The Lymphatic SystemChapter 20

Figure 20.1

The Lymphatic System

Lymphatic System - Series of vessels, tissues and organs performing 2 major functions:

1. Vessels return to the blood any fluids and proteins that leaked into tissues

2. Tissues and organs house cells for immune functioning

Figure 20.1

Lymphatic Vessels

Lymphatic vessels – the series of vessels draining lymph, returning it to the blood in a unidirectional flow

Lymph – the protein-rich, interstitial fluid that has entered the lymphatic vessels

Lymphatic capillaries – smallest vessels near capillary beds that intake lymph

• Minivalves – flap-like openings between endothelial cells making lymphatic capillaries highly permeable

• Collagen filaments – anchor endothelial cells to surrounding connective tissue, encourages minivalve opening

Figure 20.1

Lacteals – specialized lymphatic capillaries of the small intestine that absorb lymph that is high in fat content, called chyle

Other Lymphatic Vessels:

Lymphatic capillaries merge to form lymphatic collecting vessels, which merge to form lymphatic trunks, which merge to form lymphatic ducts, which drain into venous circulation

• The larger vessels have tunics and valves similar to veins

Lymphatic Vessels

Figure 20.2a

Lymphatic Vessels

Lymph Transport

• A unidirectional, low-pressure flow returning protein-rich fluid and absorbed fats to the blood (about 3L/day)

• No pump – lymph transport relies on body movements and the respiratory and muscular ‘pumps’ seen in veins

Figure 20.3

Lymphatic Tissue

Lymphoid cells:

Lymphocytes – the T cells and B cells that fight disease-causing pathogens

Plasma cells – specialized B cells that produce antibodies

Macrophages – phagocytize any foreign cells or debris

Dendritic cells – specialized phagocytes with cellular projections

Reticular cells – produce the web of reticular fibers (stroma) that support the lymphoid cells

Figure 20.3

Lymphatic Tissue

Lymphatic Tissue:

• Lymphatic tissue - largely reticular connective tissue, it houses large populations of the lymphoid cells on a stroma of reticular fibers

• Diffuse lymphatic tissue – scattered reticular tissue found in and around all body organs

• Lymphatic follicles (nodules) – dense masses of reticular tissue, often with a lighter germinal center of dendritic and B cells

Figure 20.4

Lymph Nodes

Lymph nodes – the most abundant lymphoid organs, scattered amongst the lymphatic vessels, and clustered in the inguinal, axillary, and cervical regions

• Filter lymph of foreign cells and debris via phagocytes

• Activate the immune system by stimulating lymphocytes

Figure 20.4a

Lymph Nodes

Lymph node structure:

• bean shaped, 2.5 cm in length

• Capsule – dense fibrous connective tissue surrounding for shape/support

• Trabeculae – connective tissue walls dividing the node into compartments

• Cortex – outer layer of lymphoid follicles

• Medulla – central region with medullary cord filled with B and T cells

• Sinuses – spaces allowing lymph movement through the node

• Afferent vessels – bring lymph into the node

• Efferent vessels – drain lymph from the node

Figure 20.5

Lymphatic Organs

• Lymph nodes – most abundant lymphoid organs, and the only ones to filter lymph

• Other lymphoid organs do NOT filter lymph, include the tonsils, thymus, spleen, appendix, and Peyer’s patches

Figure 20.6

The Spleen

Spleen

• Largest lymphoid organ, in left side of abdomen

• Capsule – dense fibrous surrounding, prone to rupture

• White pulp – regions performing immune surveillance via lymphocytes and macrophages

• Red pulp – area of enlarged veins and macrophages performing erythrocyte breakdown

• Also stores platelets and salvages iron

Figure 20.7

The Thymus

Thymus – a glandular structures in the lower neck that does not contain reticular tissue, but still functions as a lymphoid organ

• Produces hormones used in the activation of T lymphocytes, peak activity in childhood, then deteriorates after puberty

• Thymic corpuscles – specialized areas for T cell destruction

Figure 20.8

Tonsils

Tonsils – a group of simple lymphoid organs encircling the throat that gather and remove potential pathogens that entered the mouth or nose

Tonsillar crypts – invaginations that trap bacteria and other debris, allowing lymphoid cells in the nearby follicles to destroy them

Figure 20.9

M.A.L.T.

Peyer’s patches – clusters of lymphoid follicles in the wall of the small intestine

Appendix – a cluster of follicles hanging from the start of the large intestine

M.A.L.T (mucosa-associated lymphatic tissue) – the appendix, Peyer’s patches, and tonsils all line the mucosa of the digestive tract, trapping and destroying bacteria and generating immunological memory