Lymphatic System

Chapter 20

An Overview

• One way system flowing towards heart• Functions

– Return fluid and proteins to venous blood– House phagocytic cells and lymphocytes– Carry absorbed fats from intestines to

blood• Components

– Lymphatic vessels– Lymph– Lymph nodes

Lymphatic Vessels• Capillaries

– Endothelial cells form minivalves– Inflammation increases permeability– Lacteals carry fat from intestines as chyle

• Collecting Vessels– Similar to veins– Varies between individuals

• Trunks– Lumbar– Bronchomediastinal– Subclavian– Jugular– Intestinal

• Ducts– Right lymphatic – Thoracic

• Cisterna chyli • Dump to venous blood

Lymph Transport

• Low pressure system w/o a pump– Similar return as veins– Arterial pulsations– Tunica media smooth muscle contraction

• Balances with blood fluid loss– Hydrostatic and colloid pressures (Chpt. 19)– ~ 3L every 24 hours

• Rate increases w/activity

Lymphocytes

• Primary fighters of immune response• Targets are antigens• T-cells – direct attack– Attack and destroy antigens

• B-cells – indirect attack– Produce antibodies from plasma cells to ‘flag’

antigens

Other Lymphoid Cells

• Macrophages– Phagocytic themselves– Activate T-cells

• Dendritic cells– Capture and move antigens to lymph nodes– Activate T-cells too

• Reticular cells– Fibroblast-like cells that form supportive network

Lymphoid Tissue

• Proliferation & surveillance sites• Primarily reticular CT (except thymus)– Diffuse lymphatic tissue• Sparse scatterings in all lymph organs,• Concentrated in lamina propria of mucus membranes

– Lymphoid follicles (nodules)• Spherically packed tissue w/o capsule

– Larger organs and few isolated patches

• Germinal centers where B cells proliferate– Enlarge w/ increased B cell division

Lymph Nodes

• Main lymphatic organs • Located along lymph vessel path– Concentrated near large collecting vessel junctions

• Inguinal region• Axillary region• Cervical region

• Functions– Filtration

• Macrophages prevent foreign molecule entrance to blood

– Immune system activation• Monitor for antigens to fight

Lymph Node Structure• Dense fibrous capsular outside

– Difference b/w node and nodule– Invaginates forming trabeculae

• Regions– Medulla

• Macrophages, T cells, B cells, and plasma cells

• Lymph sinuses: capillaries where macrophages ‘hunt’– Leaking antigens activate lymphocytes in

tissue

– Cortex• Dense nodules w/germinal centers• Transient T cells

Lymphatic Circulation

• Enters node in afferent lymphatic vessels

• Large subscapular sinus to smaller, cortical sinuses

• Enter medulla• Exit at hilum via efferent

lymphatic vessels– Fewer slows flow– Allows lymphocytes &

macrophages to work

Spleen

• Largest lymphatic organ• Functions– Lymphocyte proliferation and surveillance – Stores products of RBC breakdown and platelets– Cleanse blood

• Remove aged/damaged blood, debris, and foreign matter

• Fetal erythrocyte production (ceases after birth)• Distinct areas– White pulp w/lymphocytes act in immune functions– Red pulp w/worn out erythrocytes and pathogens

Thymus

• Bilobed organ at base of neck– More pronounced when young– Corresponds w/importance of immune function

• T lymphocyte maturation only– Lacks B cells– Doesn’t directly fight antigens

• Thymocytes secrete thymosin and thymopoietin to signal T cell maturation

Tonsils

• Lymphatic tissue ring around pharynx– Palatine: largest and most likely infected– Lingual– Pharyngeal (adenoids)– Tubal

• Follicles w/germinal centers• Gather and remove pathogens from pharynx– Crypts are deep invaginations to trap and destroy

• Tonsil stones

– Produces ‘memory’ immune cells for future attacks

Mucosa-Associated Lymphatic Tissue (MALT)

• Collections of lymphatic tissue to protect external environment openings

• Peyer’s patches– In walls of small intestine– Destroy bacteria before it leaves intestines– Generate ‘memory’ lymphocytes

• Appendix– Junction of small and large intestine– Similar function as Peyer’s patches

• Lymphoid nodules– In walls of bronchi

Homeostatic Imbalances

• Tonsillitis: inflammation of tonsils• Lymphangitis: vasa vasorum of lymph vessels congested

w/blood• Lymphedema: blockage prevents return to blood• Buboes: inflamed lymph nodes • Splenectomy: removal of a ruptured spleen• Appendectomy: removal of appendix• Elephantiasis: lymph vessels clogged by worms causing

increased swelling• Hodgkin’s disease: malignant B-cells• Non-Hodgkin’s lymphoma: any lymphoma, but Hodgkin’s