Veins
The walls of veins are thinner than the walls of arteries, while their
diameter is larger. In contrast to arteries,
the
layering in the wall of veins is not very distinct. The tunica intima is
very thin. Only the largest veins contain an appreciable amount of
subendothelial connective tissue.
Internal and external
elastic laminae are absent or very thin. The tunica media appears
thinner than the tunica adventitia, and the two layers tend to blend into each
other. The appearance of the wall of veins also depends on their location. The
walls of veins in the lower parts of the body are typically thicker than those
of the upper parts of the body, and the walls of veins which are embedded in
tissues that may provide some structural support are thinner than the walls of
unsupported veins.
Venous vessels originate from the capillary network which coalesce into the
smallest venous vessels the
Venules.
They are larger than capillaries. Small venules are surrounded by pericytes.
A few smooth muscle cells may surround larger venules. The venules merge to
form
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Small to
medium-sized veins
which contain bands
of smooth muscle in the tunica media. The tunica adventitia is well
developed. In some veins (e.g. the veins of the
pampiniform plexus in the spermatic cord) the tunica adventitia
contains longitudinally oriented bundles of smooth muscle.
Aside from most veins in the head and neck, small to medium-sized veins are
also characterised by the presence of valves.
The valves are formed by loose, pocket-shaped folds of the tunica intima,
which extend into the lumen of the vein. The opening of the pocket will point
into the direction of blood flow towards the heart. One to three (usually
two) pockets form the valve. Blood flowing towards heart will pass the
pockets. If the flow reverses, blood will fill the pockets which will occlude
the lumen of the vein and prevent the return of blood into the part of the
vein preceding the valve. The ability of the valves to prevent backflow
depends to some extent on the state of contraction (tone)
of the smooth muscle in the wall of the vein.
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The largest veins of the abdomen and thorax
do contain some subendothelial connective tissue in the tunica intima, but
both it and the tunica media are still comparatively thin. Collagen and elastic
fibres are present in the tunica media. The tunica adventitia is very wide, and
it usually contains bundles of
longitudinal
smooth muscle. The transition from the tunica adventitia to the surrounding
connective tissue is gradual. Valves are absent.
Vasa vasorum are more frequent in the walls of large
veins than in that of the corresponding arteries - probably because of the
lower oxygen tension in the blood contained within them.
sections of
veins - H&E, van
Gieson or elastin (in combination with other stains)
Like arteries, veins and venules are present in many
sections. The basic features of their structure are however more difficult to
identify because of the thin walls of veins - in particular in small veins. It
is best to resort to sections of large veins, which together with large muscular arteries often have their
"own section" in teaching collections.
Vein, human -
H&E
The tunica intima is very narrow and the internal elastic lamina is difficult
to identify - even in elastin stained sections. A few elastic fibres below
the endothelium form only a very thin and incomplete internal elastic lamina.
Smooth muscle is present in the tunica media, but it is organised less
regular than in the artery. The tunica media is, again as compared to the
artery, very thin and there is no sharp border between the tunica media and
the tunica adventitia. The tunica adventitia of the largest veins contains
coarse collagen fibres, elastic fibres and longitudinal bundles of smooth
muscle. Small and medium sized veins will not contain smooth muscle in the
tunica adventitia.
Draw part of the wall of the vein, label the tunics and
indicate the presence of smooth muscle, fibres and their types.
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sections
containing small to medium sized veins - H&E, van Gieson or trichrome
Unless a specifically prepared slide is available, I
would recommend looking at skin slides, in which I have found quite a few nice
valves in the veins located at the border between dermis and hypodermis.
Vein Valve -
H&E
Unless the section has been specifically prepared to illustrate valves, you
will have to search for a while - probably through several sections. If a
vein with valves is present in the section, it should be easy to identify.
Valves are only found in small to medium-sized veins. You should see one or
two bands of tissue in the lumen of the vein. Each band is formed by two
apposing layers of tunica intima. The bands may share their origin from the
inner aspect of the wall of the vein or they may have separate origins.
Folding of the tissue bands forming the valves is variable.
Draw the vein and valve and label the tunics which you can
identify. Try to indicate the position of the section in the schematic
drawing of the valve.
You can observe the function of vein valves. Put your
finger firmly on one of the veins which are visible on the back of your hand.
Use another finger to stroke over the vein in the direction of the wrist. The
vein will empty and not backfill (closed valves!) until you release the first
finger.
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![thumbnail]( "The walls of veins")
Title:
The walls of veins
by:
om
at
2013-02-15T03:17:00+07:00
Rating: 4.8
of 5 Reviews
The walls of veins