Biomedical Engineering Reference
In-Depth Information
2.3
Lymph Nodes
The lymphatic system is comprised of a series of capillaries, ducts, nodes and
organs including the spleen and thymus, and is involved in the production and
release of lymphocytes into the blood. Lymphatic capillaries lie in interstitial
spaces along with blood capillaries. They collect excess interstitial fluid, extravasated
protein and lipids absorbed from the digestive tract to form a creamy white fluid
that flows from peripheral capillaries into lymphatic ducts that finally drain into the
thoracic lymph duct and the right lymph duct at the base of the neck. From here,
lymph fluid drains directly into the systemic circulation.
The main difference between blood and lymph capillaries is that while blood
capillaries have tight junctions between endothelial cells and a fibrous basement
membrane that limits the passage of macromolecules and particulate nanomedici-
ness, lymphatic capillaries have wide intercellular junctions and a loose basement
membrane. Blood flow is approximately 100-1000 times faster than that of lymph,
so uptake of small molecules from the interstitium is preferentially via the blood,
whereas the uptake of macromolecules is via the lymph. Prior to emptying into the
systemic circulation, lymph fluid filters through lymph nodes placed at strategic
positions to remove foreign particles preventing their access to the systemic circula-
tion. The lymph nodes are therefore the main sites of deposition for nanomedicines
that are absorbed from the interstitium, either after subcutaneous administration or
after extravasation from the blood stream.
Lymph nodes are located at the end of afferent lymph capillaries that enable
lymph flow in one direction via valves that are positioned along the entire length of
the capillary system (Fig.
5
). Lymph nodes contain an outer subcapsular sinus with
Afferent lymph
vessel
Trabeculae
Macrophage
Lymphocyte
Subcapsular
sinus
Medulla
Medullary
sinus
Cortical
sinus
Capsule
Efferent lymph vessel
Fig. 5
Diagram of the microstructure of a lymph node
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