Ch. 20 - Lymphatics

functions

• Returns fluids that have leaked from vascular system back to the blood

• Essential in body’s defense mechanisms

• Essential in resistance to disease

three parts

consists of:

• Lymphatic vessels

• Lymph

• Lymph nodes

Lymphatic Vessels

elaborate networks of drainage vessels that collect excess protein-containing interstitial fluid and return it to bloodstream

Lymph

interstitial fluid that enters lymphatic vessels

Lymph Nodes

small organs that cluster along lymphatic vessels and cleanse lymph fluid

• Secondary lymphoid organs that cleanse lymph and activate immune system; bean shaped (2.5 cm)

Lymphatic Capillaries

weave between tissue cells and blood capillaries in loose connective tissues of the body

• Absent from bones, bone marrow, and teeth

• Limited locations in meninges of CNS

• Minivalves – endothelial cells of walls not tightly joined; overlap forming flaps

• Collagen filaments anchor endothelial cells to surrounding structures – increase in interstitial fluid volume opens minivalves

Lacteals

special lymphatic capillaries that transport absorbed fat from small intestine to bloodstream

Collecting Lymphatic Vessels

same three tunics as blood vessels, but thinner and with more valves

Lymphatic Trunks

formed when largest collecting vessels unite; drain fairly large areas of body; named for region they drain

Right Lymphatic Duct

drains lymph from right upper limb and right side of head & thorax

Thoracic Duct

larger than right lymphatic duct; drains lymph from rest of body

Cisternae Chyli

enlarged sac; beginning of thoracic duct; located between last thoracic and second lumbar vertebrae

Lymph Transport

• Lymph system lacks a pump (like venous return)

• Active skeletal muscles

• Pressure changes in thorax during breathing

• Valves prevent backflow

• Lymphatic vessels bundled together in connective tissue sheaths with blood vessels

• Pulsations of nearby arteries also promote lymph flow

• Smooth muscle in walls contracts rhythmically, helping to pump lymph along

• Lymph transport is sporadic and slow

• Physical activity or passive movements increase lymph flow

Lymphoid Cells

• B Lymphocytes

• T Lymphocytes

• Plasma Cells

• Macrophages

• Dendritic Cells

• Reticular Cells

B Lymphocytes

protect body by producing plasma cells that produce antibodies

T Lymphocytes

manage immune response and directly attack & destroy infected cells

Plasma Cells

secrete antibodies into the blood

Macrophages

phagocytize foreign substances; help activate T cells

Dendritic Cells

capture antigens and bring them back to lymph nodes

Reticular Cells

fibroblast-like cells that produce the reticular fiber stroma (network that supports other cell types in lymphoid organs and tissues)

Lymphoid Tissue

• Houses lymphocytes and provides a site where they can be activated and proliferate

• Furnishes an ideal surveillance vantage point for lymphocytes and macrophages

• Three types: reticular, diffuse, follicles (nodules)

Reticular CT

dominates all lymphoid organs except thymus; macrophages live on fibers; lymphocytes temporarily occupy space

Diffuse Lymphoid Tissue

loose arrangement of lymphoid cells and some reticular fibers; found in almost all body organs; larger in lamina propria of mucous membranes

Lymphoid Follicles

solid, spherical bodies consisting of tightly packed lymphoid cells and reticular fibers; germinal center where B cells proliferate and produce plasma cells

lymph node structure

• capsule

• trabeculae

• cortex

• medulla

• medullary cords

• lymph sinuses

capsule

dense fibrous outer layer

Trabeculae

connective tissue strands that extend inward from capsule to divide node into compartments

cortex

superficial part contains densely packed follicles with germinal centers with dividing B cells; deeper part houses T cells in transit; dendritic cells abundant

Medulla

contains B & T lymphocytes

Medullary Cords

thin inward extensions from cortical lymphoid tissue

Lymph Sinuses

large lymphatic capillaries spanned by criss crossing reticular fibers; contain macrophages that phagocytize foreign matter in lymph

Lymph Circulation

• Lymph enters convex side of lymph node via afferent lymphatic vessels, moves through subscapular sinuses, through medullary sinuses, then exits at concave hilum via efferent lymphatic vessels

• Fewer efferent vessels draining node than afferent vessels feeding it; flow of lymph through node stagnates, allowing time for lymphocytes and macrophages to carry out functions

• Lymph passes through several nodes before completely cleansed

spleen

• Size of fist; left side of abdominal cavity below diaphragm

• Largest lymphoid organ 

• Site for lymphocyte proliferation and immune surveillance and response

• Extracts aged and defective blood cells and platelets from blood

• Recycles breakdown products of red blood cells

• Stores blood platelets and monocytes for release into blood when needed

• May be site of erythrocyte production in fetus

thymus

• Bilobed; inferior neck and superior thorax

• T lymphocyte precursors mature to become immunocompetent lymphocytes

• Prominent in newborns, atrophies after puberty

Mucosa-Associated Lymphoid Tissue

located in mucous membranes throughout the body

• tonsils

• Peyer’s patches

• appendix

Tonsils

ring of lymphoid tissues around entrance to pharynx; swellings of mucosa; palatine, lingual, pharyngeal, tubal; remove pathogens entering pharynx

Peyer’s Patches

large clusters of lymphoid follicles; similar to tonsils; located in wall of distal portion of small intestine

Appendix

tubular offshoot of first part of large intestine; high concentration of lymphoid follicles; prevents bacteria from breaching intestinal wall