4.1 - Anatomy of the Lymphatic System

Lymphatic system

Consists of lymph, lymphatic vessels, lymphoid tissue, and lymphoid organs; closely associated with the cardiovascular system

Functions of the lymphatic system

Takes up excess tissue fluid (lymph) and returns it to the bloodstream; lymphatic capillaries absorb fats and transport them to the blood; assists cells associated with immunity in defending the body against disease

Lymph

Excess tissue fluid not returned via the venules of the cardiovascular system that enters lymphatic capillaries

Direction of lymph flow

One way, from the lymphatic capillary system to the subclavian veins, where it joins venous circulation to return to the heart

Interstitial fluid

Fluid between the cells where tissue fluid begins; most, but not all, is returned to the heart via the veins of the cardiovascular system

Lymphatic capillaries

Take up fluid not returned through the veins; closely connected to the capillaries of the cardiovascular system; flow into lymphatic collecting vessels

Fluid taken up by lymphatic capillaries

Plasma fluid forced out of circulatory capillaries under great pressure that has not been reabsorbed; it bathes the cells and assists capillaries in delivering glucose, oxygen, salts, amino acids, and other nutrients

Path of lymph from capillaries

Lymphatic capillaries, then larger lymphatic vessels, eventually emptying into the venous blood of the cardiovascular system

Lymphatic (lymph) vessels

Extend throughout most sections of the body; have one-way flow valves similar in structure to the large veins of the cardiovascular system

Areas that typically lack lymphatic vessels

Places with no blood vessels, such as the bone marrow and epidermis

Function of lymphatic vessel valves

Prevent the backward flow of lymph

What drives return of lymph to circulation

Solely the squeezing action of skeletal muscles, which squeezes fluid one way through the lymphatic vessels

Lymphatic trunks

Formed when large lymphatic vessels combine; drain lymph from specific regions of the body and converge into larger vessels

Lymphatic ducts

Receive lymph from the lymphatic trunks and return it to venous circulation in the subclavian veins

Lymphoid organs

Lymph nodes, spleen, thymus gland, tonsils, red bone marrow, appendix, and MALT (mucosa-associated lymphoid tissue)

Tissues of lymphoid organs

Contain lymphocytes (T cells and B cells), dendritic cells, and macrophages that specialize in fighting diseases that enter the body

Lymph nodes

Hundreds found at junctions of lymphatic vessels; act as a filtering center that rids flowing lymph of infectious organisms and other debris

Sinuses of a lymph node

Cortical, subcapsular, and medullary; lymph passes through these in series

Spleen

Extracts old or defective blood cells and platelets; removes debris, foreign matter, bacteria, viruses, and toxins from blood; contains lymphocytes and macrophages that carry out its functions; curves around the left side of the stomach

Thymus gland

Located on the anterior surface of the heart (in the thorax); secretes thymosin and thymopoietin, which enable T cells to mature and function in the immune system

Thymosin and thymopoietin

Hormones secreted by the thymus that enable T lymphocytes to mature and function as part of the immune system

Mature T cells

Attack body cells that are cancerous or infected with pathogens

Thymus changes over the lifespan

Prominent in newborns and largest and most active during childhood; growth stops during adolescence; withers to a mass of fibrous-fatty tissue by old age

Tonsils

Group of small lymphoid organs in the lateral, posterior portions of the throat; gather and destroy bacteria inhaled in air or food, which work their way into lymphoid tissue where most are destroyed

Appendix

Tube connected to the cecum of the large intestine; contributes to immune function by storing beneficial gut bacteria and generating lymphocytes to combat infections

Red bone marrow

Site where all lymphocytes (T cells and B cells) develop, from immature cells to stem cells

Maturation of T cells

Develop in the red bone marrow, then mature in the thymus

Maturation of B cells

Develop in the red bone marrow and continue to mature in the bone marrow

Peyer's patches

Clusters of MALT in the intestines containing WBCs that assist in pathogen identification and destruction in the digestive system

MALT

Mucosa-associated lymphoid tissue; Peyer's patches are clusters of it in the intestines