BIOL!421: Chapter 21 (The Lymphatic System)

YOUTUBE VIDEO: THE LYMPHATIC SYSTEM OVERVIEW ANIMATION
The lymphatic system is a drainage system that removes excess fluid from body tissues, and returns it to the bloodstream.
-subsystem of both the circulatory system and immune system

Blood plasma containing nutrients moves out of the capillaries at the arterial end of capillary beds, while tissue fluid containing wastes reabsorbs back into the bloodstream at the venous end.
-however, not all of the fluid is drawn back to the bloodstream
-15% is left in the issues, and would cause swelling if accumulated
-lymphatic system picks up this excess fluid, and returns it back to the circulatory system

Lymphatic system is a one-direction, open-ended network of vessels
-lymphatic vessels begin as lymphatic capillaries made of overlapping endothelial cells, which contain flaps that function as a one-way valve
-when fluid accumulates in the tissue, interstitial pressure increases and pushes the flaps inward; as a result, this opens large gaps between the cells and allows fluid to flow into the lymphatic vessels
-as pressure inside the capillaries increases, the endothelial cells are pressed back outwards and closes the gap (which prevent backflow)
-large gaps allow bacteria, immune cells (macrophages), and other large particles to enter bloodstream

Once inside the lymphatic vessels, the recovered fluid is called lymph
-lymph flow is enabled by the same forces that faciliate blood flow in the veins
-lymph flow starts from capillaries → larger lymph capillaries, and encounters lymph nodes, which filters the fluid before it enters the blood stream
-lymph nodes: small bean-shaped structures scattered throughout the lymphatic network, and are most prominent in areas where vessels converge

Lymph nodes contain macrophages and dendritic cells that directly “swallow up” any pathogen (like bacteria or viruses), and lymphocytes (T cells and B cells involved in adaptive immune response)
-adaptive immune responses: process responsible for producing activated lymphocytes and antibodies specific to the invading pathogen)
-lymphocytes travel through lymphs to be distributed wherever they are needed to fight infections

Lymphatic system includes primary lymphoid organs like thymus and bone marrow, which are the sites of lymphocyte production, maturation, and selection.
-selection: process in which lymphocytes learn to recognize and destroy pathogens, without attacking the body’s own cells
-mature lymphocytes eventually leave the primary organs and travel to the secondary lymphoid organs like the lymph nodes, spleens, and lymphoid nodules (this is where they encounter pathogens and become activated)

LYMPHATIC SYSTEM OVERVIEW
The lymphatic system consists of the thymus, spleen, network of lymph vessels, ducts, and nodes.
-protects the body by removing pathogens

Tissues in the body are surrounded by lymphatic vessels and capillaries of the circulatory system.
-as water and substances are exchanged between tissues and bloodstream, unwanted substances enter the lymphatic network and travel towards nodes

Nodes are filled with lymphocytes, which are the body’s defense cells
-these cells support immunity by eliminating pathogens that can cause infections
-clean lymphatic fluid travels → thoracic cavity, → reenters bloodstream

LYMPHATIC SYSTEM STRUCTURES / FUNCTIONS
-structures of the lymphatic system: consists of lymph vessels and lymphatic tissues (spleen, thymus, lymph nodes)
-thymus located above the heart

-functions of the lymphatic system: regulates body’s immune responses by producing and distributing lymphocytes, which are cells that combat disease
-lymphatic system drains from lower regions to upper regions, valves push the fluids upwards and out

LYMPHATIC CIRCULATION / LYMPHATIC VESSELS AND VEINS / DUCTS
Lymph circulates throughout the body
-it begins as interstitial fluid between cells → filters into lymphatic capillaries → flows into larger vessels and trunks → eventually returns back to venous blood

-lymph empties into bloodstream from thoracic duct and right lymphatic duct, which is located at the junction between the subclavian and internal jugular veins
-lymph: interstitial fluid containing bacteria, cellular debris, plasma proteins, and other cells

-thoracic duct (or left lymphatic duct) collects lymphs from the left upper body and entire body beneath the ribs; begins at the cisterna chyli
-right lymphatic duct collects lymphs from the right upper body
-both pathways return to subclavian veins
(subclavian vein → superior vena cava → right atrium of the heart)

LYMPH TRUNKS
Lymph trunks are major lymphatic vessels that empty into the thoracic and right lymphatic ducts. (before entering ducts)
-includes jugular trunks and nodes, subclavian trunks, bronchomediastinal trunks, lumbar trunks, and intestinal trunk and nodes

LYMPHOCYTE PRODUCTION (B CELLS AND T CELLS)
B cells and T cells are lymphocytes that develop in red bone marrow inside spongy bone.
-their main production and maturation sites are in vertebrae, sternum, ribs, pectoral girdle, pelvic girdle (maturation: having receptors for antigens)
-after maturing, they enter bloodstream and colonize lymphatic tissues (spleen and lymph nodes); some stay in bloodstream until needed
-function of B cells: lymphocytes that produce antibodies

T cells are lymphocytes that mature in the thymus.
-after maturing, they enter bloodstream and colonize lymphatic tissues (spleen and lymph nodes); some stay in blood until needed
-function of T cells: lymphocytes that destroy infected cells

-spleen and liver are where they are stored
-bone marrow and thymus are where they mature

SPLEEN
Spleen contains macrophages, that consume abnormal cells in order to contribute to immune response.
-spleen also stores iron from blood cells

-spleen is surrounded by dense connective tissue capsule, trabeculae strands from the capsule extend into the spleen
-spleen is composed to two types of lymphatic tissue (white pulp contains the lymphocytes, white blood cells and surrounds central artery to make macrophage process easier; and red pulp surrounds venus sinus of blood and also filters out red blood cells)
-spleen is responsible for immune surveillance of blood and removal of damaged blood cells and platelets

LYMPH NODE DISTRIBUTION/HISTOLOGY
Lymph nodes are clustered in the torso area, groin area, and intestine area.
-function: filters lymph before it reenters bloodstream
(lymph enters node through afferent vessels and passes through a network of chambers including the trabecula, medillary sinuses)
-filtered lymph leaves the node through efferent vessels
-medulla (in the center) has activated lymphocytes, cortex have nonactivated lymphocytes
-germinal centers contain macrophages and

PHYSICAL DEFENSES
The skin and mucous membranes, first line of defense, provide physical barrier to invading pathogens
-tears, saliva, urine, and gastric juices also aid in the destruction and removal of microbes and bacteria

INNATE IMMUNITY / INNATE IMMUNE RESPONSE
Innate immunity is a fast and general defense against invading pathogens, does not have specific pathogens for specific infections
-once infection occurs, blood vessels dilate and white blood cells pass through pores and enter the infected tissues
-some of the white blood cells turn into macrophage phagocytes to consume the invading microbes
-other white blood cells don’t turn into macrophage and simply “hug” the microbe to kill it

-has physical barriers to pathogens
-can create inflammation of skin to fight infection

-white blood cells can pass through pores, red blood cells cannot pass through pores due to their larger size

PHAGOCYTOSIS
When bacteria or other pathogens are present in the body, certain white blood cells become macrophage phagocytes in order to remove pathogens
(phagocytosis: cell process that protects the body by removing pathogens)
-macrophage encases the microbe, and releases vacuoles filled with lysosomes that fuse together with the microbe to digest and kill the pathogen

TYPES OF WHITE BLOOD CELLS
White blood cells (leukocytes) are components of blood that defends the body against disease
-their numbers increase during infection or inflammation
-if greater number of WBC, assumption is that body is fighting infection

-70% of WBC is neutrophils with hugging mechanism
-lymphocytes are 2nd most common (B cells produce antibodies and attaches to pathogens to kill them, T cell kills cell and attaches to our cells and pathogen and kills both)
-eosinophil act as antihistamine, whereas basophil produce histamine
-monocyte develop into macrophage, to fight against infection

ADAPTIVE IMMUNITY
Cells in the body can fight unknown invaders by adapting an antibody response or a T cell mediated response.
-B cell is activated and divides into plasma cells and memory cells, plasma cell releases antibodies that eliminate the pathogen, memory B cells stay on the body to prepare for next invasion
-T cell contains receptors that attaches to antigen, and destroys both our cell and pathogen (makes memory of antigen, so that it can recognize it)

B CELLS
B cells develop and mature in red bone marrow.
-then become active in the spleen, lymph node, blood stream
-they produce antibodies (that recognize antigens on microbes and act as tags that identify the invaders)

T CELLS
T cells develop in red bone marrow and mature in the thymus.
-then become active in the spleen, lymph node, blood stream
-once activated, they seek out and destroy infected cells

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