CHAPTER 20-THE LYMPHATIC SYSTEM
I. FUNCTIONS OF THE LYMPHATIC SYSTEM
A. Draining Excess Interstitial Fluid
B. Transporting dietary lipids
1. Lymphatic vessels transport lipids and lipid-soluble vitamins (A, D, E, and K) absorbed in
the gastrointestinal tract throughout the body.
C. Immune responses
II. THE LYMPHATIC SYSTEM
A. This system is composed of two primary parts:
1. Lymphatic Vessels which transport fluids back to the blood that have escaped from the blood.
2. Lymphatic Organs which are scattered throughout the body.
III. TYPES OF LYMPHOID CELLS
A. Lymphocytes-serve as the primary cells of the immune system.
1. These develop in red bone marrow and mature into one of two types of immunocompetent cells:
a. T Cells (T Lymphocytes)-these manage immune responses and some directly attack
and destroy foreign cells.
b. B Cells (B Lymphocytes)-protect the body by producing plasma cells. Plasma cells secrete
antibodies into the blood. Antibodies attach to and immobilize antigens until they can be
destroyed.
B. Lymphatic Macrophages-protect the body by phagocytizing foreign substances and by activating T Cells.
C. Dendritic Cells-are also phagocytes on foreign substances.
D. Reticular Cells-produce a network of protein fibers that support cells in the lymphoid organs.
IV. LYMPHATIC VESSELS
A. These vessels remove interstitial fluid and proteins and return them to the bloodstream. Once the
Interstitial fluid enters lymphatic vessels, it is known as lymph.
B. The Lymphatic Vessels
1. These vessels begin as Lymphatic Capillaries which are small vessels located in the spaces
between cells. Lymphatic capillaries are typically closed at one end (unlike capillaries).
a. Lymphatic capillaries are not found in bone marrow or nervous tissue.
b. Lymphatic capillaries are composed of very thin endothelial cells whose ends overlap.
1) The ends of these cells are easily pushed open when fluid pressure is great on
their external surfaces; thus allowing fluid to enter the lymphatic capillary.
2) When fluid pressure is greatest on the inside of the lymphatic capillary, the
ends of the endothelial cells are forced shut, preventing fluid from leaking back
into tissue spaces.
c. Proteins in the spaces surrounding tissues can easily enter lymphatic capillaries for
transport through the body. Likewise, cell debris and pathogens can enter these capillaries
for transport through the body.
d. Lymph flows from lymphatic capillaries into lymphatic collecting vessels.
2. Lymphatic Collecting Vessels
a. These are larger than lymphatic capillaries.
b. Lymphatic collecting vessels have the same three tunics as veins, but the collecting vessels
are thinner-walled and have more internal valves than veins.
c. In general, these vessels in the skin travel along with superficial veins and the deep
lymphatic vessels of the body travel with deep arteries.
3. Lymphatic Trunks –formed by the union of the largest lymphatic collecting vessels.
a. These drain fairly large areas of the body. These are named for the area of the body that
they drain: lumbar trunk, subclavian trunk, intestinal trunk.
4. Lymphatic Ducts-receive lymph from lymphatic trunks.
a. Two major lymphatic ducts:
1) Right Lymphatic Duct-drains lymph from the right upper arm and the right side of
the head and thorax.
2) Thoracic Duct-large, receives lymph from the rest of the body.
C. The lymphatic system lacks a pump to push fluids into and through lymphatic vessels. What moves fluid
through the lymphatic system?
1. Milking action of skeletal muscles
2. Pressure changes in the thorax during breathing
3. Numerous valves in lymphatic vessels
4. Pulsations of nearby arteries
V. LYMPHATIC ORGANS
A. Lymph Nodes-cluster along the major lymphatic vessels of the body.
1. These act to filter lymph as it moves through the lymphatic system.
2. Lymph nodes are located throughout the body but they cluster in the inguinal, axillary and
and cervical regions of the body (these are areas where lymphatic collecting vessels converge
to form lymphatic trunks).
3. Lymph Nodes have two primary functions:
a. Filtering lymph
1) Macrophages in the nodes act to destroy and remove microbes and foreign debris
that is carried by lymph.
b. Activating the Immune System
1) Lymphocytes in the nodes monitor the lymphatic system for foreign invaders and
they attack such creatures when they are present.
4. Structure of a Lymph Node
a. They are typically small and bean-shaped.
b. Each node is surrounded by a thick Capsule. Trabeculae (connective tissue strands) extend
inward to divide the lymph node into a number of chambers or compartments.
c. The Cortex-outer portion of a node. This area contains follicles where dividing B cells are
located.
d. The Medulla-contains medullary cords which are inward extensions of lymphatic tissue.
1) The medullary cords contain plasma cells.
e. Lymph Sinuses fill the central portion of the node. These sinuses contain numerous
lymphocytes and macrophages, both of which provide protection to the body.
5. Circulation in the Lymph Nodes
a. Lymph enters lymph nodes via afferent lymphatic vessels. Lymph then moves into a
baglike structure known as the subcapsular sinus. Next, lymph moves through a series
of smaller sinuses and finally exits the lymph node at its hilus (an indention) near
efferent lymphatic vessels.
b. Generally, there are fewer efferent lymphatic vessels than afferent lymphatic vessels;
therefore, lymph flow slows substantially. This allows time for macrophages and
lymphocytes to cleanse and filter lymph of foreign debris and bacteria.
c. Lymph has to pass through several lymph nodes before it is completely cleansed.
6. Homeostatic Imbalance-Buboes
B. The Spleen-the largest lymphoid structure in the body. Located in the left side of the abdominal cavity
beneath the diaphragm.
1. It is served by the splenic artery and vein.
2. The spleen is the site of lymphocyte proliferation and immune response. It also has a role in
cleansing the blood by removing old and worn blood cells and platelets from the body.
3. Other Functions of the Spleen include:
a. Storage of red blood cell breakdown products for future use and processing.
b. Serving as the site of red blood cell production in the fetus (this function ends after
birth).
c. Storing blood platelets.
4. Structure of the Spleen
a. It is surrounded by a capsule. The spleen contains trabeculae that support large numbers of
macrophages and lymphocytes. These lymphocytes are primarily stored in a region known
as the white pulp.
b. Red Pulp-involved primarily in removing red blood cells. Splenic cords make up much of
the red pulp of the spleen. These structures contain macrophages that aid in the breakdown
of red blood cells.
5. Can a person live without a spleen? Can the spleen regenerate itself?
C. The Thymus Gland-located in the inferior neck and extending into the superior thorax.
1. This gland secretes the hormones thymosin and thymopoietin. These two hormones force
T Lymphocytes to fight specific pathogens in the immune response (a property known as
immunocompetence).
2. The thymus gland is largest and most active during childhood. Following adolescence, its
growth stops and it gradually starts to atrophy.
3. The thymus gland is primarily composed of densely packed lymphocytes. These lymphocytes
are located in the cortex. The thymus gland does not contain B cells.
D. Tonsils-are considered to be the simplest of all the lymphoid organs.
1. These form a ring around the pharynx and they are named according to location.
2. Types of Tonsils:
a. Palatine Tonsils-paired, located on either side at the posterior end of the oral cavity.
1) These are the largest and most commonly infected tonsils.
b. Lingual Tonsils-paired, located at the base of the tongue. These are referred to as the
adenoids if they are enlarged.
c. Pharyngeal Tonsil-located at the posterior wall of the nasopharynx.
d. Tubal Tonsils-surround the openings of the auditory tubes into the pharynx.
3. The tonsils gather and remove pathogens entering the pharynx via food or inhaled air.
4. Internally, tonsils contain invaginations known as Crypts. The walls of these crypts are
covered by epithelial tissue. Bacteria and other pathogens are often trapped and destroyed
in the epithelial cells covering the crypts.
E. Aggregates of Lymphoid Follicles
1. Peyer’s Patches-structurally similar to tonsils, these are located in the small intestine.
a. These are involved in destroying bacteria that enter into the small intestine and
generating memory lymphocytes that provide long-term immunity.
2. Appendix-located in the large intestine, is structurally similar to Peyer’s Patches.
VI. RELATED CLINICAL TERMS