Lymphatic Vessels and Capillaries

Lymphatic Vessels

• Lymphatic vessels contain no pump
• Fluid flows from high to low pressure
• Lymphatic vessels have lower pressure than blood vessels
• Lymph flows unidirectionally
• Lymph flows toward the heart
• Blood flows both ways
• Lymph carries fats and proteins etc that are introduced into circulation

Function of Lymphatic Vessels

• Some proteins made by cells are dumped into the lymphatic vessels and then the circulatory system
• Fats and large molecules go into lymphatic vessels before entering circulation

Lymphatic Capillaries

• Lymphatic capillaries are dead end vessels that do not have lymphatic beds
• They are located throughout the body
• Lymphatic capillaries are formed by overlapping endothelial cells, possessing only one tunic
• They are similar to but unlike blood capillaries do not have tight junctions
• These cells are all lightly linked together by collagen fibers
• The pressure of interstitial fluid causes the cells to open to allow fluid to move in forming a mini-valve system

Fluid Transport

• Fluid flows into the lymphatic capillary from interstitial space
• Fluid flows from high to low pressure
• Lymph moves back towards the heart due to interstitial pressure
• Lymphatic capillaries does not occur in bones, teeth, or the nervous system but are everywhere else

Lacteals

• Lacteals are lymphatic capillaries of the intestines
• Lacteals absorb certain fats, proteins and large molecules
• The body does not absorb proteins via the digestive system as they are broken down into amino acids
• Lymph in lacteals takes on the name "chyle"
• Lacteals increase nutrient absorption in the small intestine and deliver nutrients to the body

Lymphatic Collecting Vessels

• Lymphatic collecting vessels have three tunics similar to veins
• The tunics are thinner because of lower pressure
• Lymphatic collecting vessels have more valves than veins because of the low pressure
• These valves also prevent backflow

Lymphatic Trunks

• Five major lymphatic trunks are named based on drainage location:
• Bronchomediastinal drains the chest and thorax
• Subclavian drains the arms
• Jugular drains the head and neck
• Lumbar drains the lower torso and legs
• Intestinal drains the abdominal cavity

Lymphatic Ducts

• The right lymphatic duct drains fluid from head, arms, and upper thorax, and does not fight gravity
• The thoracic duct receives fluid from south of the right lymphatic duct
• The thoracic duct originates at the cisterna chyli
• The cisterna chyli is a swelling in the lumbar region
• Both connect to veins where the pressure is lower to allow for the introduction of lymph into the circulatory system

Lymph Flow

• The order of lymphatic transport is: Lymphatic Capillary, Collecting Vessel, Trunk, then Duct (CVTD)

Factors Assisting Lymphatic Vessels

• The high number of valves prevent backflow
• Skeletal muscle contraction squeezes lymphatic vessels
• Physical activity increases the flow of lymph
• Moderate exercise can help process lymph faster and increase recovery rate
• Respiratory pump
• Pressure changes in chest cavity when we breathe allow for regulation of lymph fluid
• Arterial pumping
• Lymphatic vessels near an artery will feel the effect from blood pressure against the wall
• Helps move the lymph fluid
• Smooth muscle contraction
• Tunica media in lymphatic vessels helps to move lymph

Lymphocytes

• Lymphocytes are B and T cells
• B cells acquire immunocompetence in bone marrow
• They become activated when something foreign binds to them
• Activated B cells produce antibodies that tag foreign substances without killing them
• T cells acquire immunocompetence in the thymus
• They lyse cells when they bind to something
• Lysis refers to piercing holes in the plasma membrane and killing the cell

Named for Where They Become Immunocompetent

• Immunocompetence is when a cell can recognize a foreign substance
• Immunocompetence does not mean that something happens, only recognition
• A cell must bind to a foreign pathogen to be activated
• B cells tag free floating pathogens in lymph and blood
• T cells attack damaged cells, which could be infected or improperly dividing

Other Lymphatic Cells

• Macrophages are phagocytic cells that were monocytes, and can activate T cells
• Dendritic cells are another type of phagocytic cell and can also activate T cells
• Reticular cells don't attack, but secrete reticular fibers instead
• Reticular fibers form a stroma, and holds all the other lymphatic cells in place

Lymphoid Tissue

• Lymphoid tissue contains germinal centers, which are also called germination centers
• Germinal centers contain large amounts of stroma with macrophages and dendritic cells attached to the fibers, and lymphocytes are suspended

Lymphatic Organs

• Lymph Node:
• Most common lymphatic organ and found throughout the human body (thousands)
• Contains lymphoid tissue
• Found along lymphatic pathways, with high concentrations in: Inguinal region, armpit, and neck
• Spleen:
• Largest lymphatic organ located behind the stomach in the abdominal cavity
• Requires a major blood supply
• Blood is filtered in the spleen, and then returns to the blood vessels