ANATOMI-Angiology

7/25/2019 ANATOMI-Angiology

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The Cardiovascular System:

Blood Vessels

Chapter 19


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Introduction

The blood vessels of the body form aclosed 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


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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


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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


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Structure of Blood Vessel Walls


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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


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Blood Vessel Walls

The innermost tunic

is the tunica intima

This tunic contains

the endothelium, the

simple squamousendothelium that

lines all vessels

Its flat cells fit closely

together, forming aslick surface that

minimizes friction as

blood moves through

the vessel lumen

Tunica

adventitia


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Blood Vessel Walls

In blood vessels

larger than 1 mm in

diameter, a sub-

endothelial layer of

loose connectivetissue, subendothelial

layer, (basement

membrane) supports

the endothelium


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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 fibersof the sympathetic

division of the

autonomic nervous

system

Tunica media


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Blood Vessel Walls

Depending on the needs of the body, the

vasomotor fibers can cause vaso-

constriction 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


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Blood Vessel Walls

The outermost layer

of a blood vessel is

the tunica externa

This tunic is

composed largely ofloosely woven

collagen fibers that

protect blood vessels

and anchor it tosurrounding

structuresTunica

externa


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Blood Vessel Walls

The tunica externa is

infiltrated with nerve

fibers and lymphatic

vessels and, in larger

vessels, a system oftiny blood vessels

These vessels, the

vasa vasorum

nourish the externaltissues of the blood

vessel wallTunica

externa


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Arteries

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


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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 arteriesand thus are called 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 thenrecoiling to propel blood onward into the

circulation when the heart relaxes


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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


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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


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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


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A feature unique

to muscular

arteries,

especially thick

sheets of elastinlie on each side of

the tunica media

An external

elastic lamina liesbetween the

tunica media and

tunica externa


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The elastin in muscular arteries, like that

in elastic arteries, helps dampen the

pulsatile pressure produced by the

heartbeat


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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 smoothmuscle with a few scattered muscle fibers

The smaller arterioles that lead into

capillary beds, are little more than a singlelayer of smooth muscle cells spiraling

around the endothelial lining


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Arterioles

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 sentdownstream to each capillary bed

Sympathetic nervous system adjusts the

diameter of arterioles throughout the body to

regulate systemic blood pressure


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Capillaries

The microscopic

capillaries are the

smallest blood vessels

In some cases, one

endothelial cell formsthe entire circum-

ference of the

capillary wall

The average length ofa capillary is 1 mm

and the average

diameter is 8-10 m


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Capillaries

Capillaries have a

lumen just large

enough for blood

cells to slip through

in single file


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Capillaries

Capillaries are the bodys 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 nitrogenouswastes that cells deposit in the fluid


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Capillaries

Given their location and the thinness oftheir walls capillaries are ideally suitedfor their role of providing access tonearly every cell

Along with the universal functions justdescribed some capillaries also performsite-specific functions Lungs: gas exchanges

Endocrine glands: pick up hormones

Small intestine: nutrients

Kidneys: removal of nitrogenous wastes


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Capillary Beds

A capillary bed is a network of the bodys

smallest vessels that run throughout

almost all tissues, especially the loose

connective tissue

This flow is also called a microcirculation


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Capillary Beds

In most body regions, a capillary bed consists

of two types of vessel a vascular shunt (meta-

arteriole) and true capillaries


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Capillary Beds

The terminal arteriole leads into a metarteriole

which is directly continuous with the thorough-

fare channel


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Capillary Beds

The thoroughfare channel joins the post-

capillary venule that drains the capillary bed


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Capillary Beds

The true capillaries number 10 to 100 per capillary

bed, depending on the organ served

Branch from metarteriole to thoroughfare channel


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Capillary Beds

A cuff of smooth muscle fibers, called a pre-

capillary sphincter surrounds the root of each

capillary at the metarteriole and acts as a valve to

regulate the flow of blood into the capillary


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Capillary Beds

When the precapillary sphincters are relaxed, blood

flows through the true capillaries and takes part in

exchanges with tissue cells


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Capillary Beds

When the precapillary sphincters are contracted,

blood flows through the shunts and bypasses the

tissue cells


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Capillary Beds

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


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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


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Capillary Permeability

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 bytight junctions and occasional desmosomes


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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


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External to the

endothelial cells,

the delicate

capillary is

strengthened andstabilized by

scattered pericytes


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The pericytes are

spider shaped cells

whose thin

processes form a

network that iswidely spaced so as

to not to interfere

with capillary

permeability


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Structurally there are three types of

capillaries

Continuous

Fenestrated

Sinusoidal


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Continuous Capillaries

Continuous

capillaries areabundant in theCNS, skin andmuscles and are

the most common They are

continuous in thesense that their

endothelial cellsprovide anuninterruptedlining


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Continuous Capillaries 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 largeenough to allow limited passage of fluids


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Fenestrated Capillaries

Fenestrated

capillaries havefenestrations(pores) spanningthe endothelial cells

Fenestratedcapillaries occuronly where thereare exceptionally

high rates ofexchange of smallmolecules betweenblood and the

surrounding tissue


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Fenestrated Capillaries The fenestrations are usually covered by a

thin diaphragm but this variety has muchgreater permeability to fluids and small

solutes

Fenestrated capillaries are found whereactive capillary absorption or filtrate

formation occurs

C i i


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Fenestrated Capillaries

Fenestrated

capillaries are foundin the small intestine

to receive digested

nutrients

These capillaries are

also found in the

synovial membranes

of joints to allowwater molecules to

exit the blood to

form synovial fluid

Intercellular

clefts

R f C ill P bili


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Routes of Capillary Permeability

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

R f C ill P bili


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Routes of Capillary Permeability

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 diffusethrough lipid containing membranes of cells

L P bili C ill i


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Low Permeability Capillaries

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 areabsent

L P bilit C ill i


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Low Permeability Capillaries

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

Si id l C ill i


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Sinusoidal Capillaries

Some organs

contain wide,leaky capillaries

called sinusoids

Each sinusoidfollows a twisted

path and has both

expanded and

narrowed regions

Si id l C ill i


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Sinusoids are

usually fenestratedand their

endothelial cells

have fewer cell

junctions than do

ordinary

capillaries

Si id l C ill i


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In some sinusoids

the intercellularcleft is wide open

Sinusoids occur

wherever there isan extensive

exchange of large

materials, such as

proteins or cells,between the blood

and surrounding

tissue

Si id l C ill i


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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

V i


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Veins

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

V i


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Veins

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

Venules, ranging from 8 to 100 m in diameter areformed when capillaries unite

The smallest venules, the postcapillary venules,

consist of endothelium on which lie pericytes

V l


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Venules

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 bodys total blood

supply is found in the veins at any one

time although the veins are normally only

partially filled with blood

V i


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Veins

Veins have three distinct tunics, but their walls arealways thinner and their lumens larger than those of

corresponding arteries

There is little smooth muscle even in the largest veins

externa

Veins


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Veins

The tunica externa is

the heaviest walllayer and is oftenseveral times thickerthan the tunica media

In the venae cavae,the largest veins,which return blooddirectly to the heart

the tunica externa isfurther thickened bylongitudinal bands ofsmooth muscle

Tunica externa

Veins


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Veins

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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Veins

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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Veins

Venous valves are

formed from folds ofthe tunica intima and

they resemble the

semilunar valves of

the heart in structure

and function

Venous valves are

most abundant in theveins of the limbs,

where the upward

flow of blood is

opposed by gravity

Veins


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Veins

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 thereturn of venous blood to the heart is the

normal movement of our body and limbs

Veins


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Veins

Another mechanism of

venous return is calledthe skeletal muscular

pump

Here contractingmuscles press against

the thin-walled veins

forcing valves proximal

to the contraction toopen 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 forblood to reach a given body region



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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



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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 coronaryartery can be 90% occluded by

atherosclerosis (plaque) before a

myocardial infarction (heart attack)occurs



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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 veinrarely blocks blood flow leading to tissue

death

Vasa Vasorum


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Vasa Vasorum

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 andveins in their tunica externa

These tiny vessels the vasa vasorum

nourish the outer half of the wall of alarge vessel with the inner half being

nourished by the blood in the lumen


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