Lymph consists of cells, mainly lymphocytes, and
plasma.
The lymphatic system collects tissue fluid and conveys
it to the bloodstream. It comprises the lymph capillaries and
vessels, the lymph nodes, and aggregations of lymph tissue in
the spleen and thymus and around the alimentary tract.
The system forms an extensive network over the body, although its fine vessels
are not easily identified.The lymph capillaries are larger than those of the
blood;
they are composed of a single layer of endothelial cells. The lymph vessels resemble
veins and possess many paired valves.
The larger collecting vessels open into the venous
system near the formation of the brachiocephalic veins.A lymph node is an aggregation of lymph tissue
along the course of a lymph vessel. It is bean-shaped, with a number of
afferent vessels entering its convex surface and an efferent vessel leaving its
hilus. It is
surrounded by a fibrous capsule from which fibrous
trabeculae pass inwards. It is filled with a reticular network of fine collagen
fibres, and the cells are either primitive or fixed macrophages. Numerous
lymphocytes and a few monocytes lie freely within the meshwork, but they are absent
peripherally, leaving a subcapsular lymph space. The cells of the outer part of
the node are densely packed and known as germinal centres. The centre of the
follicle and the hilar regions of the
node contain loosely packed lymphocytes.Lymph aggregations elsewhere in the
body consist of a
mixture of follicles and loosely packed
lymphocytes.Bacterial infections produce inflammatory responses in the
regional lymph nodes. In many malignant diseases
neoplastic cells spread via the lymph vessels to the regional lymph nodes,and
there develop to such an extent as to completely replace
the normal tissue of the lymph node and occlude lymph
flow.The stagnation of lymph within the tissues due to obstruction of flow
produces a swelling of the tissues known as lymphoedema. Lymphoedema may also
occur in
subjects who are born with a defective lymphatic
system, this
being termed primary lymphoedema; acquired obstruction
is called secondary lymphoedema. The
term lymphadenopathy is used to describe a generalized enlargement of the lymph
nodes, although they are not glands in the strict
definition of the term.
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