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
• Returns interstitial fluid and leaked plasma proteins back to the blood
• Lymph – interstitial fluid once it has entered lymphatic vessels

Lymphatic System: Overview

Figure 20.2a

Lymphatic System: Overview

Figure 20.1a

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
• Do not allow lymph to escape from the capillaries

Lymphatic Capillaries

Figure 20.1b

Lymphatic Capillaries

• During inflammation, lymph capillaries can absorb:
• Cell debris
• Pathogens
• Cancer cells
• Cells in the lymph nodes cleanse and “examine” this debris
• Lacteals – specialized lymph capillaries present in intestinal mucosa
• Absorb digested fat and deliver chyle to the blood

Lymphatic Collecting Vessels

• Have the same three tunics as veins
• Have thinner walls, with more internal valves
• Anastomose more frequently
• Collecting vessels in the skin travel with superficial veins
• Deep vessels travel with arteries

Lymphatic Trunks

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

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Lymphatic Trunks

Figure 20.2b

Lymph Transport

• The lymphatic system lacks a pumping organ
• Vessels are low-pressure conduits
• Uses the same methods as veins to propel lymph:
• Pulsations of nearby arteries
• Contractions of smooth muscle in the walls of the lymphatics

Lymphoid Cells

• Lymphocytes are the main cells involved in the immune response
• Two main varieties:
• T cells
• B cells

Lymphocytes

• T cells and B cells protect the body against antigens
• Antigen – anything the body perceives as foreign
• Bacteria and their toxins; viruses
• Mismatched RBCs or cancer cells

Lymphocytes

• T cells
• Manage the immune response
• Attack and destroy foreign cells
• B cells
• Produce plasma cells, which secrete antibodies
• Antibodies immobilize antigens

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
• Immune system activation – monitor for antigens and mount an attack against them

Structure of a Lymph Node

• Nodes are bean shaped and surrounded by a fibrous capsule
• Trabeculae extended inward from the capsule and divide the node into compartments
• Nodes have two histologically distinct regions: a cortex and a medulla

Figure 20.4a

Structure of a Lymph Node

Figure 20.4a, b

Lymph nodes from class

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
• Only lymph nodes filter lymph

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
• Immune surveillance and response
• Cleanses the blood

Additional Spleen Functions

• Stores breakdown products of RBCs for later reuse
• Spleen macrophages salvage and store iron for later use by bone marrow
• Site of fetal erythrocyte production (normally ceases after birth)
• Stores blood platelets

Structure of the Spleen

Figure 20.6a, b

Thymus

• A bilobed organ that secretes hormones (thymosin and thymopoietin) that cause T lymphocytes to become immunocompetent
• Size of the thymus varies with age:
• In infants, it is found in the inferior neck and extends into the mediastinum where it partially overlies the heart
• It increases in size and is most active during childhood
• It stops growing during adolescence and then gradually atrophies
• The thymus differs from other lymphoid organs in important ways
• It functions strictly in T lymphocyte maturation
• It does not directly fight antigens

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Thymus

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
• 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, forming blind-ended crypts
• Crypts trap and destroy bacteria and particulate matter

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, preventing them from breaching the intestinal wall
• Generate “memory” lymphocytes for long-term immunity

MALT

• MALT – mucosa-associated lymphatic tissue:
• Peyer’s patches, tonsils, and the appendix (digestive tract)
• Lymphoid nodules in the walls of the bronchi (respiratory tract)
• MALT protects the digestive and respiratory systems from foreign matter