• LYMPHATIC SYSTEM

• Importance of the Lymphatic System

• Two most important functions
  • Maintenance of fluid balance in the internal environment.
  • Immunity
• Importance of the lymphatic system in maintaining fluid balance.
  • Plasma filters into the interstitial spaces from blood flowing through the capillaries.
  • Much of this interstitial fluid is absorbed by tissue cells or reabsorbed by the blood before it flows out of the tissue.
    • A small amount of interstitial fluid is left behind.
      • If this would continue over even a brief period of time, the increased interstitial fluid would cause massive edema.
  • This edema would causes tissue destruction or death
  • This problem is avoided by the presence of lymphatic vessels that act as "drains" to collect the excess fluid and return it to the venous blood just before it reaches the heart.
• The lymphatic system is a specialized component of the circulatory system.
  • Consists of a moving fluid (lymph) derived from the blood and tissue fluid and a group of vessels (lymphatics) that returns lymph to the blood.
  • Lymphatic vessels parallel veins.
• OH-21.2 A,B The Lymphatic System includes
  • Lymph nodes located along the paths of collecting vessels.
  • Isolated nodules of lymphatic patches in the intestinal wall.
  • Specialized lymphatic organs such as the tonsils, thymus, and spleen.
• The lymphatic system differs from the circulatory system in that the lymphatics do not form a closed ring or circuit.
  • Instead, begin blindly in the intercellular spaces of the soft tissues of the body.

• Lymph and Interstitial Fluid

• Definitions:
  • Lymph--clear, watery fluid found in the lymphatic vessels.
  • Interstitial fluid--fluid that fills the spaces around cells.
    • In some cases, it is part of the semifluid ground substance.
    • In others, it is bound water in a gelatinous ground substance.

• Distribution of Lymphatic Vessels OH-Lymphatic Vessels, OH_The Lymphatic System

• Originate as microscopic blind-end vessels called lymphatic capillaries.20825-26

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  • Those originating in the villi of the small intestine are called lacteals.
  • Wall of the lymphatic capillary consists of a single layer of flattened endothelial cells.
    • Networks of lymphatic capillaries are located in the intercellular spaces and are widely distributed throughout the body.
  • Lymphatic capillaries merge with other lymphatics to eventually form the main lymphatic trunks.OH-Routes for Drainage
    • Right lymphatic ducts
      • Receives lymph from the right upper quadrant and drains into the right subclavian vein.
    • Thoracic duct 18062
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  • Lymph from the entire body except for the right upper quadrant drains into the thoracic duct which drains into the left internal jugular vein.
  • Originates as a dilated structure the cisterna chyli in the lumbar region of the abdominal cavity. 20820
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  • Has an average diameter of 5 mm.

• Structure of Lymphatic Vessels

• Lymphatic vessels resemble veins in structure with these exceptions:
  • Lymphatics have thinner walls.
  • Lymphatics contain more valves.
  • Lymphatics contain lymph nodes located at certain intervals along their course.

• Functions of Lymphatic Vessels

• Lymphatics play a critical role in homeostasis.
  • The high degree of capillary permeability permits large molecular weight substances which cannot be absorbed by the blood capillary to be removed from he interstitial spaces.
    • Proteins that accumulate in the interstitial spaces can return to the blood only via lymphatics
      • Above is of great clinical importance.
    • If anything blocks lymphatic return, blood protein and blood osmotic pressure fall below normal and fluid imbalance and death will result.
      • Lacteals (lymphatics in the villi of the small intestine) serve an important function in the absorption of fats and other nutrients
    • The lymph found in lacteals after digestion contains 1-2% fat and is called chyle

• Circulation of Lymph OH-Relationship of Lymphatic System to Cardiovascular
• System 20827

• Water and solutes continually filter out of capillary blood into interstitial fluid.
  • To balance this outflow, fluid continually reenters blood from the interstitial fluid.
• Each day about 50% of the total blood proteins leak out of the capillaries into the tissue fluid and return to the blood via the lymphatic vessels.
• OH-Routes for Drainage of Lymph From lymphatic capillaries, lymph flows through progressively larger lymphatic vessels to eventually reenter blood at the junction of the internal jugular and subclavian veins. 20831

• The Lymphatic Pump

• Although there is no muscular pumping organ connected with the lymphatic vessels to force lymph onward as the heart forces blood, still lymph moves slowly and steadily along its vessels.
  • Lymph flows through the thoracic duct and reenters the general circulation at the rate of 125 mL/hour.
    • Occurs despite the fact that most of the flow is against gravity or "uphill".
  • It moves through the system in the right direction because of the large number of valves that permit fluid flow only in one direction.
  • The movement is due to breathing movements and skeletal muscle contractions.

• Activities that result in central movement or flow are called lymphokinetic actions.
  • X-rays show that lymph pours into the central veins most rapidly at the peak of inspiration.
• The mechanism of inspiration, resulting from the descent of the diaphragm, causes intraabdominal pressure to increase as intrathoracic pressure decreases.
  • This simultaneously causes pressure to increase in the abdominal portion of the thoracic duct and to decrease in the thoracic portion.
  • Research has shown that thoracic duct lymph is literally "pumped" into the venous system during inspiration.
    • The rate of flow of lymph into venous circulation is proportional to the depth of inspiration.
  • The total volume of lymph that enters the central veins during a given time period depends on both the depth of inspiration and the overall breathing rate.
• Contracting skeletal muscles also exert pressure on the lymphatics to push the lymph forward.

a. During exercise, lymph flow may increase as much as 10-15x.

• In addition, segmental contraction of the walls of the lymphatics themselves, results in lymph being pumped from one valve segment to the next.
• Other pressure generating factors:
  • Arterial pulsations
  • Postural changes
  • Passive compression (massage) of the body soft tissues.

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