sinusoids have large, irregularly shaped
lumens and are usually fenestrated
Their endothelial lining is modified to
exhibit fewer tight junctions and larger
intercellular clefts
Sinusoidal Capillaries
These structural modifications allow large
molecules (proteins) and even blood cells to
pass between the blood and the surrounding
tissues
In the liver, the endothelium of the sinusoids
is discontinuous and large macrophages
called Kupffer cells form part of the lining
In organs such as the spleen, phagocytes
located outside the sinusoid stick cytoplasmic
extensions through the inter-cellular clefts
into the sinusoidal lumen to get at their prey
Capillary Beds
Capillaries form interweaving networks called
capillary beds
This flow is called a microcirculation
Capillary Beds
In most body regions, a capillary bed consists
of two types of vessel a vascular shunt (metaarteriole) and true capillaries
Capillary Beds
The terminal arteriole leads into a metarteriole
which is directly continuous with the thoroughfare channel
Capillary Beds
The thoroughfare channel joins the postcapillary venule that drains the capillary bed
Capillary Beds
The true capillaries number 10 to 100 per capillary
bed, depending on the organ served
Branch from metarteriole to thoroughfare channel
Capillary Beds
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
When the precapillary sphincters are relaxed blood
flow through the true capillaries and takes part in
exchanges with tissue cells
Capillary Beds
When the precapillary sphincters are contracted,
blood flows through the shunts and bypasses the
tissue cells
Capillary Beds
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
Venous System
Blood is carried from the capillary beds
toward the heart by veins
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
Venules, ranging from 8 to 100 m in diameter are
formed when capillaries unite
The smallest venules, the postcapillary venules,
consist entirely of endothelium around which a few
fibroblasts congregate
Venules
Venules are extremely
porous and fluid and
white blood cells
move easily from the
bloodstream through
their walls
Veins
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 or elastin in the tunica
media, which tends to be thin in even the largest veins
Veins
The tunica adventitia
is the heaviest wall
layer and is often
several times thicker
than the tunica media
In the venae cavae
which return blood
directly to the heart
the tunica adventitia
is further thickened
by longitudinal bands
of smooth muscle
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 can be blood reservoirs called
capacitance vessels
Veins
Because blood pressure within veins is
low, they can be much thinner walled
than arterioles without danger of bursting
However, the low-pressure conditions
demand some special adaptations to help
return blood to the heart at the same rate
as it was pumped into circulation
On adaptation is the large diameter
lumen of veins
Veins
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
Vascular Anastomoses
Where vascular channels unite 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
Arterial anastomoses are abundant in
abdominal organs and around joints,
where active movement may hinder blood
flow through one channel
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
End of Material
Chapter 20