The Role of Lymphatic System in Cellular Nutrition and Immunity

Every single cell in our body tissues and organs needs nutrition and clearing away of its waste products for survival and vital functioning. Lymphatic system plays a vital role in the circulation and regulation of interstitial fluid or tissue fluid. As the blood passes through blood capillaries in the tissues; plasma or tissue fluid oozes out through the porous walls of blood capillaries. The tissue fluid or the interstitial fluid fills the spaces or interstices between the cells of different tissues and organs. The blood circulates only through the blood vessels but the tissue fluid circulates through the actual tissue and carries food, oxygen and water from the blood stream to each individual cell and carries away its waste products like carbon dioxide, water and urea and pours out all these in the blood stream for final disposal. Lymphatic system is pump less system and runs parallel to the circulatory system and is comprised of following components.

Components of the Lymphatic System:

1. Lymphatic capillaries: These are hair like fine vessels in the spaces in the tissues and gather up excess fluid from the tissues. Lymphatic capillaries unite to form lymphatic vessels.
2. Lymphatic vessels: These are similar to veins in structure but carry lymph instead of blood. They are finer and more in number than the veins and are provided with unidirectional valves, to prevent the back flow of lymph or the tissue fluid. Lymphatic vessels are present in all tissues except the central nervous system. These run in the subcutaneous tissue and pass through one or more lymphatic nodes.
3. Lymph nodes or lymphatic nodes: Lymph nodes are numerous in number and vary in size from a pinhead to an almond. Lymphatic vessels which bring lymph to them are called afferent vessels. afferent vessels divide up within the node and discharge the lymph into the mesh of the lymph node. The lymph is collected again into a fresh vessel known as efferent vessel, which ultimately empties into a lymph duct. Lymph nodes consist of cells similar to white blood cells and are encapsulated by connective tissue. Lymph nodes filter out bacteria, provide fresh lymphocytes for the circulation and also produce some antibodies and antitoxins and boost up immunity.
4. Lymphatic ducts: These are major lymph channels. There are two lymphatic ducts, the thoracic duct and the right lymphatic duct. The thoracic duct is larger and all the lymphatic vessels from the lower limbs, and abdominal and pelvic organs empty into it. The thoracic duct empties into the left subclavian vein. The right lymphatic duct is comparatively small vessel formed by union of lymphatic vessels from the right side of the head, thorax and the right upper limb at the root of the neck. The right lymphatic duct is about one centimeter long and empties into the right subclavian vein.
5. Spleen, the master lymphatic organ: The spleen is the largest nodule of the lymphoid tissue. It is deep purplish red in color and lies high up at the back of the abdomen, on the left side behind the stomach and is enclosed in a capsule of connective tissue. It is composed of fibrous meshwork filled with pulp like material known as splenic pulp. It is a source of fresh lymphocytes for the blood stream, an area for the destruction of worn red blood cells (RBCs) and a legendary organ for fighting out circulatory infections.

Functions of Lymphatic System:

1. Restoration of constant stream of fresh interstitial fluid or lymph in the interstitial spaces as depicted in the diagram given below:

2. Regulation excess proteins in the tissue fluid and passing that back to the blood stream.
3. The lymph nodes filter out the bacterial infection and harmful substances from the lymph before pouring it back into the blood stream.
4. Lymphatic vessels in the abdominal organs assist in the absorption of digested fat.
5. Lymph nodes also produce fresh lymphocytes for the circulation.

Disseminated Intravascular Coagulation: Pathophysiology and Diagnosis

Disseminated intravascular coagulation (DIC) should be recognized as consumptive coagulopathy since it is not a primary disease. It is always a complication of an underlying disease that not only triggers it but also fuels it. Disease or trauma associated tissue injury with a release of thromboplastic material into the circulation is the major cause of DIC. The clotting system as well as the fibrinolytic system (bleeding system) are involved in the pathophysiology of disseminated intravascular coagulation. Clinically, coagulopathy could be recognized as acute hemorrhagic DIC and subacute or chronic DIC. A third type of consumptive coagulopathy could be recognized with fibrinolysis. Disseminated intravascular coagulation is an acquired coagulation disorder in which formation of microthrombi, consumption of coagulation factors, activation of fibrinolysis and a bleeding tendency may occur consecutively or simultaneously. In brief, it is a systemic pathologic process characterized by a disseminated (generalized) activation of clotting and/or fibrinolytic systems in the circulatory system of the patient. The common pathway of all inciting causes (independent of etiologies) is the formation of thrombin and plasmin (fibrinolysin).

Thrombin plays a vital role in DIC. The alterations of coagulation system detected in the laboratory during DIC reflect the multiple actions of thrombin. Thrombin cleaves fibrinogen to release fibrinopeptide-A (FPA) and fibrinopeptide-B (FPB). Subsequently the remaining fibrin monomers may combine with fibrinogen and circulate as soluble fibrin monomer complexes (SFMC) or polymerize to form fibrin microthrombi. Thrombin also activates factor XIII (fibrin stabilizing factor) to form factor XIIIa, and the factor XIIIa creates bridges, linking any two adjacent monomers of fibrin. Thrombin activates procoagulant cofactors, factors VIII and V, to participate in the process of its own generation. Thrombin also plays a regulatory role by activating protein-C, which acts as an anticoagulant to inactivate factors VIIIa and Va. In brief, thrombin alone accounts for decreased levels of fibrinogen and factors II, V, VIII & XIII and decreased count of platelets in patients with DIC.

Screening tests for DIC are: Prothrombin time (PT), Partial thromboplastin time (PTT), Fibrinogen assay and Platelet count. Platelet count, PT, Fibrinogen assay and Determination of Antithrombin-III (AT-III) should always be done to diagnose consumptive DIC.

Confirmatory tests for DIC are: Fibrin monomer assay (it measures thrombin cleaved fibrinogen), Detection of fibrin split products (i.e. detection of plasmin-cleaved fibrinogen or fibrin) and Detection of D-dimer (i.e. detection of plasmin-cleaved cross-linked fibrin). Activation of coagulation could be assessed by the detection of soluble fibrin monomer complexes(SFMC). Detection of fibrinogen degradation products (FDPs) is indicative of reactive fibrinolysis.

Medical conditions which may lead to 'Acute Hemorrhagic DIC':

• Infections: Typhoid fever, Gram-positive and Gram-negative septicemia, viremia, parasites etc.
• Tissue injury: Renal allograft rejection, snake bite, heat stroke, brain injury, crush injury, necrotizing enterocolitis, hemolytic transfusion reaction etc.
• Malignancy: Acute promyelocytic leukemia.
• Obstetric: Amniotic fluid embolism, eclampsia, abruptio placentae, hypertonic saline abortion.
• Other causes: Severe liver disease.

Medical conditions which may lead to 'Subacute Chronic DIC':

• Vascular: Chronic renal disease, connective tissue disorders, venous thrombosis, arterial embolization, pulmonary embolus etc
• Obstetric: Retained dead fetus.
• Malignancy: Mucin-producing adenocarcinomas.