A&P The Lymphatic System

lymphatics

1. Network of lymphatic vessels

● Hundreds of nodes are found throughout body. Most are embedded deep in connective tissue in clusters along lymphatic vessels. Some are nearer to body surface in inguinal, axillary, and cervical regions of body where collecting vessels converge into trunks.

11. Where are lymph nodes found?

● Found everywhere except in bones, teeth, bone marrow, and CNS (CNS uses CSF for drainage).

4. Where is the lymphatic system found in the body? Where is it NOT found?

a. Reticular Connective Tissue

A type of loose connective tissue. i. Macrophages live on reticular fibers. Spaces between fibers offer a place for lymphocytes to occupy when they return from patrolling body.

1. Cleansing the lymph

Act as lymph "filters". Macrophages remove and destroy microorganisms and debris that enter lymph. Prevent unwanted substances from being delivered to blood.

○ Pharyngeal tonsil

Also called adenoids; located in posterior wall of nasopharynx.

a. Primary organs

Areas where T and B cells mature (red bone marrow and thymus). T and B cells originate in bone marrow, but only B cells matures there; T cells mature in thymus.

b. Secondary organs

Areas where mature lymphocytes first encounter their antigen and become activated. i. Nodes, spleen, MALT (mucosa-associated lymphoid tissue) and diffuse lymphoid tissues.

● Thymus

Bilobed lymphatic organ found in inferior neck. ● Extends into mediastinum and partially overlies heart.

● Lymphatic capillaries

Blind-ended vessels that weave between tissue cells and blood capillaries. ○ Absent from bones, teeth, bone marrow, and CNS (CNS uses CSF for drainage). ○ Similar to blood capillaries, but more permeable. ○ Can take up larger molecules and particles that blood capillaries cannot. ■ Example: Proteins, cell debris, pathogens, and cancer cells. ■ Can act as route for pathogens or cancer cells to travel throughout body. ○ Increased permeability due to two specialized structures. ■ Endothelial cells overlap loosely to form one-way minivalves. ■ Minivalves are anchored by collagen filaments to matrix, so increases in ECF volume opens minivalves even more. ● Decreases in ECF cause minivalves to close.

d. Dendritic Cells

Capture antigens and deliver them to lymph nodes; also help activate T cells.

b. Lymphocytes

Cells of the adaptive immune system; mature into one of two main types.

3. Lymph nodes

Cleanse lymph.

● Lymphangitis

Condition in which lymphatic vessels appear as painful red lines under the skin. ○ Caused by inflammation of larger lymphatic vessels that contain vaso vasorum. ■ Vaso vasorum become congested with blood. ○ Larger lymphatics, like blood vessels, receive their nutrients from branching vasa vasorum.

b. Cortex area

Cortex contains rapidly dividing lymphocytes (the bulk of thymic cells) and scattered macrophages.

a. Lumbar vessels

Drain lymph from the legs, the lower abdominal cavity, and the pelvic organs.

e. Intestinal trunk

Drains lymph from the abdominal viscera (internal abdominal organs).

c. Subclavian vessel

Drains lymph from the arms.

b. Bronchomediastinal vessel

Drains lymph from the chest

d. Jugular trunks

Drains lymph from the neck and head.

g. Thoracic duct

Drains rest of body. i. In about half of individuals, starts out as an enlarged sac, cisterna chyli.

f. Right lymphatic duct

Drains right upper arm and right side of head and thorax.

2. Lymph

Fluid in vessels

c. Lymph sinuses

Found throughout node. i. Consist of large lymphatic capillaries spanned by criss crossing reticular fibers. ii. Macrophages reside on fibers, checking for and phagocytizing any foreign matter.

h. Cisterna chyli

Is a dilated sac at the lower end of the thoracic duct which lymph from the intestinal trunk and two lumbar lymphatic trunks flow.

iv. Blood thymus barrier

Keeps immature T lymphocytes isolated from any antigens to prevent premature activation.

○ Palantine

Located at the posterior end of oral cavity. ■ Largest of tonsils and most often infected.

b. Diffuse Lymphoid tissue

Loose arrangement of lymphoid cells and some reticular fibers. i. Found in virtually every body organ. ii. Larger collections in lamina propria of mucous membranes.

○ Lingual tonsil

Lumpy collection of follicles at base of tongue

● Larger lymphatic vessels

Lymph capillaries drain into increasingly larger vessels called collecting lymphatic vessels. ○ Consist of collecting vessels, trunks, and ducts. ○ Have structures and tunics similar to veins, except: ■ Have thinner walls, with more internal valves. ■ Anastomose more frequently. ○ Collecting vessels in skin travel with superficial veins, but deep vessels travel with arteries.

b. Trabeculae

Lymph nodes contain trabeculae. i. Surrounded by external fibrous capsule. ii. Capsule fibers extend inward as trabeculae that divide node into compartments.

iv. MALT (mucosa-associated lymphoid tissue)

Lymphoid tissues in mucous membranes throughout body. 1. Protects from pathogens trying to enter body. 2. Found in mucosa of respiratory tract, genitourinary organs, and digestive tract. 3. Largest collections of MALT found in Tonsils, Peyer's patches, and Appendix. 4. MALT is populated by lymphocytes such as T cells and B cells, as well as plasma cells and macrophages, each of which is well situated to encounter antigens passing through the mucosal epithelium. In the case of intestinal MALT, M cells are also present, which sample antigen from the lumen and deliver it to the lymphoid tissue.

e. Stroma

Made up of epithelial cells, not reticular fibers. i. Provide environment in which T lymphocytes become immunocompetent.

i. T-Cells (T lymphocytes

Manage immune response, and some also attack and destroy infected cells.

c. Medullary areas

Medulla contains fewer lymphocytes and thymic corpuscles.

e. Medulla

Medullary cords extend inward from cortex and contain B cells, T cells, and plasma cells.

i. Stroma

Network-like support that acts as scaffolding for immune cells.

2. Immune system activation:

Offer a place for lymphocytes to become activated and mount an attack against antigens.

● Appendix

Offshoot of first part of large intestine. ○ Contains a large number of lymphoid follicles. ● Location aids in functions (like Peyer's patches). ○ Destroy bacteria, preventing them from breaching intestinal wall. ○ Generate "memory" lymphocytes.

c. Macrophages

Phagocytize foreign substances and help activate T cells.

ii. Lymph nodes

Principal secondary lymphoid organs of body. Hundreds of nodes are found throughout body. 1. Most are embedded deep in connective tissue in clusters along lymphatic vessels. 2. Some are nearer to body surface in inguinal, axillary, and cervical regions of body where collecting vessels converge into trunks.

ii. B-Cells (B lymphocytes

Produce plasma cells, which secrete antibodies.

e. Reticular Cells

Produce reticular fibers called stroma in lymphoid organs.

● Lymphedema

Severe localized edema. ○ Caused by anything that prevents normal return of lymph to blood. ■ Examples: Tumors blocking lymphatics or removal of lymphatics during cancer surgery. ○ Lymphedema may improve if some lymphatic pathways remain and enlarge.

vi. White pulp

Site where immune function occurs. 1. Contains mostly lymphocytes on reticular fibers. 2. White pulp clusters are found around central arteries. Appear as islands of white in a sea of red pulp.

vii. Red pulp

Site where old blood cells and bloodborne pathogens are destroyed. 1. Rich in RBCs and macrophages that engulf them. 2. Composed of splenic cords (reticular tissue) that separate blood-filled splenic sinusoids (venous sinuses).

c. Lymphoid follicles

Solid, spherical bodies consisting of tightly packed lymphoid cells and reticular fibers. i. Contain germinal centers of proliferating B cells. ii. May form part of larger lymphoid organs (nodes). iii. Isolated aggregations of Peyer's patches and in appendix.

○ Lacteals

Specialized lymph capillaries present in intestinal mucosa. Absorb digested fat and deliver fatty lymph (chyle) to the blood.

v. Trabeculae

Spleen also has trabeculae

iv. Capsule

Spleen is encased by fibrous capsule.

d. Cortex

Superficial area of cortex contains follicles with germinal centers that are heavy with dividing B cells. i. Deep cortex houses T cells in transit. T cells circulate continuously among blood, lymph nodes, and lymph. ii. Abundant numbers of dendritic cells are closely associated with both T and B cells. Play a role in activating both lymphocytes.

○ Tubal tonsils

Surround openings of auditory tubes into pharynx.

ii. Thymus

T cells mature here

d. Thymic corpuscles

Thymic corpuscles are where regulatory T cells develop. i. Regulatory T cells: Type of T cell that helps to prevent autoimmunity.

a. Thymic lobules

Thymus is broken into lobules that contain outer cortex and inner medulla.

i. Red Bone Marrow

Where T and B cells originate. Only B cells mature here.

iii. Spleen

Works as part of the lymphatic system to protect the body, clearing worn-out red blood cells and other foreign bodies from the bloodstream to help fight off infection.

23. Main function of the thymus?

a. Functions as lymphoid organ where T cells mature.

Immune system cells found in lymphoid tissue. ii. (2) Supporting cells that form lymphoid tissue structures.

a. Lymphoid cells consist of:

i. Served by splenic artery and vein, which enter and exit at the hilum.

a. The spleen is served by what artery and vein?

spleen

blood-rich organ about size of fist, located in left side of abdominal cavity, just below stomach. largest lymphoid organ. encased by fibrous capsule and also has trabeculae. iii. Histologically, consists of two components; White pulp and Red pulp.

h. Thymus differs from other lymphoid organs in important ways:

i. Has no follicles because it lacks B cells. ii. Does not directly fight antigens. iii. Functions strictly in T lymphocyte maturation. iv. Blood thymus barrier: Keeps immature T lymphocytes isolated from any antigens to prevent premature activation.

f. Afferent and efferent side and functions:

i. Lymph enters convex side of node via afferent lymphatic vessels. ii. Travels through large subcapsular sinus and then into smaller sinuses found throughout cortex and medulla. iii. Lymph then enters medullary sinuses. iv. Finally exits concave side at hilum via efferent lymphatic vessels. 1. Presence of fewer efferent vessels causes flow to somewhat stagnate; allows lymphocytes and macrophages time to function. 2. Lymph travels through several nodes.

spleen function

i. Site of lymphocyte proliferation and immune surveillance and response. ii. Cleanses blood of aged blood cells and platelets; macrophages remove debris. 1. Stores breakdown products of RBCs (e.g., iron) for later reuse. 2. Stores blood platelets and monocytes for release into blood when needed. 3. May be site of fetal erythrocyte production.

g. What lymphocytes are present?

i. T cells and lymphocytes (the bulk of thymic cells).

g. Subcapsular sinus

i. The space between the capsule and the cortex which allows the free movement of lymphatic fluid and so contains few lymphocytes. It is continuous with the similar lymph sinuses that flank the trabeculae.

f. Why aren't there follicles?

i. Thymus has no follicles because it lacks B cells.

13. Lymph node anatomy: a. Relative shape and size?

i. Vary in shape and size, but most are bean shaped. 1. Small, less than 2.5 cm (~1 inch).

1. Antibodies

mark antigens for destruction by phagocytosis or other means.

● Lymphatic vessels

offer a one-way system, ensuring lymph flows only toward heart.

2. T cells and B cells

protect against antigens (anything the body perceives as foreign).

● Lymphoid organs and tissues

provide structural basis of immune system by housing phagocytic cells and lymphocytes. Spleen, Thymus, Tonsils, Lymph nodes and other lymphoid tissues.

● Lymphatic system

returns fluids leaked from blood vessels back to blood

○ Lymphatic trunks

which are formed by union of largest collecting vessels, drain large areas of body. Named for regions of body they drain: ■ Paired lumbar. ■ Paired bronchomediastinal. ■ Paired subclavian. ■ Paired jugular trunks. ■ Single intestinal trunk.

20. What are Peyer's Patches?

● Clusters of lymphoid follicles in wall of distal portion of small intestine. ○ Also called aggregated lymphoid nodules. ● Structurally similar to tonsils. ● Location aids in functions. ○ Destroy bacteria, preventing them from breaching intestinal wall. ○ Generate "memory" lymphocytes.

14. What are buboes

● Inflamed, swollen, tender lymph nodes that result when nodes are overwhelmed by what they are trying to destroy. ○ Condition often referred to as swollen "glands". ○ Buboes are sometimes pus-filled. ○ Bubonic plague was named after chief clinical feature of this disease.

○ Milking action of skeletal muscle. ○ Pressure changes in thorax during breathing. ○ Valves to prevent backflow. ○ Pulsations of nearby arteries. ○ Contractions of smooth muscle in walls of lymphatics. ● Physical activity increases flow of lymph; immobilization of area keeps needed inflammatory material in area for faster healing.

● Lymph system is a low-pressure system like the venous system. ● Lymph is propelled by same mechanisms:

○ Lymphatic capillaries. ○ Larger lymphatic vessels.

● Lymph vessels (lymphatics) include

○ Lymphatic vessels (lymphatics): Elaborate network of drainage vessels. ○ Circulates ~ 3L interstitial fluid per day. ○ Once interstitial fluid enters lymphatics, it is called lymph.

● Lymphatic system returns interstitial fluid and leaked plasma proteins back to blood via:

24. What happens to the thymus as we age?

● Most active and largest in size during childhood. ● Stops growing during adolescence, then gradually atrophies. ● Still produces immunocompetent cells, though more slowly.

17. Tonsils

● Simplest lymphoid organs. ● Form ring of lymphatic tissue around pharynx. Appear as swellings of mucosa.

● Splenectomy

● The spleen has a thin capsule, so direct blow or severe infection may cause it to rupture, spilling blood into peritoneal cavity.Surgical removal of ruptured spleen. ○ Once standard treatment to prevent hemorrhage and shock, but has been discovered spleen can often repair itself. ○ Frequency of emergency splenectomies has decreased dramatically due to spleens ability to heal itself. ○ If spleen must be removed, liver and bone marrow take over most of its functions. ○ In children younger than 12, spleen will regenerate if a small part is left.

19. Main function of tonsils?

● Tonsils function is to gather and remove pathogens in food or air. ● Contain follicles with germinal centers and scattered lymphocytes. ● Are not fully encapsulated. ● Overlying epithelium invaginates, forming tonsillar crypts. ○ Bacteria or particulate matter enters crypts, where they are trapped and destroyed. ○ Risky to lure bacteria into tissues, but allows immune cells to become activated and build memory cells against these potential pathogens.