Learning Objectives - Lymphatic System

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1

List the functions of the lymphatic vessels.

The function of the lymphatic vessels, or lymphatics, is an elaborate system of drainage vessels that collect the excess protein-containing interstitial fluid and return it to the bloodstream.

2

Describe the structure and distribution of lymphatic vessels.

• The lymphatic collecting vessels have the same three tunics as veins, but the collecting vessels are thinner walled, have more internal valves, and anastomose more.

• The lymphatic vessels form a one-way system in which lymph flows only toward the heart. This transport system begins in microscopic blind-ended lymphatic capillaries. These capillaries weave between the tissue cells and blood capillaries in the loose connective tissues of the body. They are absent from bones and teeth, bone marrow, and the entire central nervous system

• Permeability due to two unique structural modifications:
o The endothelial cells forming the walls of lymphatic capillaries are not tightly joined. Instead, the edges of adjacent cells overlap each other loosely, forming easily opened, flaplike minivalves .
o Collagen filaments anchor the endothelial cells to surrounding structures so that any increase in interstitial fluid volume opens the minivalves, rather than causing the lymphatic capillaries to collapse.

3

Describe the source of lymph and mechanism(s) of lymph transport.

Once interstitial fluid enters lymphatics, it is called lymph

• Lymphatic capillaries which empty into Lymphatic collecting vessels which empty into Lymphatic trunks and ducts

• The lymphatic system lacks an organ that acts as a pump. lymphatic vessels are low-pressure conduits, and the same mechanisms that promote venous return in blood vessels act here as well—the milking action of active skeletal muscles, pressure changes in the thorax during breathing, and valves to prevent backflow.

• lymph transport is sporadic and slow. About 3 L of lymph enters the bloodstream every 24 hours, a volume almost exactly equal to the amount of fluid lost to the tissue spaces from the bloodstream in the same time period.

4

Describe the basic structure and cellular population of lymphoid tissue. Differentiate between diffuse and follicular lymphoid tissues.

Lymphoid (lymphatic) tissue is an important component of the immune system, mainly because it
• houses and provides a proliferation site for lymphocytes and
• furnishes an ideal surveillance vantage point for lymphocytes and macrophages.
Lymphoid tissue, largely composed of a type of loose connective tissue called reticular connective tissue, dominates all the lymphoid organs except the thymus. Macrophages live on the fibers of the reticular network. Huge numbers of lymphocytes that have squeezed through the walls of postcapillary venules coursing through this tissue temporarily occupy the spaces of this network. Then, they leave to patrol the body again. 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.

Lymphoid Cells
• Lymphocytes, the main warriors of the immune system, mature into one of the two main varieties of immunocompetent cells that protect the body against antigens
T cells (T lymphocytes) - manage the immune response, and some of them directly attack and destroy infected cells.
B cells (B lymphocytes) – protect the body by producing plasma cells, daughter cells that secrete antibodies into the blood

• Lymphoid Macrophages - play a crucial role in body protection and in the immune response by phagocytizing foreign substances and by helping to activate T cells.

• Dendritic Cells - capture antigens and bring them back to the lymph nodes.

• Stroma - network that supports the other cell types in the lymphoid organs and tissues

• Reticular Cells - fibroblast-like cells that produce the reticular fiber stroma

Diffuse lymphatic tissue - consisting of a few scattered reticular tissue elements, is found in virtually every body organ, but larger collections appear in the lamina propria of mucous membranes and in lymphoid organs.

Lymphoid follicles (nodules)- they lack a capsule, but follicles are solid, spherical bodies consisting of tightly packed reticular elements and cells.

5

Describe the general location, histological structure, and functions of lymph nodes.

• Nodes cluster along the lymphatic vessels of the body. Large clusters of lymph nodes occur near the body surface in the inguinal, axillary, and cervical regions, places where the lymphatic collecting vessels converge to form trunks

• Lymph nodes have two basic functions, both concerned with body protection.
o As lymph is transported back to the bloodstream, the lymph nodes act as lymph “filters.” Macrophages in the nodes remove and destroy microorganisms and other debris that enter the lymph from the loose connective tissues, effectively preventing them from being delivered to the blood and spreading to other parts of the body.
o They help activate the immune system. Lymph nodes and other lymphoid organs are strategically located sites where lymphocytes encounter antigens and are activated to mount an attack against them

• Lymph nodes vary in shape and size, but most are bean shaped and less than 2.5 cm (1 inch) in length. Each node is surrounded by a dense fibrous capsule from which connective tissue strands called trabeculae extend inward to divide the node into a number of compartments. The node’s internal framework, or stroma, of reticular fibers physically supports the ever-changing population of lymphocytes.
Two histologically distinct regions
o cortex -superficial part of the cortex contains densely packed follicles, many with germinal centers heavy with dividing B cells. Dendritic cells nearly encapsulate the follicles and abut the deeper part of the cortex, which primarily houses T cells in transit. The T cells circulate continuously between the blood, lymph nodes, and lymph, performing their surveillance role.
o Medulla - medullary cords are thin inward extensions from the cortical lymphoid tissue, and contain both types of lymphocytes plus plasma cells. Throughout the node are lymph sinuses, large lymph capillaries spanned by crisscrossing reticular fibers. Numerous macrophages reside on these reticular fibers and phagocytize foreign matter in the lymph as it flows by in the sinuses. Additionally, some of the lymph-borne antigens in the percolating lymph leak into the surrounding lymphoid tissue, where they activate lymphocytes to mount an immune attack against them.

6

Name and describe the other lymphoid organs of the body. Compare and contrast them with lymph nodes, structurally and functionally.

• Spleen is about the size of a fist and is the largest lymphoid organ. The spleen provides a site for lymphocyte proliferation and immune surveillance and response. Important are its blood-cleansing functions. Besides extracting aged and defective blood cells and platelets from the blood, its macrophages remove debris and foreign matter from blood flowing through its sinuses. The spleen also performs three additional, and related, functions.
1.It stores some of the breakdown products of red blood cells for later reuse (for example, it salvages iron for making hemoglobin) and releases others to the blood for processing by the liver.
2. It stores blood platelets.
3. It is thought to be a site of erythrocyte production in the fetus (a capability that normally ceases after birth).
White Pulp - Areas of the spleen composed mostly of lymphocytes suspended on reticular fibers , involved with the immune functions
Red Pulp - essentially all remaining splenic tissue, that is, the venous sinuses (blood sinusoids) and the splenic cords, regions of reticular connective tissue exceptionally rich in macrophages. Red pulp is most concerned with disposing of worn-out red blood cells and bloodborne pathogens

• Thymus - important functions primarily during the early years of life. The thymus is where T lymphocytes become able to defend us against specific pathogens in the immune response.
In addition to its lack of follicles, the thymus differs from other lymphoid organs in two other important ways. First, it functions strictly in maturation of T lymphocyte precursors and thus is the only lymphoid organ that does not directly fight antigens. Second, the stroma of the thymus consists of epithelial cells rather than reticular fibers. These epithelial cells provide the physical and chemical environment in which T lymphocytes can become immunocompetent.

• Tonsils are the simplest lymphoid organs. They form a ring of lymphatic tissue around the entrance to the pharynx (throat), where they appear as swellings of the mucosa. The tonsils gather and remove many of the pathogens entering the pharynx in food or in inhaled air.
Palatine Tonsils are located on either side at the posterior end of the oral cavity. These are the largest of the tonsils and the ones most often infected.
Lingual Tonsil - lumpy collection of lymphoid follicles at the base of the tongue.
Pharyngeal Tonsil (referred to as the adenoids if enlarged) is in the posterior wall of the nasopharynx.
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