Lymphatic System and Urinary System

• The ureters transport urine from the kidneys to the bladder.
• Lymphatic vessels exchange interstitial fluid and blood, including nutrients, wastes, and gases
• Hydrostatic and colloid osmotic pressures at capillary beds force fluid out of blood, with most being reabsorbed and the rest becoming part of interstitial fluid
• Lymphatic vessels carry fluid and plasma that escaped back to the blood to ensure the cardiovascular system has enough blood
• Lymphatic vessels have a one-way system where lymph flows towards the heart
• Lymph capillaries are found between tissue cells and blood capillaries in loose connective tissues, absent from bones, teeth, bone marrow, and the central nervous system
• Lymph capillaries are permeable due to endothelial cells being loosely joined and having fine collagen filaments anchoring them
• Proteins enter lymphatic capillaries as they cannot enter blood capillaries
• Lacteals are special lymphatic capillaries that absorb digested fats from the intestine
• Lymphatic vessels get bigger and thicker walled, starting in capillaries and progressing to collecting vessels, trunks, and ducts
• Lymph is delivered to either the right lymphatic duct or the thoracic duct in the thoracic region
• Lymphatic vessels transport lymph similar to how blood vessels transport blood, with pressure changes and valves to prevent backflow
• Smooth muscle in walls of lymphatic trunks and thoracic duct contract rhythmically to help pump the lymph
• Lymph transport is sporadic and slower than veins, with surrounding tissues moving causing lymph to move
• Anything blocking the return of lymph to blood causes severe localized edema or lymphedema
• Lymphocytes are the main cells in the immune system, produced in red bone marrow and become T or B lymphocytes
• B lymphocytes produce plasma cells that secrete antibodies into blood or other body fluids to protect against antigens
• Macrophages and dendritic cells phagocytize foreign substances and activate T cells
• Lymphoid tissue houses and provides a site for lymphocytes to proliferate, made up of reticular connective tissue
• Diffuse lymphoid tissue lacks a capsule and is found in virtually every body organ
• Lymphatic follicles lack a capsule and are solid, spherical bodies with germinal centers, found within larger lymphoid organs like lymph nodes
• Lymph nodes filter lymph, remove microorganisms and debris, and activate the immune system
• Lymph nodes have a cortex with packed follicles containing germinal centers and a medulla
• Cortex:

  • Packed follicles
  • Most follicles have many germinal centers with many B cells
  • Dendritic cells nearly enclose the follicles and border the deeper part of the cortex
  • Deeper part of the cortex primarily houses traveling T cells that circulate between blood, lymph nodes, and lymphatic stream
• Medulla:

  • Medullary Cords:
  • Thin inward extensions from cortical lymphoid tissue
  • Contain both types of lymphocytes and plasma cells
  • Defines the Medulla
  • Lymph Sinuses:
  • Large lymph capillaries that crisscross reticular fibers
  • Macrophages live on reticular fibers and phagocytize foreign matter in the lymph
  • Some lymph bearing antigens in the filtering leak into the surrounding lymphoid tissue where lymphocytes launch an immune attack
• Circulation in the Lymph Nodes:

  • Lymph enters the convex side of a lymph node via afferent lymphatic vessels
  • Moves through the subcapsular sinus into multiple smaller sinuses that cut through the cortex and enter the medulla
  • Lymph flows like a river through the sinuses and exits the node at its hilus through efferent lymphatic vessels
  • The flow of lymph through the node stagnates due to fewer efferent vessels that drain the node than afferent vessels that feed it, allowing time for lymphocytes and macrophages to carry out their function
• • Lymph passes through several nodes before it is cleaned
• Homeostatic Imbalance:

  • Lymph Nodes can be overwhelmed by antigens, leading to inflammation, swelling, and tenderness (swollen glands)
  • Infected lymph nodes can be called Buboes
  • Lymph nodes can become secondary cancer sites, particularly in metastasizing cancers that enter lymphatic vessels and get trapped there
  • Cancer-infiltrated lymph nodes are swollen to help distinguish cancerous lymph nodes from those not infected
• Other Lymphoid Organs:

  • Common feature of all lymphoid organs is their composition of reticular connective tissue
  • Spleen:
  • Size of a fist, largest lymphoid organ
  • Located in the left side of the abdominal cavity beneath the diaphragm
  • Site for lymphocyte proliferation and immune surveillance and response
  • Cleans Blood
  • Stores some breakdown products of RBCs for later use and releases blood for processing by the liver
  • Site of erythrocyte production in the fetus
  • Stores blood platelets
  • Contains both lymphocytes and macrophages
• Thymus:

  • Important during early years
  • Secretes hormones thymosin and thymopoietin which cause T lymphocytes to become immunocompetent
  • Size varies per age, big in newborns, increases in size during childhood, stops growing by adolescence
  • Thymus can be compared to a cauliflower, with outer cortex and inner medulla
  • Thymus lacks B cells and strictly functions in T lymphocyte maturation
• Tonsils are a Ring of lymphatic tissue around the entrance to the throat

  • Types: Palatine, Lingual, Pharyngeal, Tubal
  • Tonsils gather and remove many pathogens entering the throat through air or food
  • Tonsils contain lymphoid tissue with germinal centers surrounded by a few lymphocytes
  • Tonsil masses aren't fully enclosed and trap bacteria and particulate matter in crypts
• Aggregates of Lymphoid Follicles:

  • Peyer’s Patches are large isolated clusters of lymph follicles found in the wall of the lower part of the small intestine
  • Lymphoid follicles heavily concentrated in the wall of the appendix
  • Peyer’s patches, appendix, and tonsils are part of Mucosa-Associated lymphatic tissue (MALT) that protects the digestive and respiratory tracts from antigens entering
• Developmental Aspects of the Lymphatic System:

  • Beginnings of lymphatic vessels and main clusters of lymph nodes are apparent from the fifth week of embryonic development
  • Arose from budding of lymph sacs from growing veins
  • Lymphatic system organs grow from the mesoderm, except for the thymus which is an endodermal derivative
  • Mesodermal mesenchymal cells migrate to differentiate into lymphatic system components
• Mesodermal mesenchymal cells migrate to different body sites and develop into reticular tissue
• Thymus is the first lymphoid organ to appear
• Forms as an outgrowth of the lining of the early throat
• Detaches and migrates towards the back into the thorax
• Becomes infiltrated with immature lymphocytes from the hematopoietic tissues
• Lymphoid organs are poorly developed before birth, but shortly after birth, they are heavily populated by lymphocytes
• Development parallels the maturation of the immune system