The lymphatic circulation as a drainage system
The lymphatic system has neither a heart nor arteries.
Its microscopic dead-end capillaries extend into most tissues, paralleling the blood
The lymphatic circulation is a drainage system. Its job
in maintaining fluid balance is to:
||collect excess interstitial
fluid and return it to the blood (approximately 3 litres daily).
||return plasma proteins to the
Once interstitial fluid enters a lymph capillary, it is
referred to as lymph.
The three main types of lymphatic vessels are lymph
capillaries, lymphatics, and lymph ducts.
Lymph capillaries are microscopic tubes located between
cells. Lymph capillaries resemble blood capillaries somewhat, but differ in important
ways. Whereas a blood capillary has an arterial and a venous end, a lymph capillary has no
arterial end. Instead, each lymph capillary originates as a closed tube. Lymph capillaries
also have a larger and more irregular lumen (inner space) than blood capillaries and are
The wall of a lymph capillary is constructed of
endothelial cells that overlap one another.
When fluid outside the capillary pushes against the
overlapping cells, they swing slightly inward--like a swinging door that moves in only one
direction. Fluid inside the capillary cannot flow out through these openings.
animation of a section through a lymph capillary shows how pressure in the interstitial
fluid surrounding the capillary pushes open the overlapping cells.
The arrows represent the direction
of flow of the lymph.
Note the internal valve which
allows the lymph to flow in one direction only.
Lymph capillaries branch and interconnect
freely and extend into almost all tissues of the body except the CNS (Central Nervous
System) and the avascular tissues such as the epidermis and the cartilage.
Lymph capillaries join to form larger vessels called
lymphatics or lymph veins. These resemble blood-conducting veins but have thinner walls
and relatively larger lumen, and they have more valves. In the skin, lymphatics are
located in subcutaneous tissue and follow same paths as veins. In the viscera, lymphatics
generally follow arteries and form plexuses (networks) around them.
At certain locations lymphatics enter lymph nodes. These
are structures that consist of lymphatic tissue.
As the lymph flows slowly through the lymph sinuses
within the tissue of the lymph node, it is filtered. Macrophages remove bacteria and other
foreign matter as well as debris.
Lymphocytes are added to the lymph as it flows through
the sinuses of a lymph node. Thus the lymph leaving the node is both cleaner of debris and
richer in lymphocytes. Lymphatics leaving lymph nodes are called efferent lymph vessels
and conduct lymph toward the shoulder region. Large lymphatics that drain groups of lymph
nodes are often called lymph trunks.
Lymphatics from the lower portion of the body converge to
form a dilated lymph vessel, the cisterna chyli, in the lumbar region of the abdominal
cavity. The cisterna chyli extends for about 6 centimetres just to the right of the
abdominal aorta. At the level of the twelfth thoracic vertebra, the cisterna chyli narrows
and becomes the thoracic duct.
Lymphatic vessels from all over the body, except the
upper right quadrant, drain into the thoracic duct. This vessel delivers the lymph into
the base of the left subcIavian vein at the junction of the left subcIavian and internal
jugular veins. In this way lymph is continuously emptied into the blood where it mixes
with the plasma. At the junction of the thoracic duct and the venous system, a valve
prevents blood from flowing backward into the duct.
Only about 1 centimetre in length, the right lymphatic
duct receives lymph from the lymphatic vessels in the upper right quadrant of the body.
The right lymphatic duct empties lymph into the base of the right subclavian vein (at the
point where it unites with the internal jugular vein to form the brachiocephalic)
An example of the pattern of lymph circulation is:
Lymph capillaries lymphatic lymph
node lymphatic cisterna
chyli thoracic duct
Lymphatic vessels and lymph nodes can be visualised by
the process of lymphangiography. A radiopaque (not transparent to x-rays) contrast
material is injected into the a lymphatic vessel. This will show up the vessel and
itís connections to other lymph vessels. The fluid is left in the system for 24 hours
and the lymph nodes can then be observed by X-rays. This technique is quite important in
the treatment of neoplasms and other disorders of the lymphatic system. The technique is
also used to locate lymph nodes for radiation therapy or for surgical removal.