1. The Lymphatic System
Chapter 21

2. Introduction
The lymphatic system supports the function of the cardiovascular and immune systems of the body
The lymphatic system consists of two semi-independent parts
A network of lymphatic vessels
Lymphoid organs scattered throughout the body
The lymphatic vessels transport fluids that have escaped from the cardio-vascular system

3. Lymphatic Vessels
As blood circulates through the body, exchanges of nutrients, wastes, and gases occur between the blood and the interstitial fluid
The fluid that remains behind in the tissue spaces, as much as 3 liters a day, become part of interstitial fluid

4. Lymphatic Vessels
These leaked fluids, as well as any plasma proteins that escape from the blood-stream, must be carried back to the blood if the cardiovascular system is to sufficient blood volume to operate properly
The lymphatics are elaborate system of drainage vessels that collects the excess protein-containing interstitial fluid and returns it to the bloodstream
Once interstitial fluid enters the lymphatics ducts it is called lymph

5. Distribution of Lymphatic Vessels
The lymphatic vessels form a one-way system in which lymph flows only toward the heart
The system begins with the lymph capillaries

6. Distribution of Lymphatic Vessels
Lymph capillaries weave between the tissue cells and blood capillaries in the loose connective tissue of the body

7. Distribution of Lymphatic Vessels
Lymph capillaries are widespread, occurring almost everywhere blood capillaries occur
Lymph capillaries are absent from bone and teeth, bone marrow, and the entire central nervous system

8. Distribution of Lymphatic Vessels
Although similar to blood capillaries, lymphatic capillaries are remarkably permeable
The great permeability is due to tow structural modifications
Minivalves
Anchoring filaments

9. Minivalves
The endothelial cells forming the walls of the lymph capillaries are not tightly joined; instead their edges loosely overlap forming easily opened, flaplike minivalves

10. Anchoring Filaments
Bundles of fine filaments anchor the endothelial cells to surrounding structures so that any increase in interstitial fluid volume separates the cell flaps, exposing gaps in the wall and allowing fluid to enter rather than the capillary collapsing

11. Lymphatic Vessels
These structural modifications create a system where the valves gap open when fluid pressure is greater in the interstitial space, allowing fluid to enter the lymphatic capillary
Pressure inside the lymphatic capillary forces the minivalve flaps together prevent-ing a leak back out

12. Lymphatic Vessels
Proteins present in the interstitial fluid are prevented from entering the blood capillaries but enter lymphatic capillaries
In addition, when tissues are inflammed, lymphatic capillaries develop openings that permit uptake of even larger particles such as cell, pathogens, bacteria, viruses, and cancer cells
Thus cancer cells can use lymphatic capillaries to travel throughout the body

13. Lymphatic Vessels
Highly specialized lymphatic capillaries called lacteals are present in the fingerlike villa of the intestinal mucosa
The lymph draining from the digestive viscera is milky white rather than clear because the lacteals also receive digested fat from the intestine
This creamy lymph, called chyme, is also delivered to the blood via the lymphatic system
This concept discussed further in Chap 24

14. The Lymphatic System
From the lymphatic capillaries, lymph flows through successively larger channels
Collecting vessels
Trunks
Ducts

15. The Lymphatic System
Collecting vessels have the same three tunics as veins, but they are thinner-walled, have more internal valves, and anastomose more
In general the collect- ing vessels in the skin travel along with superficial veins of the CV system while deep vessels of the trunk travel with arteries

16. The Lymphatic System
The lymphatic trunks are formed by the union of the largest collecting vessels, and drain fairly large areas of the body
The trunks are named for the areas from which they collect lymph
Lumbar
Bronchomediastinal
Subclavian

17. The Lymphatic System
Lymph is delivered to one of two large ducts in the thoracic region
The right lymphatic duct drains lymph from the upper arm and the right side of the head and thorax
The larger thoracic duct receives lymph from the rest of the body

18. The Lymphatic System
Each terminal duct empties the lymph into the venous circulation at the junction of the internal jugular vein on its side of the body

19. Lymph Transport
Unlike the cardiovascular circulation, the lymphatic system lacks an organ that acts as a pump
Under normal conditions, lymphatic vessels are very low pressure conduits
Compression of skeletal muscle, pressure changes associated with respiration and valves to prevent back flow, aid the movement of lymph
Smooth muscle in the lymphatic duct contracts rhythmically to move lymph along

20. Lymph Transport
About 3 liters of lymph enters the blood- stream every 24 hours, a volume that almost equal to the amount of fluid lost to the tissue spaces from the bloodstream in the same time period
Movement of the adjacent tissues are extremely important in propelling lymph through the lymphatics
Physical activity or passive movement increase lymph flow

21. Lymphoid Cells
In order to understand some of the basic aspects of the lymphatic system’s role in body protection and immunity it is necessary to understand the components
Lymphoid cells
Lymphoid tissues

22. Lymphoid Cells
Infectious microorganisms, such as bacteria and viruses, that manage to penetrate the body’s epithelial barrier begin to quickly proliferate in the underlying loose tissue
These invaders are fought off by the inflammatory response by phagocytes (macrophages) and lymphocytes

23. Lymphoid Cells
Lymphocytes, the main warriors of the immune system, arise in red bone marrow
They then mature into one of the two main varieties of immunocompetent cells
T cells (T lymphocytes)
B cells (B lymphocytes)
These cells act to protect the body against antigens (bacteria and their toxins, viruses, mismatched RBC’s, or cancer cells

24. Lymphoid Cells
Activated T cells manage the immune response and some of them directly attack and destroy foreign cells
B cells protect the body by producing plasma cells, daughter cells that secrete antibodies into the blood
Antibodies immobilize antigens until they can be destroyed by phagocytes

25. Lymphoid Cells
Lymphoid marcophages play a crucial role in body protection and in the immune response by phagocytizing foreign substances and helping to activate T cells
Dendritic cells found in lymphoid tissue also activate T cells
Reticular cells are fibroblast cells that produce the reticular fiber stroma that supports the other cells types in the lymphoid organs

26. Lymphoid Tissue
Lymphoid tissue is an important component of the immune system because it
Houses and provides a proliferation site for lymphocytes
Furnishes an ideal surveillance vantage point for both lymphocytes and macrophages

27. Lymphoid Tissue
Lymphoid tissue, a type of loose connective tissue called reticular connective tissue, dominates all lymphoid organs except the thymus
The dark staining areas present the connective tissue fibers

28. Lymphoid Tissue Macrophages live on the fibers of the network
Within the spaces of this network are huge numbers of lymphocytes
Macrophage
Lymphocytes
Reticular
fiber

29. Lymphoid Tissue
Lymphocytes squeeze through the walls of capillaries and venules to reside temporarily in the lymphoid tissue and then leave to patrol the body
The cycling of lymphocytes between the circulatory vessels, lymphoid tissues, and loose connective tissues of the body ensures that lymphocytes reach infected or damaged sites quickly

30. Lymphoid Organs
Lymphoid organs as exemplified by lymph nodes, the spleen, and the thymus are discrete collections of lymphoid tissue
The exact pattern of the lymphoid tissue differs in the various lymphoid organs

31. Lymphoid Organs
Lymphoid organs are discrete, encapsulated collections of diffuse lymphoid tissue and nodules
The exact pattern of lymphoid tissue differs in the various lymphoid organs

32. Lymph Nodes
As lymph is transported back to the bloodstream, it is filtered through lymph nodes that cluster along the lymphatic vessels of the body

33. Lymph Nodes
There are hundreds of lymph nodes that are usually imbedded in connective tissue an not seen
Large clusters of lymph nodes occur near the body surface in the inguinal, axillary, and cervical regions of the body
Located where vessels form large trunks

34. Lymph Nodes
Lymph nodes have two basic functions, both concerned with body protection
They act to filter lymph
Phagocytic macrophages in the nodes remove and destroy microorganisms and other debris that enter the lymph from the loose connective tissue effectively preventing further spread
They play a role in activating the immune system
Lymphocytes in the lymph nodes monitor the lymphatic stream for the presence of antigens and attack them

35. Lymph Nodes
Lymph nodes are small (2.5 cm), bean shaped structures surrounded by a fibrous capsule of connective tissue

36. Lymph Nodes
Trabecula are connective tissue strands that extend inward to divide the node into compartments

37. Lymph Nodes
Its internal of framework of reticular fibers physically supports the ever-changing population of lymphocytes

38. Lymph Nodes
Two histologically distinct regions in a lymph node are the cortex and the medulla

39. Lymph Nodes
The outer cortex contains densely packed follicles, many with germinal centers heavy with dividing B cells

40. Lymph Nodes
Dendritic cells nearly encapsulate the follicles and abut the rest of the cortex, which primarily houses T cells in transit
The T cells circulate continuously between the blood, lymph nodes, and lymphatic stream, performing their surveillance role

41. Lymph Nodes
Medullary
cords
Medullary cords are thin inward extensions of the cortex containing lymphocytes and plasma cells

42. Lymph Nodes
Throughout the node are lymph sinuses which are large lymph capillaries spanned by reticular fibers
Numerous marcophages reside on these reticular fibers and phagocytize foreign matter in the lymph as it flows by the sinuses
Lymph borne antigens in the lymph leak into the surrounding reticular tissue, where they activate some of the strategically positioned lymphocytes to mount an immune response

43. Circulation in Lymph Nodes
Lymph enters the convex side of a lymph node through a number of afferent lymphatic vessels

44. Circulation in Lymph Nodes
Subcapsular sinus
Lymph moves through a large, baglike sinus, the sub- capsular sinus, into a number of smaller sinuses that cut through the cortex and enter the medulla

45. Circulation in Lymph Nodes
Lymph meanders through these sinuses and finally exits the node at its hilus, via efferent lymphatic vessels

46. Circulation in Lymph Nodes
Because there are fewer efferent vessels draining the node than there afferent vessels feeding it, the flow of lymph through the node stagnates somewhat, allowing time for the lymphocytes and macrophages to carry out their protective functions
In general, lymph passes through several nodes before its cleansing process is completed

47. Lymph Nodes: Clinical
Inflammation of a node caused by a large number of bacteria trapped in a node
Inflammation results in swelling and pain
Lymph nodes can become secondary cancer sites, particularly in metastasizing cancers that enter lymphatic vessels and become trapped
Cancer infiltrated nodes are swollen but not painful

48. Read about other lymphoid organs on pages 750 - 753
End of Chapter 21
Read about other lymphoid organs on pages

49. Other Lymphoid Organs

50. Spleen

51. Peyer’s Patch

52. Thymus

53. Tonsils

54. Aggregates of Lymphoid Nodules

55. Developmental Aspects of the Lymphatic System