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Cardiovascular System: Myocardium and Heart
The cardiovascular system can be anatomically subdivided into the heartand the
blood vessels, and the latter category is further subdivided into different vessel
types.
Arteries are blood vessels which conduct blood away from the heart veins are blood
vessels which conduct blood towards the heart. These two ma!or categories are
bro"en down further, and there are subcategories of arteries and veins based on
si#e, construction of the vessel wall, etc. However, the direction of blood flow is the
criterion which separates arteries from veins, and which defines a vessel as being in
one or the other ma!or class. $ou may have heard the hoary old chestnut that
%Arteries carry o&ygenated blood, and veins carry deo&ygenated blood.% TH'S 'S
()T T*+. Some arteries carry deo&ygenated, and some veins carry o&ygenated
blood. The direction of flow is the only criterion for classification.
.
The Heart
-egin with slide /, and start by
holding it up to the light. This is
an entire heart from some small
animal 0most li"ely a rat1 which
has been sectioned from the ape&
to the cranial end of the atria.
After orienting yourself, place it
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on the microscope under
low power. $ou will easily
be able to ma"e out at
least two, and probably
three of the chambers if
you have a favorablesection, you may be able
to see all four. The atria
and ventricles are
separated from each
other. *unning from the
ape& to the cranial end
you will see the septum
dividing the right and left
sides. There should be at least one of the heart valves in your section, connecting the
atria and ventricles. &ternal to the heart proper, you should have a cross section of
one of the great vessels 0the pulmonary artery1 and some adipose connective tissueas well.
Turn now to slide . This is the heart of yet another unfortunate rodent, sectioned
transversely, below the level of the atrioventricular !unctions. This slide graphically
illustrates the thic"ness of the wall of the left ventricle and ventricular septum. The
cross section at right shows a %slug% of blood fro#en in its passage from the left
atrium to the left ventricle, and the left A23 valve is open. )n this slide you should
be able to ma"e out most of the features identified in the previous slide, e&cept those
which are out of the plane of the section.
4hile both atria and ventricles are composed of the same type of speciali#ed musclecells, those of the atria tend to be less numerous, thinner, and more elongated than
those of the ventricles. The ventricles of the heart are more stoutly constructed than
the atria because they have more wor" to do. All the atria do is pass blood to the
ventricles below: the right ventricle sends blood to the lungs against the resistance of
the pulmonary circulation, and the the left ventricle has to deal with the entire
system circulation5s resistance to flow. 4hile the resistance of the lung capillaries to
blood flow is considerable, that of the entire peripheral circulation is much higher.
Myocardium
The heart is a large mass of muscle. Cardiac muscle is a variant form of striated muscle,with distinct differences from the skeletal form; and some unique structures that make it
work properly, day in and day out, until death. The term "myocardium," (from Greek,
myos muscle ! kardio heart is specifically the mass of muscle that comprises the#ulk of the organ.
)n slides / and at low power, you can easily observe the pattern of the muscle
cells of the myocardium. Myocardial cells 0cardiac myocytes1 are much smaller than
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the myofibers of s"eletal muscle, and they don5t form long cylindrical structures the
way myofibers do.4hile s"eletal myofibers can in some cases be metersin length,
it5s a rare cardiac myocyte that e&ceeds /66 7m in length, and perhaps 86296 7m
wide. The shape of the myocyte is pretty irregular, with stumpy pro!ections coming
off of it where it contacts other cells. Myocytes also differ from s"eletal myofibers in
that cardiac myocytes are mononuclearand not syncytial.)ne nucleus per myocyteis the rule, not hundreds as in a s"eletal myofiber, and that one nucleus is centrally
located.
Here5s scanning
electron
microscope
image of a
single cardiac
myocyte, which
has been
mechanicallyseparated from
the mass of the
myocardium.
This beautiful
image 0at about
9666&1 shows
the irregular
shape of the
myocyte uite
well, and the
points at whichthis cell is in contact with others via the intercalated discs are indicated by arrows.
Also visible as wispy strands of material on the surface are fine collagen fibrils of
the intercellular collagen networ". ;ust as in s"eletal muscle, the 'C( transmits
force throughout the mass, and the hierarchical arrangement of endomysium2
perimysium2epimysium applies.
(ote that the surface of the myocyte appears to be ridged or grooved. These ridges
are the sites of
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but remain connected to each other: one is more heavily stained than its partner, so
that the boundary where the two are !oined 0an intercalated dis", see below1 is clear.
As is obvious in this image, each cell has a single centrally located nucleus. +nli"e
s"eletal muscle, neither myocardium nor the cells of which it5s composed are
syncytial. The cells are physically isolated from each other, a fact which was in
dispute before the electron microscope settled the matter once and for all about/=96. 0(evertheless, than"s to the >aw )f ?ersistence )f rroneous 'nformation,
you5ll stillfind the statement in te&ts that %the heart is functionally syncytial,%
something that sets my teeth on edge.1 Though there is undoubtedly constant
communication and coordination between heart muscle cells, they are structurally
distinct entities, and the term %syncytium% is inappropriate in this conte&t.
Than"s to the difference of cellular architecture, the histological appearance of
myocardium is very different from that of s"eletal muscle.$ather than forming neat#undles of parallel fi#ers with well%defined striations, myocardium forms an
anastomosing network with a sort of "spongy" appearance. The contacts #etween
myocytes are such that a #ranching network with #lood vessels in the spaces #etweenthem is the result.
$ou will see some areas where the myocardial cells are oriented end2to2end in long
rows parallel to the plane of the section 0i.e., they are cut longitudinally1 and others
where they run at right angles 0and thus are cut transversely1. There will also be
areas where the orientation with respect to the plane of the section is less well
defined these are obliue sections. The myocardial muscle bundles are oriented in
such a way as to ma"e most efficient use of the force of contraction.
These two images give a pretty good idea of the appearance of myocardium in the
light microscope. The one at left is stained with toluidine blue at about @66& the one
at right with H at roughly B66&.
Myocardium has a %stringy% loo" compared to s"eletal muscle. 4hile s"eletal
muscle cells are very large and lie ne&t to each other in parallel bundles, the smaller
cardiac muscle cells are butted together at their ends, and irregularly shaped,
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numerous blood vessels between them. The effect is to create an anastomosing
networ" of fibers rather than a solid phalan& of muscle fascicles. The single nucleus
of each myocardial cell is uite clearly visible in these images, especially on the
right.
This drawing will clarify the architecture of the tissue. (ote the branching, and
compare it to the actual specimens shown in the longitudinal view. At lower right in
the drawing, the cells are shown in cross section. Compare the drawing to the actual
specimen shown at right, and to the sections of smooth muscle in &ercise /6: at first
glance this field could be confused with smooth muscle, but they can be told apart
fairly easily. An important difference between the appearance of this tissue
compared to smooth muscle is that in cardiac muscle almost all the cell profiles are
appro&imately the same si#e, which isn5t true of smooth muscle. urthermore, most
of the cell profiles will have a nucleus inside. Cardiac myocytes are so short and the
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nucleus ta"es up such a proportionally greater percentage of their length 0compared
to smooth muscle cells1 that the chances of intersecting a nucleus with the plane of
the section is pretty good. The individual cells are clearly defined by their CT
envelope 0the endomysium1 and most of them show a nuclear profile.
The smaller, denser nuclei outside the cells are fibroblasts that ma"e and maintainthe intercellular collagen networ". There5s a blood vessel crossing the field about
one uarter of the way down from the top edge. Cardiac muscle is even better
served by the circulatory system than s"eletal muscle is. +nli"e s"eletal muscle,
cardiac muscle can5t incur an %o&ygen debt,% because it doesn5t have the lactic acid
pathway to generate AT? when o&ygen levels are low. Conseuently it5s uite prone
to ano&ia, and the capillary beds are e&tensive to minimi#e the possibility of it
happening.
'ntercalated Dis"s
The most prominent feature of myocardium, and the one that5s absolutelydiagnostic for it, is the intercalated disk. The 'D is a speciali#ed cell2to2cell
adhesionEcommunications site. 't demarcates the beginning of one myocyte and the
end of the ne&t, and information is pass across it from cell to cell. These structures
are found only in cardiac muscle.
The 'D5s are vital to normal myocyte function and understanding how they are put
together is important. 'n these special areas there5s a considerable degree of
interdigitation of myocyte plasma membrane, but there is noactual fusion of
cytoplasm. The speciali#ed apparatus of the 'D allows myocytes to act as ifthey
were a true syncytium there is in fact no actual cytoplasmic continuity these are
distinct and individual cells.
The true nature of the 'D wasn5t fully understood until the transmission electron
microscope was available for their study. This instrument revealed that 'D5s are
physically held together by large numbers of desmosomes, with gap !unctions
between the myocytes forming a region that permits electrical communication in the
form of ion flu&es. This flu& maintains the coordination of the waves of contraction
between one cell and the ne&t.
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'ntercalated discs are readily seen on slide 9@9, in regions where the myocardium is
cut longitudinally 0they won5t be visible in cross sections1. Since the 'D5s are located
at the ends of the cell, and since that5s where the first and last sarcomeres of any
given myofibril are, you can thin" of the 'D as a sort of thic"ened %terminal
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&amine the lining of the chambers on slide /. The entire cardiovascular system,
including all chambers of the heart and all blood vessels, is lined with a simple
suamous epithelial covering.This lining is among the most metabolically active
tissues in the body. 'n blood vessels, this is 0by convention1 called %endothelium,%
but in the heart the special term endocardiumis used instead. 0There isn5t a nic"el5s
worth of difference between them, but the terminology is so entrenched it couldnever be eradicated.1 'n any
event, %endocardium% is an apt
name because it5s literally
%inside the heart.% The
endocardium covers the valve
cusps, too, as you can easily
verify on this slide. 'f you
traverse the thic"ness of the
myocardium and e&amine the
outersurface of the wall, you
will see a similar 0though lesswell defined1 layer of simple
suamous epithelium. Again, by
convention, this has a special
name it5s the epicardium, which
translates as %upon the heart.%
Anatomically, this is the visceral
layer of the pericardial sac.
The epicardium is the inner portion of the
CT envelope that surrounds the heart. 'tcorresponds to the peritoneum of the
abdominal cavity in origin 0from the
mesoderm lining the embryonic coelom1
and function 0to allow the heart to move
freely in its cavity without adhesion to
surrounding structures1. Here you see it as
a thin serous membrane overlying the
outer surface of the organ. A stain for
connective tissue would reveal a very
delicate sub2epicardial CT layer, mainly
reticular fibers.
?ur"in!e ibers
Slide 9F@ is another chun" of myocardium. Most of the features of myocardium are
visible, but this section also shows ?ur"in!e fibers0;ohannes vangelista von
?ur"in!e, /GFG2/F=, a C#ech anatomist, physiologist and microscopist1. These fibers
are part of the conducting system of the heart.
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The conducting system transmits the signal to contract from its origin in the
%pacema"er% to the rest of the myocardium. The ?ur"in!e fibers are not neurons.
They5re speciali#ed muscle cells, and they loo" li"e it. They can be distinguished
from the myocardium by their si#e 0they5re much larger than regular myocytes1 and
by their
staining0they tend
to be more
lightly
stained
than true
myocardium1. 4ith the ?AS stain they5re strongly positive, since they contain large
amounts of glycogen.
?ur"in!e fibers are not contractile, at least not to any significant e&tent. They
contribute nothing to the force generation of myocardium. The cells of the ?ur"in!e
fibers are organi#ed into several tracts that lead away from the atrioventricularnode and form nerve2li"e structures. There5s a bundle along both sides of the
septum, one distributing to the right ventricle and one to the left ventricle. The
?ur"in!e fibers are larger and less well stained than the ordinary contractile
myocardium because of the relatively protein2poor cytoplasm, which doesn5t ta"e up
the eosin stain very well. Hence the fibers loo" pale and washed out in comparison
to the protein2pac"ed myocytes. ?ur"in!e fibers do have some of the other features
of cardiac myocytes, including a centrally2located single nucleus, and even
intercalated dis"s.
-ut despite their strictly conductive function, they are muscle cell derivatives, so
you5re uite li"ely to see striations and other indications of %contractility% if youe&amine them at higher power. There should be two areas of ?ur"in!e fibers in your
slide, one at each edge of the section. 'n this slide also note the ramifications of the
muscle, and the intercalated discs.
The ?ur"in!e fibers ramify into the mass of ventricular myocardium, and from their
termini the signal is spread through the 'D system. *ecall that all muscle tissue is
%e&citable% and this property can be used to carry information. That5s what5s
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happened here. 4hat is a secondary function in contractile myocytes has been
transmuted into the main role of these not2uite2muscle cells. The fibers run
through the sub2endothelial connective tissue they can5t generate any force because
internally they have only a few disorgani#ed actin and myosin fibrils, nothing li"e
the very regular arrays of sarcomeres in the contractile cells.
Slide =, shown at left, is
interesting because it contains the
atrioventricular node, the
beginning of the ?ur"in!e fiber
conduction apparatus.
This image is ta"en with the atrial
region at the top of the field. At
the upper left is the base of one of
the great vessels leading from the
heart 0probably the aorta1 andbelow that a triangular2shaped
region, the trigona fibrosa. The
trigona fibrosa is part of the so2
called %cardiac s"eleton,% the
dense fibrous CT to which the
intercellular collagen networ" is
ultimately anchored. 'f you loo"
at this structure at high
magnification it5s very similar in
appearance to cartilage. The
trigona fibrosa lies between theatrial and ventricular parts of the
organ and the fibrous rings of the
great vessels are attached to it so
is the top level of the 'C(. 4hen
force is e&erted by contraction of
myocardium, this is the %#ero
point% against which the force is e&erted. 4hen contraction occurs, the different
directions of muscle tracts impart a compressive, twisting motion to the heart
grossly 0you will be able to see this in surgery1. The motion has been compared to
%wringing out% a wet cloth, and it serves the same function: e&pulsion of the
ma&imum amount of blood per stro"e. Contraction of the myocardial cells leads to
reduction in volume of the heart chambers, and conseuently to e!ection of all 0or
almost all1 of the blood therein. 'mpairment of this action results in decreased
efficiency of pumping and eventual failure.
Alongside the trigona, and lyingas its name impliesbetween the atrium and the
ventricle, is the A23 node. 't loo"s li"e a bit of atrial myocardium, splon"ed up
against the denser and more massive ventricular type. 0This slide also gives you a
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good comparison of the two types of myocardium side by side1. This isn5t the
%pacema"er,% 0that5s the sino2atrial or S2A node1 but it receives signals from it and
transmits them via the conducting fibers to the mass of ventricular myocardium.
Histologically it loo"s li"e the rest of the atrial myocardium, and in the absence of
the landmar"s used here, would be very difficult to identify in a section.
3alves of the Heart
Slide /6@ is of a valve cusp. >oo" at it at low magnification and at high
magnification and e&amine its construction. The cusp of a valve has a core of CT,
much of it composed of elastic fibers. There
may be some smooth muscle wor"ed into it as well. The outer surfaces, e&posed to
the blood flow, are covered with the epithelium of the endocardial lining, li"e the
rest of the system. (ote that there5s an e&cursion of myocardium into the base of the
valve cusp this terminates before the end of the cusp. 't5s continuous with the
myocardium of the ventricle.
-lood Supply
' hope it hasn5t escaped your notice that the entire mass of the myocardium is shot
through with blood vessels of varying si#es and shapes, and that there5s an e&tensive
degree of vasculari#ation. 'n fact, the heart pumps blood to itself before it sends any
to other parts of the body. The first branches off the aorta are the coronary arteries
of the myocardial circulation. After you have completed the section on blood vessels
you may want to come bac" and classify some of these.
Arteries
>et us now leave the pumping station and e&amine some of the pipes: the arteries
and veins. *emember the definition: arteries pump blood away from the heart
veins carry blood towards the heart. (o other criteria for classification of a vessel as
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one or the other e&ists.Capillaries are those small vessels within the tissues from
which o&ygen transfer ta"es place.
Arteries may be subclassified by type. Conducting or elastic arteries are large ones,
with very strong and relatively elastic walls, whose function is to %conduct% the bul"
of the blood to regions of the body where it5s to be distributed. &les include theaorta, subclavian, and pulmonary arteries. )nce the blood has reached the region of
distributionsay, the limbsit will be handled by smaller 0but still fairly large1
distributing or muscular arteries, which send it to sub2regions.As the distribution
area gets more and more limited the arteries become smaller. 'n very local areas you
will see small arterioles, essentially mini2arteries with a wall considerably less
muscular than the larger ones %upstream.%
lastic or Conducting Arteries
lastic arteries are constructed li"e fire hoses. *ightly so, because they have the
same function: to carry a stream of liuid under high pressure. Hence they5redesigned to minimi#e internal friction and flow resistance and to ma&imi#e the
strength of the wall.
The lumen is lined with a thin suamous epithelial layer 0the tunica intima1 which
may be li"ened to the rubber bore of the hose li"e the rubber, it offers a smooth and
unimpeded passage for the flow of blood. The tunica mediais a region of elastic and
collagen fibers. lastic arteries 0the aorta most of all1 must withstand an enormous
head of pressure to pump against the peripheral systemic resistance: conseuently
the wall is heavily reinforced to prevent bursting, !ust as the wall of a fire hose has
reinforcing cords in it. The elastic fibers allow some stretching and %springiness% in
response to the pressure, and the collagen fibers limit the degree of stretchpermitted. 0'n some disease or deficient nutritional statesfor e&le lathyrism, a
copper deficiency caused by certain plantsthe wall may be wea"ened, resulting in
an aneurysmwhich may lea", or burst with fatal effects.
To complete the analogy, the collagenous tunica adventitiaof the aorta is the fabric
covering on the outside of the hose. ;ust as the fireman needs a firm grip to control
the hose, so must elastic arteries be anchored down to the surrounding structures, to
prevent them from moving around as pressure varies internally. The tunica
adventitia of conducting arteries is scanty, and collagenous in nature.
Slide 86 is a fine e&le of an elastic or conducting artery. 't is, in fact, a section ofthe aorta. The section on this slide is stained with the 3erhoeff5s stainfor elastic
fibers. 't renders these fibers blac", and you will see that the wall of this vessel is
shot through with elastic fibers. The spaces between the elastic fibers are mostly
occupied with collagen, and some small amount of smooth muscle 0see below1. The
amount of elastic fiber infiltration is so great that no internal or e&ternal elastic
laminae can be identified.
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This is a section of the aorta, a nice e&le of an elastic artery. The elastic fibers
normally aren5t easily seen in an H preparation li"e this one, but in this case, the
reinforcement is so heavy that the concentric layers of elastic CT show up as distinct
iridescent rings, some of which are mar"ed by arrows. There5s a large clot of blood
in the field. The bore of this vesselli"e the rest of the cardiovascular systemis
lined with a simple suamous epithelium.
The innermost of the numerous elastic layers in this artery has its own name: the
internal elastic lamina.'t5s the one right up against the tunica intima. The internal
elastic lamina is much more easily seen in muscular arteries 0see below1 than in the
elastic ones, though. 'n an elastic artery li"e this one it gets lost in the %bac"ground%
of do#ens of similar layers.
As is true of most elastic arteries, the tunica adventitia on this one is a relatively
small contributor to the wall5s thic"ness.
There5s more than elastic fibers in the wall of this type of artery. -etween the elasticlayers there is a considerable amount of collagen and some smooth muscle,
permitting the artery to e&pand under pressure and recoil to original diameter when
the pressure drops again. Collagenous components in the wall prevent over2
e&pansion and resist bursting of the vessel.
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+sing stains specific for elastic fibers reveals how e&tensive the reinforcement of the
wall can be. Here are two e&les, again from the aorta. The one at left has been
stained with a combination of the 3erhoeff and the 3an Iieson stain at about B66&,
the e&tent of the elastic component 0in blac"1 and the collagenous reinforcementbetween the elastic fibers is easily visible. This field is from the outer edge of the
tunica media: the almost2completely red area at the bottom is the tunica adventitia.
At right, the 3erhoeff stain has been used and at about @66& it clearly show elastic
fibers but not the collagen or smooth muscle, which is unstained by this method.
3ery large elastic arteries have their own internal blood circulation system and
nervous supply. They have to: the fibroblasts and other cells that "eep the wall in
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good shape are so far from the blood supply that diffusion won5t serve their needs,
and the density of the wall and its fibrillar components also impede diffusion. The
smooth muscle in the wall is innervated so that the C(S can control blood pressure
and initiate contractions when needed. The vasa vasorum0and in the case of nerve
fibers, the nervi vasorum1 i.e., the %vessels of the vessels% and the %nerves of the
vessels% are a constant feature of big vessels.
Muscular Arteries
)nce you get
the blood out
to the ma!or
regions of
the body,
there5s a
transition in
the structureof the
arterial wall.
The
proportion
of elastic
fibers
decreases,
and the
proportion
of smooth
muscleincreases.
Some elastic
fibers and
collagen
fibers will
always be
present, but
eventually
the great
bul" of the tunica media will be smooth muscle, and at that point we5re dealing with
muscularor distributingarteries, whose function is to %distribute% blood supply to
their regions of responsibility, such as a limb. The artery on slide /68, at right, is a
dandy e&le of what a muscular or distributing artery should loo" li"e. This is
the femoral artery the femoral vein 0see below1 and the femoral nerve are there as
well. There may be a branch point off this artery on your slide.
(ote that the wall of the artery is mostly tunica media, and that this tunic is almost
entirely smooth muscle.lastic CT is present, to be sure, but not nearly to the e&tent
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that it is in the elastic arteries. There is also a noticeable internal elastic lamina,
stained bright pin", and a less well defined e&ternal elastic lamina, though at the low
magnification of this image it5s hard to ma"e out. The endothelium of the tunica
intima is easily visible. The tunica adventitia grades off into the surrounding
connective tissue, but is fairly sharply defined. The internal elastic lamina is !ust
barely visible in this image. 't5s the undulating pin" line immediately below thelining endothelium. 't mar"s the innermost limit of the muscular tunica media. The
e&ternal elastic lamina is less regular and can5t be made out at all. (ote the
prominent collagenous tunica adventitia here. 'n muscular arteries the tunica
adventitia is often most of the wall5s overall thic"ness. The e&ternal CT investment
anchors this artery to the surrounding CT.
The internal elastic lamina is most easily seen in smaller muscular arteries. 'n theseit usually stands out as a bright pin" undulating band !ust below the lining epithelial
cells and their supporting CT. 'n life, the artery is always under some pressure, but
when death occurs the tonus of the wall causes a partial collapse, so that the
undulation is something of an artifact.
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'n the e&le at left, the '> is indicated this e&le also clearly shows the
separation of the lining epithelium from the '> by the sub2endothelial CT. A few
layers of smooth muscle constitute the tunica media in this small artery.
The smooth
muscle of the wallof distributing
arteries ma"es
them very
e&tensible, and
also provides for a
counter force to be
e&erted against
the pressure of
filling. As the
vessel e&pands the
smooth musclecells are stretched:
in reaction to this
they begin to
contract. The
pea" of their
contraction comes
at about the point where systole ends and diastole begins thus the contraction of the
arterial walls dampens out the pulsations of the flow to provide a more or less steady
supply of blood at normal pressure into the capillary beds. (ervous input can also
control this to some e&tent, independent of the mechanical force of stretching. As
distance from the heart increases, the force reuired to dampen the oscillations isless, and smaller arteries can handle it the interval between pea"s also lengthens
and there is a much more uniform flow rate.
't5s
useful to
put the
two
ma!or
arterial
types
side by
side,
stained
to reveal
the wall
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components. )n slide 86, this has been done using the 3erhoeff stain. A small
muscular artery is near the large elastic one, to ma"e the point that as the artery
si#e decreases, the proportion of elastic fibers in the wall decreases as well. The wall
of the small artery has almost no elastic tissue in it, but the the internal elastic
lamina, however, is very clearly defined against the bac"ground of the muscular
tunica media.
't5s possible to combine several staining methods to show allthe ma!or components
of the wall, including the smooth muscle. 'n the image below, the 3erhoeff and
Masson5s stains have been combined. lastic tissue is stained blac" smooth muscle
is red and collagen is green. The tunica media 0TM1 is almost entirely muscle, with
a few minor strea"s of green collagen in it. The inner elastic lamina 0'>1 stands out
prominently and the outer elastic lamina 0)>1 demarcating the end of the tunica
media is also easily visible. The tunica adventitia 0TA1 is a mi&ture of green collagen
fibers and blac" elastic fibers interwoven with each other to provide strength and
resilience.
Slide F demonstrates the brachial artery and vein. The artery has the typical
structure of a distributing artery, and it has a very nice tunica intima and tunica
media. The vein shows the typical structure of tunica intima, scanty tunica media,
and a thic" CT tunica adventitia. There are also valves present.
Arteries 'n >ongitudinal Section
Since arteries are pretty common in almost any microscope slide you5ll loo" at, it5s
important to be able to recogni#e them when they aren5t cut in cross section, as all
the previous e&les have been.
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Here are two e&les from tissue sections. The left section is from slide 8B the
right from slide /BF. Since the smooth muscle in the wall of an artery is oriented
with the long a&is of the smooth muscle cells aroundthe long a&is of the vessel
proper, in cross sections, the muscle cells are cut in longitudinal view, but in long
sections of the vessels, as here, the cells are cut in crosssection. Compare these
profiles 0both at about B66&1 to the sections of smooth muscle in &ercise /6, andyou5ll be able to ma"e out the boundaries of the smooth muscle easily.
Aneurysms
Sometimes the wall of an arteryespecially a big one li"e the aorta, which is
sub!ected to all or nearly all the pressure the heart can generateis wea"ened by
disease, malnutrition, age, or some other affliction. +nder a good head of pressure,
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blood begins to dissect the layers of reinforcing material, separating them and
causing the side of the artery to bulge, e&actly as a garden hose does when its wall is
wea"ened. This bulge is an aneurysm. Chronic high blood pressure ma"es this more
li"ely, though an aneurysm can occur even when systemic blood pressure is lower
than normal. (eedless to say, an aneurysm that bursts will really spoil your plans
for the wee"end. 'f it5s a big one that blows out, such as the aorta, death will berapid.
3eins
3eins are
those
vessels
leading
blood bac"
towards
the heart.As a rule,
they have
much
thinner
walls than
arteries
do, though
in cross
sectional
area
they5reusually
larger
than the
corresponding artery, because they have to carry the same volume of blood at a
lower pressure. Since veins are on the post2capillary side of the circulatory loop,
operating at much lower pressures, there5s less need for burst resistance. Thin walls
are also important because much of the pressure that drives blood through veins is
generated notby the heart, but by contraction of the muscles in the region of the
vein. This %suishes% the blood bac" through the vein. -ecause they have low
pressures, some veins have venous valves in them to prevent bac" flow. This is
especially true of medium si#ed veins in the e&tremities, as they have to lift bloodagainst gravity.
0'f an animal is held totally immobile for a long period of time, the return of the
blood to the heart is diminished, because contraction of the muscles of the limbs no
longer pushes blood bac" to the heart. Since the heart can only put out what it ta"es
in, the net result of decreased venous return is decreased arterial output. At some
point the output declines to the point where the brain is no longer receiving
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sufficient blood, and the animal loses consciousness. This happens surprisingly
often. Soldiers on parade, when forced to remain at the position of %attention% for
long periods, will from time to time "eel over and uite literally %drop out% of the
ran"s as they
faint.1
3enous 3alves
3eins of a
certain si#e
usually have
valves to
prevent bac"
flow. 3eins of
this type are
usually found
in thee&tremities, the
valves allowing
blood to move
bac" towards
the heart with
less effort.
3enous
pressures are
so low, and the
propelling
force has tofight against
not only the resistance of the vessels but the relentless pull of gravity, that valves in
the vein allow the blood to be pushed up above and when the valve closes without
running bac" down when pressure slac"s off.
The venous valve has a great deal of structural similarity to the valves of the heart.
There5s a CT core with epithelium on both sides. Two valve flaps meet in the center
of the vessel. $ou can find such valves on slide F, and when you do, note especially
the direction of the valve: it5s designed to permit blood to flow in one direction only.
?ooling of blood occurs on the superior side of the valve flaps. 'n animals with long
lives and vertical postures, the continual stress that the weight of this blood imposes
sometimes causes outpouching of the vein. 'f there is a wea"ening of the wall on the
downstream side of the valve, the blood will push it out into a small bubble
analogous to the aneurysm in an artery. The result is what we call %varicose veins%
that many peopleespecially those who wor" on their feetdevelop in later life.
This image from slide F shows a fairly large vein, cut more or less in cross section.
The flaps on both sides of one of its valves are visible.
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Capillaries
Capillaries are small vessels, whose walls are thin enough to allow the diffusion of
nutrients, o&ygen and carbon dio&ide across them.They are %where the action is% in
gas and nutrient e&change: with a few e&ceptions, no cell of the body is very far
from one, because access to the blood is an absolute reuirement: cell death wouldresult from ano&ia or the loss of nutrient and waste transport.
Capillaries are by far the most numerous class of vessels, though they5re so small
they can only be appreciated in microscopic sections. There are two types: closed or
continuous capillaries, and fenestrated capillaries.
The closed type is found in locations where rapid %bul"% transfer of materials
between the blood and the tissue it served isn5t needed, such as in muscles. Closed
capillaries can move material in and out using a process of seuential endocytosisand e&ocytosis, as well as simple diffusion for small molecules.
enestrated capillaries, which have actual pores in their walls, are located wherever
immediate movement of materials is a functional necessity, such as in endocrine
organs and in the "idney. They don5t demonstrate the %transcytosis% activity of the
closed type, because bul" movement of ions, hormones, nutrients, etc. through the
pores is ample
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Here5s a scanning
electron micrograph of
a capillary running
alongside muscle
fibers. This is a closed
capillary. This stri"ingimage shows the
appro&imate si#e of a
capillary very well:
they5re !ust about the
same si#e as the
erythrocytes that flow
through them, maybe
/6 7m in luminal
diameter. 'n this
picture you can see an
erythrocyte pee"ingout at the bro"en end,
and the anchoring
fibrils of connective
tissue that attach it to
the surrounding muscle mass 0*1.
Compare this to the diagram above: the wall is made of very thin suamous cells
which are sealed together at the edges by desmosomes in other words, there5s only a
tunica intima, and not much of that beyond the lining epithelium and the basement
membrane it rests on. 'n a section it5s normal to see parts of several of the mural
cells in the same plane, since there5s uite a bit of interdigitation between theirregular borders of ad!acent cells.
$ou may also see a second cell type, the pericyte. Technically this isn5t part of the
capillary wall. ?ericytes %embrace% the wall but never ma"e contact with the blood,
as the mural cells do they5re believed to have some sort of contractile function.
They5re so closely associated with the mural epithelium they share a basement
membrane with it.
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Despite
their small
si#e,
capillaries
are so
numerousthat
they5re
easy to
find. They
often will
have blood
cells in
them,
which also
ma"es
them easyto spot.
This one is
from
s"eletal
muscle on slide /B. S"eletal muscle has an abundant blood supply, and this capillary
is lying between two of the large muscle cells. A couple of erythrocytes are visible,
and the bore diameter of the capillary is !ust large enough to accommodate them
0double arrow1. A white blood cell 0probably a neutrophil1 is visible in the left end of
the field. As it happens there are no capillary cell nuclei nor pericyte nuclei visible,
they5re out of the plane of the section but the wall itself can be see. The cytoplasm
of the mural cells is very scanty, and hence the precise limits of the cell and the CT
around it are hard to ma"e out.
The capillary at right is
from white fat. 't5s cut in
cross section. The plane of
section has passed through
the nucleus of the mural
cell, and the lumen is filled
with the cytoplasm of an
erythrocyte.
All blood vessels, of
whatever type, are derived
from embryonic
mesoderm, the only one of
the three layers of the
embryo that has angiogenic
potential. 't shouldn5t
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therefore be too surprising to find that connective tissues, which are also of
mesodermal origin, are usually well supplied with blood vessels. That5s the case
here: white fat is a connective tissue, and the fibrous CT that separates each fat cell
0the clear spaces in this image1 from the others is a second type of CT. This
capillary5s function is to move the components of the fat stored in it into and out of
the fat cell, and to serve the needs of this tissue for nutrient and waste transport. 'nyoung animals, which haven5t had time to accumulate the %wear and tear% pigment
lipofuscin in their fat cells 0see &ercises @ and 8 for a discussion of lipofuscin1 the
presence of so many capillaries give the fat a pin"ish2white color. 'n albinos, animals
that lac" melanin, it5s the blood circulating in capillaries of the eye that5s the source
of its pin" color.
A fenestrated capillary has %holes%
in it the word comes fromfenestra,
the >atin for %window.% These holes
are actually pores, places where the
plasma membrane is perforated, toallow for the movement of bul"
materials from the capillary lumen
to the e&terior, and vice versa. This
type of capillary is found in places
where the rapid movement of
materials is vital to function, such as
in endocrine organs, where
movement of hormones into the
blood is carried out. enestrated
capillaries are also found in the
"idney, again a place where uic"transit between two compartments is
the design criterion.
't5s not possible to see the
fenestrations in a light microscope
preparation. To appreciate the
nature of these pores, an electron microscopic image is reuired, because they5re
below the level of the light microscope5s resolution. Such an image is provided here,
at about F6,666 diameters. This e&le is from the "idney, but similar capillary
profiles could be found anywhere fast movement of large molecules is important to
normal function. The capillaries of the glomerulus 0the tuft of blood vessels that fills
the renal corpuscle1 use hydrostatic pressure from the arterial supply to filter blood
plasma through the pores in the formation of urine 0for details, see &ercise @81. As
this image ma"es clear, the fenestrations are actual openings in the capillary wall.
The arrows show the direction of flow of materials, driven by the higher pressure in
the capillary lumen than in -owman5s space. *emember, the imageshown here is
two2dimensional, but the cellisn5t. The %brea"s% indicated are really more or less
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circular perforations in the plasma membrane, and they have depth they lie on both
sides of the plane of the section.
Sinusoids
There5s onemore category
of blood vessels
to be dealt
with. These are
somewhat li"e
capillaries, and
might be
considered as a
sub2set.
Sinusoidsare a
form of large,irregular
capillary2type
vessel they
have the thin,
intima2only
wall
construction of
a capillary and
may be
fenestrated.
Sinusoids arefound where
slow flow,
intimate contact between blood and tissue, and rapid e&change of materials are
reuired. $ou5ll find them on slide /@8 in the liver, between plates of hepatic cells.
Sinusoids are
flattened and
irregular in shape,
as the image at
right ma"es clear.
This scanning M
picture is a plastic
cast of the
sinusoids in the
placenta of a goat,
another place
where slow flow
and efficient
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transfer of materials is important to normal function. 'n this method the blood
spaces are filled with plastic the plastic is allowed to harden, and the tissue digested
away to leave an accurate impression of the shape of the filled space.