Lymphatic System
Chapter 14
Lymphatic System
• Consists of 2 semi-independent parts:
– Lymphatic vessels
• Transport any fluids back to the blood that have
escaped the blood’s vascular system
– Lymphoid tissues & organs
• House phagocytic cells & lymphocytes which aid in the
body’s defense mechanisms & resistance to disease
Lymphatic Vessels
• Also known as lymphatics
• An elaborate system of drainage vessels that
collect the excess protein-containing
interstitial fluid & return it to the bloodstream
• The fluid in the lymphatics is called lymph
Distribution & Structure of Lymphatic
Vessels
• Vessels form a one-way system that transports
lymph toward the heart
• Transport system begins in microscopic lymphatic
capillaries
– Weave between the tissue cells & blood capillaries in the
loose connective tissues of the body
– Widespread; absent from bones & teeth, bone marrow,
& entire CNS
Distribution & Structure of Lymphatic
Vessels
• Lymphatic capillaries are extremely permeable
• This is due to:
– Endothelial cells forming the walls aren’t tightly joined,
overlap each other loosely
– Collagen filaments anchor the endothelial cells to
surrounding structures
• Any increase in interstitial fluid volume opens the the
minivalves
Distribution & Structure of Lymphatic
Vessels
• From the lymphatic capillaries, lymph flows
through successively larger & thicker-walled
channels
• 1st collecting vessels
• 2nd lymphatic trunks
• 3rd the largest vessels - ducts
Distribution & Structure of Lymphatic
Vessels
• Lymphatic collecting vessels have the same 3
tunics as veins
• Are thinner-walled, have more valves, &
anastomose more
• Lymphatics travel along with superficial veins
in the skin
Distribution & Structure of Lymphatic
Vessels
• Lymphatic trunks are formed by the union of
the largest collecting vessels & drain fairly
large areas of the body
• Named mostly for the body regions from
which they collect lymph
– Paired lumbar, bronchomediastinal, subclavian,
jugular trunks
– Single intestinal trunk
Distribution & Structure of Lymphatic
Vessels
• Lymph is eventually delivered to one of two
large ducts in the thoracic region
• Right lymphatic duct
– Drains lymph from the right upper arm and the
right side of the head & thorax
• Thoracic duct
– Larger than the right lymphatic duct
– Receives lymph from the rest of the body
Lymphoid Cells
• Lymphocytes are made in red bone marrow
• Mature into one to the two main varieties of
immunocompetent cells
– T cells (T lymphocytes)
– B cells (B lymphocytes)
These protect the body from antigens - anything the
body perceives as foreign Ex) bacteria, viruses, cancer
cells
Lymphoid Cells
• Activated T cells:
– Manage the immune response
– Some directly attack & destroy infected cells
• B cells:
– Protect the body by producing plasma cells
– Daughter cells secrete antibodies into the blood
• Immobilize antigens until they can be destroyed by
phagocytes or other means
Lymphoid Cells
• Lymphoid macrophages:
– Phagocytizing foreign substances
– Activate T cells
Lymphoid Tissue
• Composed mostly of a type of loose connective
tissue called reticular connective tissue
• All of the lymphoid organs & cells are made of this
type of tissue EXCEPT the thymus
• Macrophages live on the fibers of the reticular
network
• Lymphocytes are in the spaces of the network
Lymphoid Tissue
• Lymphocytes squeeze through the postcapillary
venules to reside temporarily in the lymphoid tissue
• They then leave to patrol the body for antigens and
return once they are done
• This cycling of lymphocytes ensures that they reach
infected or damaged sites quickly
Lymphoid Tissue
• Comes in various “packages”
• 1) Diffuse lymphatic tissue – consists of a few
scattered reticular tissue elements
• 2) Lymphoid follicles – a solid, spherical bodies
consisting of tightly packed reticular elements &
cells
– Found forming part of larger lymphoid organs like lymph
nodes
Lymphoid Tissue
• 3) Germinal centers – follicular dendritic cells
& B cells reside here
– Provide a site for B cells to divide & produce
plasma cells
Lymph Nodes
• Principal lymphoid organs
• Filters lymph
• Large clusters occur near the body surface in
inguinal, axillary, & cervical regions
– Where lymphatic collecting vessels converge to
form trunks
Lymph Nodes
• 2 basic functions
– Lymph filters  macrophages remove & destroy
microorganisms & other debris
– Help activate the immune system  lymphocytes
monitor the lymphatic stream for the presence of
antigens & mount an attack against them
Other Lymphoid Organs
• Spleen, thymus, tonsils & Peyer’s patches of
the intestine
• Spleen
– Largest lymphoid organ (size of a first)
– Extracts aged & defective blood cells & platelets
– Macrophages remove debris & foreign matter
from blood flowing through it
Other Lymphoid Organs
• Thymus  primary functions during early life
– Secretes thymopoietin & thymosins causing T
lymphocytes to become immunocompetent
– Immunocompetent: function against certain
pathogens in the immune response
– Prominent in newborns & highly active in first year
of life
Thymus
• Differs from other lymphoid organs in 2 ways:
– Does not directly fight antigens
– The stroma of the thymus consists of epithelial
cells rather than reticular fibers
Other Lymphoid Organs
• Tonsils  simplest lymphoid organs, form a
ring of lymphatic tissue around the entrance
of the pharynx
• Palatine tonsils – located on either side at the
posterior end of the oral cavity
– Largest of the tonsils & ones most often infected
Types of Tonsils
• Lingual tonsils – at the base of the tongue
• Tubal tonsils – surround the openings of the
auditory tubes into the pharynx
– Gather & remove many of the pathogens entering the
pharynx in food or in inhaled air
• Pharyngeal tonsils – referred to as adenoids if
enlarged
– In the posterior wall of the nasopharynx
Other Lymphoid Organs
• Peyer’s patches
– Large isolated clusters of lymphoid follicles
– Located in the wall of the distal portion of the
small intestine
– Heavily concentrated in the wall of the appendix
Peyer’s Patches & Appendix
• In an ideal position to:
– 1) destroy bacteria in the intestine
– 2) generate many “memory” lymphocytes for
long-term immunity