Unit 7 - Hemostasis

HEMOSTASIS

  • Definition: The process of turning free-flowing blood into a stationary clot.

BLOOD COMPONENTS

  • Blood consists of:

    • Liquid component: Plasma

    • Cellular Elements:

    • Erythrocytes (Red Blood Cells)

    • Leukocytes (White Blood Cells)

    • Thrombocytes (Platelets)

    • Buffy Coat: Contains most WBCs and platelets when spun down in EDTA blood tubes.

CELLULAR PARTICIPANTS IN CLOTTING

  • Responsible cells for forming a clot:

    • Canine Blood: Contains RBCs and platelets (Source: Eclinpath - Cornell University College of Veterinary Medicine)

TESTS AND ANALYSIS OF BLOOD

  • Spinning down EDTA blood tubes shows three layers:

    • Red Blood Cells (45%)

    • Plasma (55%)

    • Platelets (<1%)

POIETINS AND THEIR ROLES

  • Glycoprotein hormones involved in blood cell production:

    • Erythropoietin: Signals for RBC production.

    • Leukopoietin: Signals for WBC production.

    • Thrombopoietin: Signals for platelet (thrombocyte) production, mostly produced in the liver.

THROMBOPOIESIS

  • Definition: The process by which platelets are created.

  • Involves multiple steps leading to the production of Megakaryocyte, from which platelets are ‘pinched off’.

PLATELETS (THROMBOCYTES)

  • Definition: The smallest functional cellular element in peripheral blood.

  • Lifespan: 10-14 days in peripheral blood.

  • Senescence: Old platelets are removed by phagocytic cells in the spleen.

FUNCTIONS OF PLATELETS

  • Roles:

    • Form a platelet plug to stop small bleeds.

    • Create a framework for fibrin to adhere during secondary hemostasis.

    • Contain substances that can activate or be released to initiate the coagulation cascade.

HEMOSTASIS SYSTEMS

  • Mechanisms to stop bleeding and prevent hemorrhage involve:

    1. Blood Vessels

    2. Platelets

    3. Plasma Coagulation Proteins

    4. Plasma Fibrinolytic Proteins

PRIMARY HEMOSTASIS

  • Involves the role of the platelets and blood vessels:

    • Function: Forms a plug (akin to a bandaid) to stop bleeding.

  • Steps:

    • Platelets adhere to the defect when blood vessels are damaged.

    • Requires Von Willebrand’s Factor (vWF) to bind platelets to the damaged endothelium.

STEP-WISE PROCESS OF PRIMARY HEMOSTASIS

  1. Adhesion:

    • Platelets bind to the damaged vessel lining due to substances released from the endothelium.

  2. Release:

    • Contents of granules are released by activated platelets.

  3. Aggregation:

    • Platelets attract more platelets to the site of injury.

  4. Contraction:

    • Platelets contract, forming a tight seal.

SECONDARY HEMOSTASIS

  • Definition: Stabilizes the platelet plug by forming a fibrin mesh, acting as cement.

  • Requires calcium as a major cofactor for coagulation.

  • Two systems involved:

    1. Extrinsic Pathway:

    • Factors are partially found in tissue; fibrin operates outside the blood vessel.

    1. Intrinsic Pathway:

    • All components are found within the blood; fibrin forms inside the vessel.

COMMON PATHWAY FOR CLOTTING

  • Results in the formation of fibrin.

  • The original clot is initially friable and requires Factor XIII (Fibrin Stabilizing Factor) for stabilization.

ROLE OF CALCIUM

  • Essential for the coagulation process.

  • Anticoagulants like EDTA work by binding calcium to prevent clot formation.

FIBRINOLYSIS

  • Definition: The breakdown of the fibrin-based clot.

  • Components: A group of plasma proteins called the Fibrinolytic system become active post coagulation.

  • Starts at the edges of the clot, ensuring vessels do not become clogged with clots.

COAGULATION TESTS

  • Key tests to evaluate coagulation include:

    1. Complete Blood Count (CBC) for platelet count.

    2. Buccal Mucosal Bleeding Time (BMBT).

    3. Prothrombin Time (PT).

    4. Activated Partial Thromboplastin Time (aPTT).

    5. Activated Clotting Time (ACT).

SPECIFIC TESTS

  • BMBT: Tests platelet function; normal results in

    • Dogs: < 4 minutes

    • Cats: < 3.3 minutes.

  • PT: Evaluates extrinsic and common coagulation; focused on assessing Factor VII.

  • aPTT: Evaluates intrinsic and common coagulation; prolonged results indicate potential issues in these pathways.

CLINICAL INDICATORS OF COAGULATION DISORDERS

  • Petechiae: Small red spots due to minor bleeding; observed in mucous membranes and skin.

SYMPTOMS OF PATIENTS WITH COAGULATION DISORDERS

  • Nosebleeds (epistaxis), blood in stool (melena), and petechiae indicate potential clotting disorders.