Download Blood Vessels

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
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 most
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 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 always 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 ehart 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 where rapid filtration of blood plasma
Sinusoidal Capillaries



Sinusoids are highly modified, leaky
capillaries found in only in certain organs
(liver, bone marrow, lymphoid tissues)
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 they 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
Typically 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 to 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 by passes 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
Venules


Venules, ranging from 8 to 100 m in diameter are
formed when capillaries unite
The smallest venules, the postcapillary venules,
consist entirely of endo- thelium 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 accomodate 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 may be vulnerable if their
blood flow is interrupted
End of Material
Chapter 20