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Functions of the Lymphatic
System
• Fluid balance
– Excess interstitial fluid enters lymphatic
capillaries and becomes lymph.
• Fat absorption
– Absorption of fat and other substances
from digestive tract via lacteals.
• Defense
– Microorganisms and other foreign
substances are filtered from lymph by
lymph nodes and from blood by spleen
Tissue Fluid
• Fluid between cells
– Formation – pressure forces water out of the
capillaries and into the interstitial space. 90%
gets reabsorbed by capillaries. 10% remains.
– Composition – water, amino acids, sugars,
fatty acids, hormones, neurotransmitters, salts
and wastes
Lymph
• Formation – tissue fluid
• Composition
– Water plus solutes from two sources
– Plasma: ions, nutrients, gases, some proteins
– Cells: hormones, enzymes, waste products
• Function –
– Returns to circulatory system via veins;
essential for fluid balance.
Lymphatic Vessels
Carry lymph away from tissues
• Lymphatic capillaries
– More permeable than blood capillaries
– Epithelium functions as series of one-way valves
Lymphatic Vessels
• Lymphatic capillaries join to form lymphatic
vessels
• Lymphatic vessels: have valves that ensure
one-way flow
• Lymph nodes: distributed along vessels and
filter lymph
• Lymphatic trunks: jugular, subclavian,
bronchomediastinal, intestinal, lumbar
• Lymphatic collecting ducts: drain tissues of
body and move lymph into major veins
Right and Left Subclavian Veins
– Right lymphatic duct: drains right side of
head, right-upper limb, right thorax
– Thoracic duct: drains remainder of the body
Flow of Lymph
• Muscular Contraction
– Skeletal muscles contract and compress the
lymphatic vessels.
– Lymph is propelled forward
– Backflow is prevented by one-way valves
• Breathing
– Inspiration – decreased pressure pulls lymph into
vessels
– Expiration – increased pressure compresses vessels
and propels lymph forward.
Lymph Nodes
• Structure – Bean shaped up to 25 cm long
• Organized into cortex and medulla with
dense connective tissue capsule
surrounding.
• Function - Only structures to filter lymph
– Substances removed by phagocytosis or they
stimulate lymphocytes to proliferate in germinal
centers.
Thymus
• Located in superior mediastinum
• grows rapidly during first year, decreases in size after
puberty.
• Site of maturation of T cells: many T cells produced here,
but most degenerate. Those that remain can react to
foreign substances, but not to healthy body tissue.
Spleen
• Located in left superior side of abdomen
• Can be ruptured in traumatic abdominal
injuries resulting in bleeding, shock,
death
• Functions
– Destroys defective RBCs
– Detects and responds to foreign
substances
– Limited reservoir for blood
Other Lymphatic Tissues
• Tonsils
• Appendix
• Peyer’s Patches
Non-Specific Defenses
• Species Resistance – a virus that affects
one species usually does not affect a
different species
– FIV – Feline aids does not affect humans
– Canine parvovirus affects dogs but not cats or
humans
– Bird flu – possible exception but only under
specific conditions that make it unlikely
Non-Specific Defenses
• Mechanical barriers –
– Skin, mucous membranes
• Chemical barriers
– Oil glands, sweat glands, stomach acid
• Interferon
– Produced by infected cells to warn
neighboring cells of an invader
• Phagocytosis – “cell eating”
– Neutrophils, macrophages engulf bacteria
Inflammation
• Description
– Response initiated by chemical mediators that
produce vasodilation and increased vascular
permeability.
• Causes
– Tissue injury by trauma or pathogens
Inflammation
• Stages
– Increased blood flow - leads to swelling
(pain), heat and redness
– Brings WBC’s to injury site to destroy
pathogens
– Brings clotting factors to injury site
– Brings oxygen and nutrients
• Function
– Prevents spread of pathogens
– Disposes of cell debris
– Sets stage for repair
Origin of Lymphocytes
• T Lymphocytes
– Red Bone Marrow but migrate to the Thymus
gland to mature
• B Lymphocytes
– Red Bone Marrow
• Antigens
– Markers on the invaders that identify them as
foreign
Functions of Lymphocytes
• Cell Mediated Immunity ( T Lymphocytes)
– Lymphokines - are produced by T cells
to direct the immune system response
by signaling between its cells.
Lymphokines attract other immune cells,
like macrophages and other
lymphocytes, to an infected site and to
help attack the invaders.
Functions of Lymphocytes
• Antibody Mediated Immunity (B Lymphocytes)
– Immunoglobulins (antibodies)
– proteins secreted by activated B Cells
• Are specific to the antigen they immobilize
Activation of B Lymphocytes
• Helper T Cell presents piece of pathogen
to B Lymphocyte
• One B Cell will recognize the antigen and
clone itself over and over (plasma cells)
• After infection has been defeated some
plasma cells will stay in the body to jump
start new attack if same pathogens tries to
invade again.
Activation of T Lymphocytes
• Cytotoxic (Killer) T Cell
– Recognizes an infected cell by the foreign
antigens and binds to it
– Injects a cocktail of lethal enzymes that
destroys the invader and the entire cell.
Activation of T Lymphocytes
• Suppressor T Cell
– These cells are
• involved in closing down immune responses after
they have successfully tackled invading organisms.
• involved in keeping in check immune responses
that may potentially attack one's own tissues
(autoimmunity). Keeps Killer T Cells in check.
Ways to Acquire
Adaptive Immunity