Additional Specialisations of Vessels
Small arteries and veins often form anastomosing networks, which provides
routes for alternative blood supply and drainage if one of the vessels should
become occluded because of pathological or normal physiological circumstances.
Some arteries are however the only supply of blood to their target tissues.
These arteries are call
end arteries. Tissues
which are supplied by end arteries die if the arteries become occluded.
The segments of the kidney and the heads of the
gastrocnemius muscle are examples of tissues supplied by end arteries.
Arteries and veins may also form
arteriovenous shunts,
which can shunt the blood flow that otherwise would enter the capillary network
between the vessels. These shunts usually contain specialisations of the smooth
muscle in the region of the shunt. Arteriovenous shunts are frequently seen in
the blood supply of distal parts of the limbs and the nose (thermoregulation)
and in the blood supply of endocrine organs.
Lymphatic Vessels
Parts of the blood plasma will exude from the blood vessels into the
surrounding tissues because of transport across the endothelium or because of
blood pressure and the fenestration of some capillaries
(this
process is partly counteracted by the higher osmotic pressure of the blood).
The fluid entering tissues from capillaries adds to the interstitial fluid
normally found in the tissue. The surplus of liquid needs to be returned to the
circulation. Lymph vessels are dedicated to this
unidirectional
flow of liquid, the
lymph. Three types of
lymph vessels can be distinguished based on their size and morphology.
Lymph capillaries
are somewhat larger than blood capillaries and very
irregularly shaped. They begin as blind-ending tubes in connective tissue. The
basal lamina is almost completely absent and the endothelial cells do not form
tight junctions, which facilitates the entry of liquids into the lymph
capillary. Temporary openings in the endothelial lining of the lymph
capillaries also allow the entry of larger particles into the lymph capillaries
(lipid droplets, which are absorbed from the lumen of the
gut do not enter blood capillaries, but enter the circulation via lymph vessels
which are found in the villi of the ileum and jejunum). Lymph
capillaries merge to form
Lymph collecting vessels
which are larger and form valves
but otherwise appear similar to lymph capillaries. The lymph is moved by the
compression of the lymph vessels by surrounding tissues. The direction of lymph
flow is determined by the valves. Lymph vessels empty intermittently into lymph nodes from which the lymph continues in efferent
lymph vessels.
Only very little lymph is
returned from the limbs if they are immobilized, which illustrates the
importance of muscular action in lymph transport. This is also the reason for
immobilizing limbs that are either infected or that have been bitten by
venomous Australians. The effect can also be observed after long intercontinental
flight when you may feel that your shoes and socks are just about one number
too small. Finally, impeded lymph drainage is one of the problems associated
with surgery which requires the removal of lymph nodes and which thereby
interrupts the lymph collecting vessels.
Eventually the lymph collecting vessels merge to
form
Lymph ducts
which contain one or two
layers of smooth muscle cells in their wall (some
textbooks call this layer the tunica media of lymph vessels). They also
form valves. The walls of the lymph ducts are less flexible in the region of
the attachment of the valves to the wall of the duct, which may give a beaded
appearance to the lymph ducts. Peristaltic contractions of the smooth muscle
contribute to the movement of lymph towards the heart in addition to the
compression of the ducts by surrounding tissues.
The largest lymph duct of the
body, the thoracic duct, drains lymph from the
lower half and upper left quadrant of the body and empties the lymph into the
circulation by merging with the vascular system close to the junction of the
left internal jugular and subclavian veins. That it is the largest lymph duct
does not mean that it is a large vessel when compared to the large arteries and
veins. It actually is not much larger (about 5mm in diameter) than one of the
superficial forearm veins.
Sections of
small intestine - H&E
Jejunum,
baboon - H&E
At low magnification you will see villi extending into the lumen of the
jejunum. Find villi which are cut longitudinally and change to medium
magnification. In some of the villi you will see fairly large open spaces,
which are surrounded by a layer of flattened endothelial cells. They should
not contain any red blood cells - if they do you are looking at a capillary,
which also should be somewhat smaller. These openings represent the blind end
of lymph capillaries which originate in the villi.
Their name, lacteals, is derived from the milky appearance of the lymph. This
appearance is caused by suspended lipid droplets which enter these lymph
capillaries.
Draw a villus containing a lymph capillary. Focus on the
features of the lymph capillary and the surrounding connective tissue.
Include, if possible, a blood capillary in your drawing.
The jejunum slide
is also good for revision. You should be able to find columnar epithelium,
goblet cells, smooth muscle, small nerves, a few ganglion cells and, of course,
lots of loose connective tissue and blood vessels.
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Title:
Lymphatic Vessels
by:
om
at
2013-02-15T03:17:00+07:00
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Lymphatic Vessels