1. Chapter 20 Lymphatic System

2. Overview of the Lymphatic System
Importance of the lymphatic system
Two most important functions— maintain fluid balance in the internal environment and promote body’s immunity
Lymph vessels act as “drains” to collect excess tissue fluid and return it to the venous blood just before it returns to the heart

3. Overview of the Lymphatic System
specialized component of the circulatory system
made up of lymph, lymphatic vessels, and isolated structures containing lymphoid tissue: lymph nodes, aggregated lymphoid nodules, tonsils, thymus, spleen, and bone marrow
Transports tissue fluid, proteins, fats, and other substances to the general circulation
Lymphatic vessels begin blindly in the intercellular spaces of the soft tissues- do not form a closed circuit

4. Lymph and Interstitial Fluid
Lymph (lymphatic fluid)
Clear, watery-appearing fluid found in the lymphatic vessels
Closely resembles blood plasma in composition

5. Lymph and Interstitial Fluid
Interstitial fluid (IF)
Complex, organized fluid that fills the spaces between the cells
Resembles blood plasma in composition
Along with blood plasma, constitutes the extracellular fluid

6. Lymphatic Vessels
Distribution of lymphatic vessels (lymphatics) (Figures 20-2 and 20-3)
Lymphatic capillaries
microscopic blind-end vessels where lymphatic vessels originate
wall consists of a single layer of flattened endothelial cells
networks branch and reconnect freely
Lymphatic capillaries merge to form larger lymphatics
drains into subclavian vein

7. Lymphatic Vessels Structure of lymphatic vessels
Similar to veins except lymphatic vessels have thinner walls, more valves, and contain lymph nodes
Lymphatic capillary wall is formed by a single layer of thin, flat endothelial cells
As the diameter of lymphatic vessels increases from capillary size, the walls become thicker and have three layers
Semilunar valves are present every few millimeters in large lymphatics and even more frequently in smaller lymphatics

8. Lymphatic Vessels Functions of the lymphatic vessels
Remove high weight substances
If anything blocks lymphatic return, blood protein concentration and blood osmotic pressure 
Lacteals absorb fats and other nutrients from the small intestine

9. Circulation of Lymph Lymph system re-circulates about 3 liters/day!
From lymphatic capillaries, lymph flows through progressively larger lymphatic vessels & eventually reenters blood at the subclavian veins
Lymphatic pump
Lymphokinesis
the movement of lymph
can be visualized
Lymph moves through the
system in right direction of
lots of valves

10. Lymph Nodes Structure of lymph nodes Oval structures
Nodes are a biological filter
Once lymph enters a node, it moves slowly through sinuses to drain into the efferent exit vessel
lined with specialized cells capable of phagocytosis

11. Lymph Nodes Functions of lymph nodes Defense functions
Filtration
Mechanical filtration— physically stops particles
Biological filtration— biological activity of cells destroys and removes particles
Phagocytosis— cells remove microorganisms and other injurious particles from lymph and phagocytose them
If overwhelmed, lymph nodes can become infected or damaged
If infection blocks nodes, fluid accumulates  swollen
Hematopoiesis— lymphoid tissue is site for final stages of maturation of some lymphocytes and monocytes

12. Lymphatic Drainage of the Breast
Clinically important because cancer cells and infections can spread along lymphatic pathways to lymph nodes and other organs of the body
Lymph nodes associated with the breast
>85% lymph from the breast enters lymph nodes of axillary region
Remainder enters lymph nodes along lateral edges of sternum

13. Tonsils
Form a broken ring under the mucous membranes in the mouth and back of the throat—the pharyngeal lymphoid ring (Figure 20-17)
Palatine tonsils— each side of throat
Pharyngeal tonsils— near posterior opening of nasal cavity (adenoids when swollen)
Lingual tonsils— near base of the tongue
Protect against bacteria that may invade
tissues around the openings between the
nasal and oral cavities

14. Thymus Location and appearance of the thymus
Primary organ of lymphatic system
Single, unpaired organ located in the mediastinum
Thymus is pinkish gray in childhood
with advancing age, becomes yellowish as lymphoid tissue is replaced by fat

15. Thymus Structure of the thymus
Pyramid-shaped lobes are subdivided into small lobules
Each lobule is composed of a dense cellular cortex and an inner, less dense, medulla
Medullary tissue can be identified by presence of thymic corpuscles

16. Thymus Function of the thymus Plays vital role in immunity mechanism
Source of blood cells before birth
Shortly after birth, thymus secretes thymosin and other regulators, which enables lymphocytes to develop into T cells

17. Spleen Location of the spleen— left hypochondrium Structure of spleen
Ovoid in shape
White pulp— dense masses of developing lymphocytes
Red pulp

18. Spleen Functions of the spleen Defense
macrophages lining spleen remove microorganisms from blood and phagocytose them
Hematopoiesis
monocytes and lymphocytes complete their development in the spleen
Red blood cell and platelet destruction
macrophages remove worn-out RBCs and imperfect platelets and destroy them
salvage iron and globin from destroyed RBCs
Blood reservoir
stores blood
FYI- As response to hemorrhage, spleen can “self-transfuse” about 200 mL of blood!

19. Cycle of Life: Lymphatic System
Dramatic changes throughout life
Organs with lymphocytes appear before birth and grow until puberty
Postpuberty
Organs atrophy through late adulthood
Shrink in size
Become fatty or fibrous
Spleen— develops early, remains intact
Overall function maintained until late adulthood
Later adulthood
Deficiency permits risk of infection and cancer
Hypersensitivity— likelihood of autoimmune conditions