Lymphatic System

Chapter 14

Lymphatic System

• Consists of 2 semi-independent parts:– Lymphatic vessels• Transport any fluids back to the blood that have

escaped the blood’s vascular system

– Lymphoid tissues & organs• House phagocytic cells & lymphocytes which aid in the

body’s defense mechanisms & resistance to disease

Lymphatic Vessels

• Also known as lymphatics

• An elaborate system of drainage vessels that collect the excess protein-containing interstitial fluid & return it to the bloodstream

• The fluid in the lymphatics is called lymph

Distribution & Structure of Lymphatic Vessels

• Vessels form a one-way system that transports lymph toward the heart

• Transport system begins in microscopic lymphatic capillaries – Weave between the tissue cells & blood capillaries in the

loose connective tissues of the body– Widespread; absent from bones & teeth, bone marrow,

& entire CNS

Distribution & Structure of Lymphatic Vessels

• Lymphatic capillaries are extremely permeable• This is due to:– Endothelial cells forming the walls aren’t tightly joined,

overlap each other loosely– Collagen filaments anchor the endothelial cells to

surrounding structures• Any increase in interstitial fluid volume opens the the

minivalves

Distribution & Structure of Lymphatic Vessels

• From the lymphatic capillaries, lymph flows through successively larger & thicker-walled channels

• 1st collecting vessels• 2nd lymphatic trunks• 3rd the largest vessels - ducts

Distribution & Structure of Lymphatic Vessels

• Lymphatic collecting vessels have the same 3 tunics as veins

• Are thinner-walled, have more valves, & anastomose more

• Lymphatics travel along with superficial veins in the skin

Distribution & Structure of Lymphatic Vessels

• Lymphatic trunks are formed by the union of the largest collecting vessels & drain fairly large areas of the body

• Named mostly for the body regions from which they collect lymph– Paired lumbar, bronchomediastinal, subclavian,

jugular trunks– Single intestinal trunk

Distribution & Structure of Lymphatic Vessels

• Lymph is eventually delivered to one of two large ducts in the thoracic region

• Right lymphatic duct – Drains lymph from the right upper arm and the

right side of the head & thorax• Thoracic duct– Larger than the right lymphatic duct– Receives lymph from the rest of the body

Lymphoid Cells• Lymphocytes are made in red bone marrow

• Mature into one to the two main varieties of immunocompetent cells– T cells (T lymphocytes)– B cells (B lymphocytes)

These protect the body from antigens - anything the body perceives as foreign Ex) bacteria, viruses, cancer cells

Lymphoid Cells

• Activated T cells:– Manage the immune response– Some directly attack & destroy infected cells

• B cells:– Protect the body by producing plasma cells– Daughter cells secrete antibodies into the blood• Immobilize antigens until they can be destroyed by

phagocytes or other means

Lymphoid Cells

• Lymphoid macrophages:– Phagocytizing foreign substances – Activate T cells

Lymphoid Tissue

• Composed mostly of a type of loose connective tissue called reticular connective tissue

• All of the lymphoid organs & cells are made of this type of tissue EXCEPT the thymus

• Macrophages live on the fibers of the reticular network

• Lymphocytes are in the spaces of the network

Lymphoid Tissue

• Lymphocytes squeeze through the postcapillary venules to reside temporarily in the lymphoid tissue

• They then leave to patrol the body for antigens and return once they are done

• This cycling of lymphocytes ensures that they reach infected or damaged sites quickly

Lymphoid Tissue• Comes in various “packages”

• 1) Diffuse lymphatic tissue – consists of a few scattered reticular tissue elements

• 2) Lymphoid follicles – a solid, spherical bodies consisting of tightly packed reticular elements & cells– Found forming part of larger lymphoid organs like lymph

nodes

Lymphoid Tissue

• 3) Germinal centers – follicular dendritic cells & B cells reside here– Provide a site for B cells to divide & produce

plasma cells

Lymph Nodes

• Principal lymphoid organs• Filters lymph • Large clusters occur near the body surface in

inguinal, axillary, & cervical regions– Where lymphatic collecting vessels converge to

form trunks

Lymph Nodes

• 2 basic functions– Lymph filters macrophages remove & destroy

microorganisms & other debris

– Help activate the immune system lymphocytes monitor the lymphatic stream for the presence of antigens & mount an attack against them

Other Lymphoid Organs

• Spleen, thymus, tonsils & Peyer’s patches of the intestine

• Spleen– Largest lymphoid organ (size of a first)– Extracts aged & defective blood cells & platelets– Macrophages remove debris & foreign matter

from blood flowing through it

Other Lymphoid Organs

• Thymus primary functions during early life– Secretes thymopoietin & thymosins causing T

lymphocytes to become immunocompetent– Immunocompetent: function against certain

pathogens in the immune response– Prominent in newborns & highly active in first year

of life

Thymus

• Differs from other lymphoid organs in 2 ways:

– Does not directly fight antigens– The stroma of the thymus consists of epithelial

cells rather than reticular fibers

Other Lymphoid Organs

• Tonsils simplest lymphoid organs, form a ring of lymphatic tissue around the entrance of the pharynx

• Palatine tonsils – located on either side at the posterior end of the oral cavity– Largest of the tonsils & ones most often infected

Types of Tonsils• Lingual tonsils – at the base of the tongue

• Tubal tonsils – surround the openings of the auditory tubes into the pharynx– Gather & remove many of the pathogens entering the

pharynx in food or in inhaled air

• Pharyngeal tonsils – referred to as adenoids if enlarged– In the posterior wall of the nasopharynx

Other Lymphoid Organs

• Peyer’s patches– Large isolated clusters of lymphoid follicles– Located in the wall of the distal portion of the

small intestine– Heavily concentrated in the wall of the appendix

Peyer’s Patches & Appendix

• In an ideal position to:– 1) destroy bacteria in the intestine– 2) generate many “memory” lymphocytes for

long-term immunity