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Chapter 22
The lymphatic system
The lymphatic system
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Consists of a network of tissues, vessels and organs that help
maintain body fluid balance.
Closely tied to the immune system in protecting the body
against foreign organisms.
Organs and tissues of the lymphatic system:
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Lymph
Lymphatic vessels
Lymphatic cells
Lymph nodes
Spleen
Tonsils
Peyer’s patches
Thymus
The lymphatic system
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Functions:
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(a). fluid recovery: reabsorbs 2-4 l/day; prevents edema.
(b). Immunity: lymph nodes and lymph organs monitor
lymph and blood for pathogens and cancer cells.
(c). lipid absorption through the lacteals in small intestine.
Distribution of lymph
nodes in body.
Lymph flows toward
the heart with the aid
of small valves
Lymphatic fluid
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How Formed: produced by filtration of plasma through
the capillaries and is based on hydrostatic and osmotic
pressures in capillaries and interstitial space.
Pathway of Lymph Flow: Flow is always toward
the heart. Begins in dead ended lymphatic capillaries
that occur everywhere capillaries exist (except: brain,
teeth, bones and bone marrow).
Lymphatic capillaries
Lymphatic capillaries
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lymph capillaries –smallest of lymph vessels; are
permeable to lymph fluid and proteins. Lymph
capillaries drain into lymph collecting vessels.
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lacteals: - highly specialized lymphatic capillaries
located in intestinal mucosa - transport absorbed fat
from intestines and empty into great thoracic duct
Lymphatic collecting vessels
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Lymphatic collecting vessels contain valves that act as
smooth muscle pumps to move lymph toward heart.
Travel with veins in superficial tissues/arteries in
deeper tissues.
Have the same three tunics as blood vessels but walls
are much thinner and lymph pressure is very low.
They also have lymph nodes scattered along their
length to filter the lymph.
Lymphatic cells
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Natural killer cells (NK cells) large lymphocytes that attack
and lyse bacteria, foreign tissue cells and infected host cells.
T lymphocytes mature in the thymus (4 types Tc, Ts, Th, Tm).
B lymphocytes mature in red bone marrow and produce
antibodies when activated.
Macrophages develop from monocytes and are phagocytic.
Dendritic cells (branched macrophages) found in skin.
Reticular cells branched cells in stroma of lymphatic organs.
Lymph nodes
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Bean shaped organs that cleanse the lymph of
pathogens
~500 lymph nodes in the body ~ 1mm to 25mm.
Node is surrounded by a fibrous capsule from which
trabeculae extend inward to split the node into
compartments called sinuses.
Lymph enters node via afferent lymphatic vessel and
exits through efferent lymphatic vessel back into
collecting vessels.
As lymph percolates thru the node macrophages
cleanse the lymph of all pathogens.
Lymph nodes
Lymph Trunk
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Formed by the convergence of several lymph collecting
vessels that drain large areas of the body.
Six major trunks in body:
Lumbar trunks- lymph from lower limb, pelvic region and
anterior abdominal wall.
Intestinal trunk- lymph from stomach, intestines and other
digestive organs (chyle).
Bronchomediastinal trunk- lymph from thoracic viscera
Subclavian trunks- lymph from upper limbs, inferior neck
and superior thoracic wall.
Jugular trunks- lymph from head and neck.
Intercostal trunk- lymph from intercostal region
Lymphatic ducts
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The lymphatic trunks drain into the largest lymphatic
vessels: the lymphatic ducts. Most people (80%) have
2 lymphatic ducts: (1) Thoracic duct (present in all
people) and (2) the Right lymphatic duct.
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The thoracic duct empties into the venous circulation
at the junction of the internal jugular and left
subclavian veins. Lymph is then incorporated into the
circulating blood, eventually cleansed of foreign and
dead matter and excreted via the liver.
Thoracic duct and
right lymphatic duct
Thoracic duct drains
right side of face and
most of left thorax and
lower body.
Right lymphatic drains
left side of head and
upper body.
Lymphoid tissue
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The most important tissue of the immune system as it is
responsible for the production and activation of the “B” and
“T” lymphocytes. Found in two primary sites:
(1) In the infected mucous membranes of the digestive,
respiratory, urinary and reproductive tracts. This tissue is
called MALT (mucosa associated lymphoid tissue).
(2) In all lymphoid organs except the thymus.
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Lymphoid tissue is a framework of reticular fibers and reticular cells
in a network of “B” and “T” lymphocytes as well as macrophages.
This network consists of a network of lymphoid follicles with
germinal centers of dividing lymphocytes
Lymphoid organs
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Primary lymphoid organs are thymus and bone
marrow
Secondary lymphoid organs include the lymph nodes,
spleen, tonsils, aggregated lymphoid nodules in the
intestine and appendix.
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The thymus and bone marrow produce “T” and “B”
lymphocytes and the secondary organs serve to collect and
destroy the infectious microorganisms where they are
removed by the liver and spleen.
Red bone marrow
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Second largest organ in the body next to skin.
Two types red and yellow; red actively produces all
blood cells; yellow is dormant and produces blood
cells in extreme emergencies.
Composed of a network of reticular fibers forming
cave like structures with reticular cells lining the walls.
Within the caves are blood cells in various stages of
development and fat cells.
Capillaries run throughout the network and carry the
developed blood cells into the circulation.
Red marrow
Spleen
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Located in the upper left quadrant of abdominal cavity
with a gastric and renal area.
Functions to remove blood borne antigens and aged
or defective blood cells.
It is a site for hematopoiesis in fetus and storage of
blood throughout life.
Consists of red and white pulp. (seen in fresh tissue).
Red pulp is sinuses engorged with rbc’s.
 White pulp is lymphocytes and macrophages collected like
sleeves along the splenic artery
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Spleen structure
Spleen anatomy & histology
Tonsils
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Located in the oropharynx, they trap bacteria and
foreign pathogens entering the mouth and nose.
Palatine tonsils – either side of and posterior end of
oral cavity
Lingual tonsils – base of tongue
Pharyngeal tonsils (adenoids)– posterior wall of
nasopharynx
Tonsils
Peyer’s patches
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Defined: large isolated clusters of lymphatic nodules
found intermingled around the ileum of the small
intestines.
Function: destroy bacteria and generates memory
lymphocytes for long term immunity against infection.
Thymus
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Site where immature lymphocytes develop into “T”
lymphocytes
Secretes thymosin and thymopoietin which stimulate
“T” cells to become immunocompetent.
Most prominent in childhood and by age 40 is ~ 5%
active.
Thymus structure & histology