20 The Lymphatic System

Lymphatic System: Overview Consists of two semi-independent parts: A network of lymphatic vessels Lymphoid tissues and organs scattered throughout the body Lymphatic vessels return interstitial fluid (fluid in the tissues) and leaked plasma proteins back into the bloodstream Once interstitial fluid has entered lymphatic vessels, it is called lymph

Lymphatic System: Overview Figure 20.2a

Lymphatic System: Overview Figure 20.1a

One-way system, lymph flows toward the heart Lymph vessels include: Lymphatic Vessels One-way system, lymph flows toward the heart Lymph vessels include: Microscopic, permeable, blind-ended capillaries Lymphatic collecting vessels Trunks and ducts

Lymphatic Capillaries Similar to blood capillaries, with modifications: Very permeable Loosely joined endothelial minivalves Withstand interstitial pressure and remain open The minivalves function as one-way gates that: Allow interstitial fluid to enter lymph capillaries Prevent lymph from escaping out of the capillaries

Lymphatic Capillaries Figure 20.1b

Lymphatic Capillaries During inflammation, lymph capillaries can absorb: Cell debris Pathogens Cancer cells Lacteals – specialized lymph capillaries present in lining of the small intestines Absorb digested fat and deliver chyle (fatty lymph) to the bloodstream

Lymphatic Collecting Vessels Have the same three tunics as veins Have thinner walls, with more internal valves Anastomose (branch off) more frequently Collecting vessels in the skin travel with superficial veins Deep vessels travel with arteries

Lymph is delivered into one of two large trunks Lymphatic Trunks Lymphatic trunks are formed by the union of the largest collecting ducts Lymph is delivered into one of two large trunks Right lymphatic duct – drains the right upper arm and the right side of the head and thorax Thoracic duct – arises from the cisterna chyli and drains the rest of the body

Lymphatic Trunks Figure 20.2b

Return of Lymph to Heart Figure 20.2a

The lymphatic system lacks a pumping organ Lymph Transport The lymphatic system lacks a pumping organ Vessels operate under low pressure Uses the same methods as veins to propel lymph: Pulsations of nearby arteries Contractions of smooth muscle in the vessel walls Contractions of skeletal muscle

Major Tissue Type: Reticular Connective Tissue Network of reticular fibers. Weblike. Loosely bound together to support lymphocytes and macrophages

Lymphocytes are the main cells involved in the immune response Lymphoid Cells Lymphocytes are the main cells involved in the immune response Two main varieties: T cells B cells Both are produced in the red bone marrow – so it is classified as a primary lymphatic organ. B cells mature in the red bone marrow. T cells must travel to the thymus gland to mature; thus the name “T”

Lymphocytes T cells B cells Manage the immune response Directly attack and destroy virus - infected cells and tumor cells B cells Produce antibodies which circulate in the blood and attack foreign substances (antigens)

Macrophages Macrophages – big “eaters” When monocytes leave the blood stream and enter tissues they transform into macrophages and “eat up” viruses, bacteria, especially in chronic infections like T.B.

EDEMA Swelling caused by abnormal amount of interstitial (between cells) fluid. Common in lower extremities when right side of the heart is damaged. Common in lungs when left side of the heart is damaged.

Principal lymphoid organs of the body Lymph Nodes Principal lymphoid organs of the body Embedded in connective tissue and clustered along lymphatic vessels Aggregations of these nodes occur near the body surface in inguinal, axillary, and cervical regions of the body Figure 20.4a

Lymph Nodes Two basic functions: Filtration – macrophages destroy microorganisms and debris ***Only part of the lymphatic system that filters lymph *** Immune system activation – monitors for antigens and mounts an attack against them

Structure of a Lymph Node Bean shaped and surrounded by a fibrous capsule Figure 20.4a, b

Circulation in the Lymph Nodes Lymph enters via afferent lymphatic vessels It then enters a large sinus and travels into smaller sinuses It meanders through these sinuses and exits the node at the hilus via efferent vessels Because there are fewer efferent vessels, lymph flow slows down in the node This allows lymphocytes and macrophages time to carry out protective functions

Lymphoid Organs Figure 20.5

Other Lymphoid Organs The spleen, thymus gland, and tonsils Peyer’s patches and bits of lymphatic tissue scattered in connective tissue All are composed of reticular connective tissue All help protect the body

Spleen Largest lymphoid organ, located on the left side of the abdominal cavity beneath the diaphragm It is served by the splenic artery and vein, which enter and exit at the hilus Functions: Site of lymphocyte proliferation (in WHITE pulp) Immune surveillance and response Cleanses the blood

Additional Spleen Functions Breaks down and stores products of RBCs for later reuse (in RED pulp) Spleen macrophages salvage and store iron for later use by bone marrow Site of fetal erythrocyte production (normally ceases after birth) Stores blood platelets Does NOT filter lymph

Structure of the Spleen Highly vascular Figure 20.6a, b

Ruptured Spleen White/Red Pulp Areas

A bilobed organ located Thymus A bilobed organ located in the inferior neck and extends into the mediastinum where it partially overlies the heart Size of the thymus varies with age: It increases in size and is most active during childhood It stops growing during adolescence and then gradually atrophies By this time, enough T lymphocytes have matured; vaccinations and antibodies formed from exposure to illnesses has occurred

Thymus The thymus differs from other lymphoid organs in two important ways Secretes hormones that cause T lymphocytes to mature, enabling them to recognize pathogens and initiate an immune response It does not directly fight antigens It is classified as a primary lymphatic organ.

Tonsils Simplest lymphoid organs; form a ring of lymphatic tissue around the pharynx Location: Palatine tonsils – either side of the posterior end of the oral cavity Lingual tonsils – lie at the base of the tongue Pharyngeal tonsil – posterior wall of the nasopharynx (referred to as ADENOIDS if enlarged) Tubal tonsils – surround the openings of the auditory tubes into the pharynx

Tonsils Lymphoid tissue of tonsils contains follicles with germinal centers Tonsil masses are not fully encapsulated Epithelial tissue overlying tonsil masses invaginates (folds in on itself), forming blind-ended crypts Crypts trap and destroy bacteria and particulate matter

INFECTED TONSILS - Tonsilitis

Aggregates of Lymphoid Follicles Peyer’s patches – isolated clusters of lymphoid tissue, similar to tonsils Found in the wall of the distal portion of the small intestine Similar structures are found in the appendix Peyer’s patches and the appendix: Destroy bacteria that entered the body via the mouth or anus, preventing them from penetrating the intestinal wall Generate “memory” lymphocytes for long-term immunity

MALT – mucosa-associated lymphatic tissue Protects passages that open to the EXTERIOR Peyer’s patches, tonsils, and the appendix (digestive tract – mouth and anus) Lymphoid nodules in the walls of the bronchi (respiratory tract – mouth and nose and ears) Mucosal lining of the reproductive and urinary system ( urethra and vagina)

Breast Cancer/Lymph Nodes Axillary lymph nodes

Lymph Node Biopsy Standard treatment for breast cancer was removal of the breast(s) and the axillary lymph nodes. Now it is sometimes possible to biopsy the sentinel nodes (those closest to the tumor). Dye is injected near the tumor and then sentinel nodes are identified if they are stained with dye. If cancer has not spread, surgery is less invasive. Sentinel node biopsy is used with other tumors, not just breast.

Post Surgery Care When multiple regional lymph nodes are removed, the patient may experience side effects such as lymphedema (swelling caused by excess fluid build-up), numbness (nerve damage), infection and difficulty moving the affected body area. Patients must avoid pressure (lifting heavy objects, having blood pressure taken, etc.) in affected area. Wear an elastic stocking to reduce swelling, especially when flying.