LYMPHATIC SYSTEM
| Lymphatic
vessels form an extensive and complex interconnected network of
channels, which begin as "porous" blind-ended lymphatic capillaries in
tissues of the body and converge to form a number of larger vessels,
which ultimately connect with large veins in the root of the neck.
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| Lymphatic
vessels mainly collect fluid lost from vascular capillary beds during
nutrient exchange processes and deliver it back to the venous side of
the vascular system (Fig. 1.29).
Also included in this interstitial fluid that drains into the lymphatic
capillaries are pathogens, cells of the lymphocytic system, cell
products (such as hormones), and cell debris.
|
In
the small intestine, certain fats absorbed and processed by the
intestinal epithelium are packaged into protein-coated lipid droplets (
chylomicrons),
which are released from the epithelial cells and enter the interstitial
compartment. Together with other components of the interstitial fluid,
the chylomicrons drain into lymphatic capillaries (known as
lacteals
in the small intestine) and are ultimately delivered to the venous
system in the neck. The lymphatic system is therefore also a major route
of transport for fat absorbed by the gut.
The fluid in most lymphatic vessels is clear and colorless and is known as
lymph.
That carried by lymphatic vessels from the small intestine is opaque
and milky because of the presence of chylomicrons and is termed
chyle.
| There
are lymphatic vessels in most areas of the body except the brain, bone
marrow, and avascular tissues such as epithelia and cartilage.
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| The
movement of lymph through the lymphatic vessels is generated mainly by
the indirect action of adjacent structures, particularly by contraction
of skeletal muscles and pulses in arteries. Unidirectional flow is
maintained by the presence of valves in the vessels.
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| Lymph
nodes are small (0.1-2.5 cm long) encapsulated structures that
interrupt the course of lymphatic vessels and contain elements of the
body's defense system, such as clusters of lymphocytes and macrophages.
They act as elaborate filters that trap and phagocytose particulate
matter in the lymph that percolates through them. In addition, they
detect and defend against foreign antigens that are also carried in the
lymph.
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| Because
lymph nodes are efficient filters and flow through them is slow, cells
that metastasize from (migrate away from) primary tumors and enter
lymphatic vessels often lodge and grow as secondary tumors in lymph
nodes. Lymph nodes that drain regions that are infected or contain other
forms of disease can enlarge or undergo certain physical changes, such
as becoming "hard" or "tender." These changes can be used by clinicians
to detect pathologic changes or to track spread of disease.
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| A number of regions in the body are associated with clusters or a particular abundance of lymph nodes (Fig. 1.30).
Not surprisingly, nodes in many of these regions drain the body's
surface, the digestive system, or the respiratory system. All three of
these areas are high-risk sites for the entry of foreign pathogens.
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| Lymph
nodes are abundant and accessible to palpation in the axilla, the groin
and femoral region, and the neck. Deep sites that are not palpable
include those associated with the trachea and bronchi in the thorax, and
with the aorta and its branches in the abdomen.
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Lymphatic trunks and ducts
a
All
lymphatic vessels coalesce to form larger trunks or ducts, which drain
into the venous system at sites in the neck where the internal jugular
veins join the subclavian veins to form the brachiocephalic veins (Fig. 1.31):
- Lymph from the right side of the head and neck, the right upper
limb, right side of the thorax, and right side of the upper and more
superficial region of the abdominal wall is carried by lymphatic vessels
that connect with veins on the right side of the neck;
- lymph from all other regions of the body is carried by lymphatic vessels that drain into veins on the left side of the neck.
| Specific
information about the organization of the lymphatic system in each
region of the body is discussed in the appropriate chapter.
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| Lymph nodes are
efficient filters and have an internal honeycomb of reticular connective
tissue filled with lymphocytes. These lymphocytes act on bacteria,
viruses, and other bodily cells to destroy them. Lymph nodes tend to
drain specific areas, and if infection occurs within a drainage area,
the lymph node will become active. The rapid cell turnover and
production of local inflammatory mediators may cause the node to enlarge
and become tender. Similarly, in patients with malignancy the
lymphatics may drain metastasizing cells to the lymph tender. Similarly,
in patients with malignancy the lymphatics may drain metastasizing
cells to the lymph nodes. These can become enlarged and inflamed and
will need to be removed if clinically symptomatic. |
Lymph nodes may
become diffusely enlarged in certain systemic illnesses (for example,
viral infection), or local groups may become enlarged with primary lymph
node malignancies, such as lymphoma (Fig. 1.32).
Figure 1.32 A. This computed tomogram with contrast, in the axial
plane, demonstrates the normal common carotid arteries and internal
jugular veins with numerous other nonenhancing nodules that represent
lymph nodes in a patient with lymphoma. B. This computed tomogram
with contrast, in the axial plane, demonstrates a large anterior
soft-tissue mediastinal mass that represents a lymphoma.
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