Lymphatic System
Identify the lymphatic vessels and discuss how lymph is transported through these structures
● lymphatic vessels carry lymph fluid in a one-way flow pattern toward the heart
● Lymphatic capillaries
○ Lymph starts here (beginning of lymphatic pathway)
○ Formed by loosely overlapping simple squamous epithelial cells which overlay
like shingles on a roof
○ Cells are anchored by collagen filaments at one end
○ Highly permeable → ET cells pivot open and close to allow fluids and larger
proteins through
○ High pressure outside the capillary opens the flaps and allows fluid to enter, high
pressure inside the capillaries pushes flaps closed and keeps fluid in
○ Pressure against vessels wall helps create one way flow
○ Found everywhere except bones, teeth, and nervous system
○ Lacteals
■ Special lymphatic capillaries found in the villi of intestines
■ Lymph fluid within lacteals is called chyle
● Lymphatic collecting vessels
○ Lymph from lymphatic capillaries travels here next
○ Composed of three tunics (like a vein but thinner)
● Lymphatic trunks
○ Drain major areas of the body
○ Larger lymphatic vessels
○ Names based on their location
■ Bronchomediastinal: thoracic cavity
■ Subclavian: head
■ Jugular: neck and head
■ Lumbar: lower body, lower limbs
■ Intestinal: abdominal cavity
● Lymphatic ducts
○ Two of them → right and thoracic
○ Right
■ Drains top part of the body (head, upper limbs, upper thoracic cavity)
○ Thoracic
■ Found on left side of the body
■ Drains everything else (everything below upper thoracic cavity)
○ Lymph collected from the body is drained into the circulatory system, both ducts
drain into the jugular subclavian junction
○ Cisterna chyli = origin of the thoracic duct
Lymphatic capillaries → lymphatic collecting vessels → lymphatic trunks → lymphatic
ducts
● Factors assisting lymphatic flow
, ○ Valves: more than in veins, prevent backflow
○ Muscular pump: muscle contraction moves lymph by putting pressure on the
vessels, physical activity increases lymph flow
○ Respiratory pump: negative pressure caused by breathing pulls fluid towards the
thoracic cavity
○ Arterial pumping: blood pushing through arteries compresses lymphatic vessels
○ Smooth muscle contraction: peristalsis, wave like contractions of the smooth
muscle in lymphatic vessel walls, needs this extra assistance because pressure
in lymphatic vessels is so low
Differentiate the lymphoid cells
● Lymphocytes
○ Specific immunity, genetically programmed to only attack certain things
○ We have the ability to attack pathogens before ever encountering thing because
of our genetics
○ Immunocompetence = developmental process where lymphocytes gain the ability
to detect their specific antigen (gain receptors), don't truly become activated until
they encounter an antigen
○ B cells
■ Become immunocompetent in bone marrow
■ Produce antibodies
■ Antibodies tag foreign things to be destroyed by other parts of the
immune system
○ T cells
■ Become immunocompetent in the thymus
■ Have ability to directly attack foreign cells
● Macrophages
○ Nonspecific, phagocytic cell
○ Monocytes after diapedesis
○ Activate T-cells → T-cells destroy macrophages after they consume a foreign
substance
○ Macrophage eats foreign thing → hangs out → t-cells destroys both
(macrophage and foreign thing)
● dendritic cells
○ Nonspecific, phagocytic cell
○ Also activates t-cells
● Reticular cells
○ Does not interact with foreign things
○ Produce a stroma → fibrous network that support all the other cells, site of
lymphoid cell collection
○ Foreign substances are filtered out of lymph as they push through the stroma so
they cannot enter the bloodstream
● Lymphoid tissue
○ Proliferation site for lymphocytes
, ○ Allows detection of infection or damage, filters
○ Nodular tissue = reticular cells, stroma
○ Macrophages attach to the stroma, lymphocytes are in interstitial space so they
can move with the lymph and process it
○ Thymus is NOT reticular
List the lymphoid organs, discuss their functions, identify their histological makeup and compare
their modes of action
● Lymph nodes
○ Principle lymphatic organs
○ Thousands in the body
○ Cluster along lymphatic collecting vessels
○ Collection of nodular tissue = filter
○ Lymphatic cells in stroma of lymph nodes cleanse lymph as it passes through
○ Help control the spread of things in the body
○ Major collections of lymph nodes in the groin, back of neck, armpits → inflame
when we have an infection
○ Action
■ Lymph enters via afferent lymphatic vessels (several entry points)
■ Filters through sinuses/space
■ Exits through efferent lymphatic vessels at hilus
■ More afferent than efferent allows lymph to have processing time within
the lymph node
● Spleen
○ Largest lymphatic organ (about the size of your fist)
○ Accumulation of lymphocytes
○ Anything foreign found in the spleen can activate an immune response
○ Filters blood → Blood runs through the spleen and the spleen removes aging or
damaged blood cells, utilizes proliferation of macrophages
○ Liver recycles RBCs but components and platelets are stored in the spleen
○ Produces RBCs in embryo until bone marrow is formed enough to take over
○ Action
■ Blood delivered via splenic artery
■ Lymph filtered by lymphocytes in white pulp
■ Lymph filtered by macrophages in red pulp
■ Cleansed fluid returns to splenic vein and is back in circulation
■ ALL BLOOD PASSES THROUGH THE SPLEEN
● Thymus
○ Shrinks with age
○ Immunocompetent site for t-lymphocytes
○ Produces hormones that signal production of WBCs
○ Stuff we are exposed to when we are young sets t-lymphocytes up for later in life
○ Action
■ Most prominent in newborns and declines with age
Identify the lymphatic vessels and discuss how lymph is transported through these structures
● lymphatic vessels carry lymph fluid in a one-way flow pattern toward the heart
● Lymphatic capillaries
○ Lymph starts here (beginning of lymphatic pathway)
○ Formed by loosely overlapping simple squamous epithelial cells which overlay
like shingles on a roof
○ Cells are anchored by collagen filaments at one end
○ Highly permeable → ET cells pivot open and close to allow fluids and larger
proteins through
○ High pressure outside the capillary opens the flaps and allows fluid to enter, high
pressure inside the capillaries pushes flaps closed and keeps fluid in
○ Pressure against vessels wall helps create one way flow
○ Found everywhere except bones, teeth, and nervous system
○ Lacteals
■ Special lymphatic capillaries found in the villi of intestines
■ Lymph fluid within lacteals is called chyle
● Lymphatic collecting vessels
○ Lymph from lymphatic capillaries travels here next
○ Composed of three tunics (like a vein but thinner)
● Lymphatic trunks
○ Drain major areas of the body
○ Larger lymphatic vessels
○ Names based on their location
■ Bronchomediastinal: thoracic cavity
■ Subclavian: head
■ Jugular: neck and head
■ Lumbar: lower body, lower limbs
■ Intestinal: abdominal cavity
● Lymphatic ducts
○ Two of them → right and thoracic
○ Right
■ Drains top part of the body (head, upper limbs, upper thoracic cavity)
○ Thoracic
■ Found on left side of the body
■ Drains everything else (everything below upper thoracic cavity)
○ Lymph collected from the body is drained into the circulatory system, both ducts
drain into the jugular subclavian junction
○ Cisterna chyli = origin of the thoracic duct
Lymphatic capillaries → lymphatic collecting vessels → lymphatic trunks → lymphatic
ducts
● Factors assisting lymphatic flow
, ○ Valves: more than in veins, prevent backflow
○ Muscular pump: muscle contraction moves lymph by putting pressure on the
vessels, physical activity increases lymph flow
○ Respiratory pump: negative pressure caused by breathing pulls fluid towards the
thoracic cavity
○ Arterial pumping: blood pushing through arteries compresses lymphatic vessels
○ Smooth muscle contraction: peristalsis, wave like contractions of the smooth
muscle in lymphatic vessel walls, needs this extra assistance because pressure
in lymphatic vessels is so low
Differentiate the lymphoid cells
● Lymphocytes
○ Specific immunity, genetically programmed to only attack certain things
○ We have the ability to attack pathogens before ever encountering thing because
of our genetics
○ Immunocompetence = developmental process where lymphocytes gain the ability
to detect their specific antigen (gain receptors), don't truly become activated until
they encounter an antigen
○ B cells
■ Become immunocompetent in bone marrow
■ Produce antibodies
■ Antibodies tag foreign things to be destroyed by other parts of the
immune system
○ T cells
■ Become immunocompetent in the thymus
■ Have ability to directly attack foreign cells
● Macrophages
○ Nonspecific, phagocytic cell
○ Monocytes after diapedesis
○ Activate T-cells → T-cells destroy macrophages after they consume a foreign
substance
○ Macrophage eats foreign thing → hangs out → t-cells destroys both
(macrophage and foreign thing)
● dendritic cells
○ Nonspecific, phagocytic cell
○ Also activates t-cells
● Reticular cells
○ Does not interact with foreign things
○ Produce a stroma → fibrous network that support all the other cells, site of
lymphoid cell collection
○ Foreign substances are filtered out of lymph as they push through the stroma so
they cannot enter the bloodstream
● Lymphoid tissue
○ Proliferation site for lymphocytes
, ○ Allows detection of infection or damage, filters
○ Nodular tissue = reticular cells, stroma
○ Macrophages attach to the stroma, lymphocytes are in interstitial space so they
can move with the lymph and process it
○ Thymus is NOT reticular
List the lymphoid organs, discuss their functions, identify their histological makeup and compare
their modes of action
● Lymph nodes
○ Principle lymphatic organs
○ Thousands in the body
○ Cluster along lymphatic collecting vessels
○ Collection of nodular tissue = filter
○ Lymphatic cells in stroma of lymph nodes cleanse lymph as it passes through
○ Help control the spread of things in the body
○ Major collections of lymph nodes in the groin, back of neck, armpits → inflame
when we have an infection
○ Action
■ Lymph enters via afferent lymphatic vessels (several entry points)
■ Filters through sinuses/space
■ Exits through efferent lymphatic vessels at hilus
■ More afferent than efferent allows lymph to have processing time within
the lymph node
● Spleen
○ Largest lymphatic organ (about the size of your fist)
○ Accumulation of lymphocytes
○ Anything foreign found in the spleen can activate an immune response
○ Filters blood → Blood runs through the spleen and the spleen removes aging or
damaged blood cells, utilizes proliferation of macrophages
○ Liver recycles RBCs but components and platelets are stored in the spleen
○ Produces RBCs in embryo until bone marrow is formed enough to take over
○ Action
■ Blood delivered via splenic artery
■ Lymph filtered by lymphocytes in white pulp
■ Lymph filtered by macrophages in red pulp
■ Cleansed fluid returns to splenic vein and is back in circulation
■ ALL BLOOD PASSES THROUGH THE SPLEEN
● Thymus
○ Shrinks with age
○ Immunocompetent site for t-lymphocytes
○ Produces hormones that signal production of WBCs
○ Stuff we are exposed to when we are young sets t-lymphocytes up for later in life
○ Action
■ Most prominent in newborns and declines with age
