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 Two parts: 1.Lymphatic vessels 2.Lymphoid tissues and organs  Functions: ◦ Transports escaped fluids back to the blood ◦ Body defense and resistance to disease © 2015 Pearson Education, Inc.

 Lymph—excess tissue fluid and plasma proteins carried by lymphatic vessels ◦ Also includes cell debris, bacteria, viruses  Lymphatic vessels ◦ pick up lymph and return it to the blood  Lymph capillaries (smallest) absorb leaked fluid  Contain minivalves to ensure one-way flow to heart  Stay closed when vessel pressure > interstitial fluid  Remain open when vessel pressure < interstitial fluid  If fluids are not picked up, edema occurs (fluid accumulates in tissues and the area swells) © 2015 Pearson Education, Inc.

Venous system Arterial system Lymphatic system Heart Lymph duct Lymph trunk Lymph node Lymphatic collecting vessels, with valves Lymph capillary Tissue fluid (becomes lymph) Blood capillaries Loose connective tissue around capillaries

(a) Tissue fluid Tissue cell Lymphatic capillary Blood capillaries ArterioleVenule

(b) Endothelial cell Fibroblast in loose connective tissue Flaplike minivalve Filaments anchored to connective tissue

 Lymph is collected through a system of vessels that gets progressively larger  Two large ducts return fluid to circulatory veins near the heart ◦ Right lymphatic duct  drains the lymph from the right arm and the right side of the head and thorax ◦ Thoracic duct  drains lymph from rest of body © 2015 Pearson Education, Inc.

Regional lymph nodes: Cervical nodes Axillary nodes Inguinal nodes KEY: Drained by the right lymphatic duct Drained by the thoracic duct Entrance of right lymphatic duct into right subclavian vein Internal jugular vein Thoracic duct entry into left subclavian vein Thoracic duct Aorta Spleen Cisterna chyli (receives lymph drainage from digestive organs) Lymphatics

 similar to veins of the cardiovascular system ◦ (have the same tunics: intima, media, externa) ◦ Thin endothelial walls ◦ Valves in larger vessels ◦ Low-pressure, pumpless system  Lymph transported is aided by: ◦ Milking action of skeletal muscles ◦ Pulsations of nearby arteries ◦ Pressure changes in thorax during breathing ◦ Smooth muscle in walls of vessels © 2015 Pearson Education, Inc.

 About 500 nodes in the body ◦ Large clusters in specific regions:  inguinal, axillary, cervical  Structure: ◦ Surrounded by a dense connective tissue capsule ◦ Compartmentalized: sinuses lined with endothelial cells ◦ Outer cortex contains lymphocytes (B cells & T cells) ◦ Inner medulla contains phagocytic macrophages  filter harmful materials before it is returned to the blood ◦ Bacteria ◦ Viruses ◦ Cancer cells ◦ Cell debris © 2015 Pearson Education, Inc.

Afferent lymphatic vessels Afferent lymphatic vessels Germinal center in follicle Efferent lymphatic vessels Cortex Follicle Medullary cord Medullary sinus Hilum Trabecula Subcapsular sinus Capsule

 Several other organs contribute to lymphatic function: ◦ Spleen ◦ Thymus ◦ Tonsils ◦ Peyer’s patches © 2015 Pearson Education, Inc.

Tonsils (in pharyngeal region) Thymus (in thorax; most active during youth) Spleen (curves around left side of stomach) Peyer’s patches (in intestine) Appendix

 left side of the abdomen, beneath diaphragm, around the stomach  Functions: ◦ Filters pathogenic debris from blood ◦ Destroys worn-out red blood cells  Iron is reused for hemoglobin mfr.  Remainder is secreted in bile  Acts as a platelet reservoir  Site of hematopoesis in the fetus © 2015 Pearson Education, Inc.

 Located low in the throat, just under the sternum  Function: ◦ (at peak levels only during childhood) ◦ Produces hormones (such as thymosin)  hormones cause T lymphocytes to gain immunocompetency © 2015 Pearson Education, Inc.

 masses of lymphoid tissue around the pharynx ◦ Palatine tonsils are most often infected  Trap and remove bacteria/foreign materials  Tonsillitis -caused by congestion with bacteria  The ability of the tonsils to trap bacteria generates a wide variety of memory lymphocytes that protect throughout life © 2015 Pearson Education, Inc.

 Found in the wall of the small intestine and appendix  Resemble tonsils in structure  Capture and destroy bacteria in the intestine ◦ Prevent bacteria from getting into blood here ◦ Generate memory lymphocytes for long-term © 2015 Pearson Education, Inc.

 Includes: ◦ Peyer’s patches ◦ Tonsils ◦ Other small accumulations of lymphoid tissue  Acts as a sentinel to protect respiratory and digestive tracts © 2015 Pearson Education, Inc.