Chapter 20

The Lymphatic System

G.R. Pitts, J.R. Schiller, and James F. Thompson, Ph.D.

Use the video clip: CH 20 - Lymph Node Anatomy for a review of lymph node structure

The Lymphatic System• Basic organization

– Lymph fluid in lymph vessels– Structures: organs with lymphatic tissue, red

bone marrow, liver and spleen

• Functions– return interstitial fluid and proteins to the

blood– transport dietary fats to adipose tissue – protect against cancer & infection

• resistance - fight off disease– nonspecific resistance - general protection against

disease– immunity - specific protection

• susceptibility - lack of resistance

Lymph Flow• More fluid moves out of the blood

capillaries by filtration than returns by reabsorption - Starling’s Law

• ≈ 3Ll day of lymph is generated

• Proteins escaped from the blood or secreted by tissues are transferred back to the blood by the lymphatics

• Lymph flow is facilitated by muscle pumps, the respiratory pump, valves, and smooth muscle (in the walls of the trunks & thoracic duct)

• Smaller vessels drain into larger vessels

Lymphatic Capillaries• “Blind ended,” covered vessels between cells, larger

than capillaries• Not found in avascular tissues (CNS, cartilage) nor in

the splenic pulp, and bone marrow

Lymphatic Capillaries• Structure/Function regulates fluid flow

– Anchoring filaments - from lymphatic endothelium attach to surrounding tissues

– Endothelial cells overlap• high hydrostatic fluid pressure separates cells, fluid into caps• hydrostatic fluid pressure in cap prevents fluid movement out

minivalve

Lymph Flow Follows Venous

Circulation

• Lymph vessels have the same organization and routing as the vascular tree

Lymphatic Flow (cont.)

• Lymphatic vessels have no separate pump (heart)

• All lymph returns to the vena cava and to the right side of the heart

Lymphatic Flow (cont.)• Lymph ducts

– Right lymphatic duct • about ½ inch long • drains lymph from

upper right side of body (arm & head)

– Thoracic (left) duct • main collecting duct

of the lymphatic system

• 38-45 cm long• drains 75% of body• begins as a dilation

known as the cisterna chyli located anterior to lumbar disk #2

Lymphatic Flow (cont.)

• Lymph returns to the venous drainage through right and left lymphatic ducts at the junction of the internal jugular and subclavian veins

Summary of Lymphatic Vessels

• Lymph Flow from smallest to largest:

• Capillaries vessels trunks ducts

• Lymph vessels anastomose and supply and drain lymph nodes along their course

Two Main Types Of Lymphocytes

• B lymphocytes = B cells– attack microbes, especially bacteria– develop into plasma cells to produce antibodies (Ab)

• bind to antigen to form antibody-antigen (Ag-Ab) complexes• complexes prevents Ag from interacting with other body cells

or molecules• memory B cells – dormant until future exposure to Ag

• T lymphocytes = T cells– regulate many immune responses– attack viruses, fungi, transplants, cancer, some

bacteria• 4 types of T cells

– cytotoxic (killer) T cells - destroy foreign invaders – helper T cells - assist B cells and cytotoxic T cells– suppressor T cells – help bring immune response to an

end– memory T cells - dormant until future exposure to Ag

Lymphocyte Development• primary lymphatic organs - site of

lymphocyte (B cell and T cell) production– bone marrow - produces B cells,

immature T cells– T cells migrate to the thymus gland to

mature or die if determined to be improper

• secondary lymphatic organs – sites of activated immune responses– lymphatic nodules (lymph follicles) – lymph nodes, spleen, tonsils

Other Lymphoid Tissue Cells

•Macrophages & Dendritic cells–Phagocytize foreign substances and cells–Transport them to lymphatic tissues–Process foreign things into individual antigens

–Present Ags to T & B lymphocyte to help activate them

•Reticular cells–Similar to fibroblasts–Produce reticular fibers (stroma) that provide the framing structure for other cells in lymphoid organs

Lymphatic Tissue - General• Stroma of reticular

connective tissue (except thymus)

• Parenchyma of macrophages, B and T lymphocytes, occasional other leukocytes

• May or may not have a connective tissue capsule

Lymphatic Organs – Thymus Gland

• Thymus Gland – Two lobes between the

sternum and the heart– Thymocytes produce hormones– Atrophies with age (starting ~20)

• Structure/Function– Outer cortex – immature T cells

• screened for functional capacity• stimulated to proliferation • stimulated to maturation

– Inner medulla• defective T cells degenerate • mature T cells move into blood

Lymphatic Organs – Lymph Nodes

•Anatomy– oval, bean shaped small

structures scattered throughout body along lymph vessels

– may be deep or superficial

– concentrated along the respiratory tree and GI tract, in the mammary glands, axillae, and groin

– filter lymph fluid to trap foreign organisms, cell debris, and tumor cells

Circulation in the Lymph Nodes• Lymph enters via a number of

afferent lymphatic vessels• It then enters a large

subcapsular sinus and travels into a number of smaller sinuses

• It meanders through these sinuses and exits the node at the hilus via efferent vessels

• The node acts as a “settling tank,” because there are fewer efferent vessels, lymph stagnates somewhat in the node

• This allows lymphocytes and macrophages time to carry out their protective functions

Only lymph nodes filter lymph!

Cancer Metastasizes To Lymph Nodes

Cancer cells from the tumor are first trapped in a lymph node

Lymphatic Organs - Spleen• largest lymphoid organ

in the body• fibrous capsule with

arteries, veins, and efferent lymph vessels

• located between stomach and diaphragm

Spleen Functions: White Pulp

• a site of immune surveillance and response• macrophages phagocytize bacteria, worn-out RBC's,

platelets• hemoglobin is recycled and components transferred to

liver• macrophage antigen-presentation and lymphocyte

activation and proliferation• some B cells mature into plasma cells

Spleen Functions: Red Pulp• Site of fetal erythrocyte

production (normally ceases after birth)

• Stores ~ 1 L of blood which can be released during an emergency (hemorrhage)

Unencapsulated Lymphatic Tissue

• Diffuse lymphatic tissue– Small scattered patchs– In nearly every organ

• Lymphoid follicles (nodules)– More organized, more

cellular clusterssmallbronchus

Mucosa-Associated Lymphatic Tissue (MALT)

found in the lamina propria of mucous membranes of the GI tract, respiratory tract, urinary tract, and reproductive

tract

Peyer’s patchesin the intestines

Lymphatic Organs - Tonsils• lymphoid tissue

beneath the mucosae

• tonsilar crypts trap micro-organisms

• immune cells destroy the micro-organisms

• palantine tonsils – largest and most frequently infected

Lymphadenopathy

Enlarged lymph nodes due to increased drainage from inflammatory lesions or infections.

Associated with malignant and nonmalignant diseases.

Lymphocyte leukemia with severe lymphadenopathy

Hodgkin’s Disease:one of thesignificant malignant lymphomas

Cervicallymph nodes

Burkitt’s Lymphoma

Commonly found in central Africa and New Guinea.

Associated with Epstein-Barr virus which causes infectious mononucleiosis in North America and Europe.

Rarer American type has extensive marrow replacement.

Cancerous cell is a B lymphocyte.

End Chapter 20