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Emily Agard

Lymphatic System Distribution & Structure of Lymphatic Vessels Lymphatic Capillaries • Begins in lymphatic capillaries which weave between tissue cells & blood capillaries • Wide-spread but absent from bones & teeth, bone marrow & CNS • Remarkably permeability due to 2 unique structural modifications: o Endothelial cells forming wall overlap loosely forming flap-like mini valves o Collagen filaments anchor endothelial cells to surrounding structures so any increase in interstitial fluid volume opens the mini valves • When fluid pressure in interstitial space > Pressure in lymphatic capillary, mini valve flaps open, allowing fluid to enter lymphatic capillary • When pressure greater inside lymphatic capillary, it forces endothelial mini valve flaps shut, preventing lymph from leaking back out as the pressure moves it along the vessel • Proteins unable to enter blood capillaries but can enter lymphatic capillaries easily • When tissues become inflamed, lymphatic capillaries develop openings that permit uptake of even larger particles (cell debris, pathogens, cancer cells) • Pathogens can use it to travel throughout the body • Threat resolved because lymph ‘detours’through lymph nodes, where it is cleansed & examined by immune cells • Special lymphatic capillaries (lacteals) transports absorbed fat from SI to bloodstream • Milky white lymph drays through theme — this fatty lymph called chyle drains from the fingerlike villi of the intestinal mucosa Larger Lymphatic Vessels • Lymph flows through larger & thicker walled channels — first collecting vessels & then largest, ducts • Collecting lymphatic vessels have the same 3 tunics but have thinner walls, more internal valves, and anastomose more • Lymphatics in the skin travel along with superficial veins, while the deep lymphatics of the trunk & digestive viscera travel with the deep arteries • Largest collecting vessels unite to form lymphatic trunks, which drain large areas of the body - lumbar, bronchomediastinal, subclavian, jugular &single intestinal trunk • Lymph is eventually delivered to 1 of 2 large ducts in the thoracic region • Right lymphatic duct drains lymph from right upper limb & ride side of head/thorax • Larger thoracic duct receives lymph from the rest of the body • It arises anterior to the first 2 lumbar vertebrae as an enlarged sac, the cisterna chyli that collects lymph from the 2 large lumbar trunks that drain the lower limbs and intestinal tract that drains the digestive organs • As thoracic duct runs superiorly, it receives drainage from left side of thorax, left upper limb, and left head • Each terminal duct empties lymph into venous circulation at the junction of internal jugular vein & subclavian vein on its own side of the body Lymph Transport • Lymphatic system lacks an organ that acts as a pump • Under normal conditions, lymphatic vessels are low-pressure conduits & same mechanisms that promote venous return in BVs act here— milking of skeletal muscles • Lymphatic vessels are usually bundled together in C.T sheaths along with blood vessels & pulsations of nearby arteries also promote lymph flow • Smooth muscle in the wall of all but the smallest lymphatic vessels contracts rhythmically helping to pump the lymph along • Even so, transport is sporadic and slow • Movement of adjacent tissues is extremely important in propelling lymph • When physical activity increases, lymph flows much more rapidly • Good reason to immobilize a badly infected body part to hinder flow of inflammatory material from that region LYMPHOID CELLS & TISSUES Lymphoid Cells • Lymphoid cells consist of immune cells found in lymphoid tissues together with the supporting cells that from ‘scaffolding’of tissues • Lymphocytes arise in bone marrow & mature into one of two — T cells or B cells — that protect against antigens • Activated T cells manage the immune response & some directly attack/destroy • B cells produce plasma cells, daughter cells that secreteAb+ into the blood, which mark antigens for destruction by phagocytes, etc. • Macrophages phagocytize foreign substances and help active T cells • Dendritic cells also do & they capture antigens and bring them back to lymph nodes • Reticular cells produce the reticular fiber stroma, which is a network that supports the other cell types in lymphoid organs and tissues Lymphoid Tissue • Lymphoid tissue (1) houses & provides proliferation site for lymphocytes and (2) furnishes an ideal surveillance vantage point for lymphocytes & macrophages • Lymphoid tissue dominates all the lymphoid organs except thymus • Macrophages live on the fibers of the reticular C.T network • Lots of lymphocytes squeeze through walls of post capillary venues coursing through this network and temporarily occupy its spaces • Then, they leave to patrol the body again • Cycling of lymphocytes b/w circulatory vessels, lymphoid tissues & loose C.T of the body ensures that lymphocytes reach infected/damaged sites quickly • Lymphoid tissues come in various ‘packages’ • Diffuse lymphoid tissue • Lymphoid follicles Lymph Nodes • Cluster along the lymphatic vessels of the body • There are
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