The lymphatic system filters and returns interstitial fluid to the blood circulation through a network of vessels and organs. It begins with lymphatic capillaries that collect fluid from tissues and convey it through lymphatic vessels to lymph nodes, where the fluid is filtered and monitored for pathogens. The lymph then passes through larger lymphatic vessels and trunks before the two main collecting ducts, the thoracic duct and right lymphatic duct, return the lymph to venous circulation. Lymph nodes throughout the body help fight infection by filtering lymph and producing immune cells.

1. The Lymphatic System
Section 1, Chapter 16
Overview
The Lymphatic system is a network of
vessels and organs that filters and returns
interstitial fluid to blood circulation.
•It prevents fluid build-up (edema)
•It protects the body against pathogens.
•It absorbs fats from the intestine and
transports them to the bloodstream

2. Lymphatic Pathways
•The lymphatic system begins at lymphatic capillaries
•Lymphatic capillaries convey lymph towards
lymphatic vessels
•Lymph passes through lymph nodes, where it
is filtered and monitored for pathogens.
•Large lymphatic vessels (collecting ducts)
ultimately return lymph to venous circulation.

3. Lymphatic Capillaries
•Lymphatic capillaries are closed-ended vessels closely interlaced
with blood capillaries.
•They collect fluid forced out of blood capillaries (interstitial fluid)
and return it to venous circulation.

4. Lymphatic Capillaries
Structure
•Simple squamous epithelium (endothelium)
anchored to connective tissue.

5. Formation of Lymph
Hydrostatic pressure moves plasma out of
blood capillaries into the interstitial space.
Hydrostatic pressure
Osmotic pressure only reabsorbs some
of the lost fluid from interstitial space.
Osmotic pressure
Blood capillary
To
venule
From
arteriole
Interstitial space
To Lymphatic vessel
lymph
l;
Lymphatic capillary
Excess fluid enters
through one-way valves

6. Lymphatic Vessels
Structure: Lymphatic vessels are similar to veins, but thinner.
3 Layers: Tunica interna – endothelium and one-way valves
Tunica media – smooth muscles and elastic fibers
Tunica externa – connective tissue
lymph
Lymphatic Vessel with a flap-like valve.
Arrow shows direction of lymph flow.

7. Lymphatic Trunks
•Larger vessels lead to lymph nodes and then to larger lymphatic trunks
•Lymphatic trunks are named for the body region they drain.
•Lumbar Trunk – drains lymph from lower limbs and pelvic organs
•Intestinal Trunk– drains abdominal viscera
•Bronchomediastinal Trunk – drains portions of the thorax
•Subclavian Trunk – drains upper limbs
•Jugular Trunk – drains portions of the neck and head

8. Lymphatic Trunks

9. Collecting Ducts
Two Collecting ducts empty lymph from lymphatic trunks into venous circulation.
• Thoracic Duct
• Originates at the abdomen and empties into the left subclavian vein.
• Drains lymph from entire lower body: lumbar and intestinal trunks
• Also drains lymph from left upper limb, and left head and neck.
• Right Lymphatic Duct
• Originates in right thorax and empties into the right subclavian vein.
• Drains lymph from right upper limb, and right head and neck.

10. Collecting Ducts
Figure 16.6 Lymphatic pathways. The right lymphatic duct drains
lymph from the upper right side of the body (in red), whereas the
thoracic duct drains lymph from the rest of the body.

11. The Lymphatic Pathway
Figure 16.7 The lymphatic pathway. This
pathway is true for both right and left sides.

12. Lymph Movement
Muscle activity largely influences the movement of lymph through the
lymphatic vessels via:
• Action of skeletal muscles
• Respiratory movements
• Smooth muscle in the larger lymphatic vessels
• Valves in the lymphatic vessels

13. Lymph Nodes
• Lymph nodes are located along the lymphatic pathways
• They contain lymphocytes and macrophages to fight invading pathogens
Lymph enters and leaves a lymph node through lymphatic vessels.
• Afferent Vessels –convey lymph towards lymph node
• Efferent Vessels – convey lymph away from lymph node.

14. Lymph Node Structure
Capsule of connective tissue (c.t.). Sinuses
• Trabiculae = c.t. partitions
• Complex channels through
which lymph circulates
• Interlaced with reticular fibers.
Hilum
• Entrance for blood vessels and nerves
• Efferent vessels leave from the hilum
Cortex (outer layer)
• Nodules = dense populations of B-cells
and macrophages
Medulla (inner layer)
• Reticulum
• T-cells
trabiculae

15. Locations of Lymph Nodes
There are about 450 lymph nodes throughout the body.
• Cervical region – filter lymph from the face and scalp
• Axillary region – filters lymph from upper limbs, thorax, and mammary glands
• Supratrochlear region – filter lymph from forearms
•Enlarges in children in response to cuts on the hand
• Inguinal region – filters lymph from lower limbs and external genitalia
• Pelvic cavity – filter lymph from internal pelvic organs
• Abdominal cavity –filter lymph from abdominal viscera
• Thoracic cavity – filter lymph from thoracic viscera

16. Locations of Lymph Nodes
Functions of Lymph Nodes
1. Filter particles from lymph before
returning it to the blood stream
2. Monitor body fluids (immune surveillance)
3. Lymphocyte production.
Figure 16.11 Major locations of lymph nodes

17. Obstruction of Lymph Flow
Axillary lymph nodes are often removed during a mastectomy to
prevent the metastasis of breast cancer to other regions in the body.
This can obstruct the drainage of lymph from the upper arm, resulting in edema.

18. MALT
Mucosa-associated lymphoid tissue (MALT) – lymphatic nodules associated
with the mucous membranes (i.e. digestive, respiratory, genitourinary tracts)
Tonsils – encapsulated lymphoid tissues
Peyer’s Patches – common in the ilium portion of the small intestine
Appendix – contains aggregations of lymphatic nodules
Individual lymphatic nodules are also found along the mucosal membrane.
Two lymphatic nodules of the Peyer’s
Patches in the small intestine.

19. Thymus
• The thymus
• Is enlarged in childhood, then atrophies in adult hood.
• It is the site of T-cell production
• Secretes Thymosin, which promotes the maturation of T-cells.
Figure 16.13 Compared to other thoracic
organs, the thymus in the fetus is large, but
in the adult is small. Figure is not to scale.

20. The Spleen
The spleen is the largest lymphatic organ.
It is located in the upper left abdominal quadrant,
just inferior to the diaphragm.
The spleen’s primary function is to filter
blood, remove old red blood cells, and
house lymphocytes.
Its structure resembles a large lymph node.

21. The Spleen
Capsule of connective tissue (c.t.).
• Trabiculae = c.t. partitions
Hilum
• Entrance for blood vessels and nerves
Venous Sinuses
• Complex channels through which blood circulates
• Interlaced with reticular fibers.
White Pulp
• Lymphatic Nodules (packed with lymphocytes)
Red Pulp
• Abundant Red Blood Cells (RBCs)
• Removes old RBCs from circulation
• Contains many lymphocytes and macrophages

22. End of Section 1, Chapter 16