PHYS 4_ platelets
Platelets; Endothelium-Platelet Relationship, and Hemostatic Mechanisms
Page 3: Hemostasis Overview
Hemostasis refers to the arrest of bleeding or prevention of blood loss after a blood vessel injury.
Achieved through interaction of vessel wall, circulating platelets, and plasma coagulation proteins.
Consequences of Imbalance:
Inadequate hemostasis leads to bleeding.
Excessive hemostasis results in inappropriate clotting (thrombosis).
Page 4: Hemostasis Events
Hemostasis mechanisms include:
Vascular constriction.
Formation of a platelet plug.
Blood clot formation via coagulation.
Fibrous tissue growth into the clot to permanently close vessel openings.
Page 5: Vascular Contraction Mechanisms
After vessel injury, smooth muscles in the wall contract, reducing blood flow.
Contraction mechanisms include:
Local myogenic spasm.
Autacoid factors from traumatized tissues and platelets.
Nervous reflexes from sensory impulses from nearby tissues.
Page 6: Nervous Reflexes and Vasoconstriction
Vascular contraction occurs due to pain nerve impulses.
Increased vasoconstriction results from:
Local myogenic contraction.
Significantly traumatized vessels lead to more prolonged spasms.
Platelets release thromboxane A2, contributing to vasoconstriction.
Page 7: Formation of the Platelet Plug
Small blood vessel cuts may be sealed by a platelet plug instead of a full clot.
Page 8: Characteristics of Platelets
Platelets (thrombocytes) are 1 to 4 micrometers in diameter.
Formed from megakaryocytes in the bone marrow.
Normal platelet concentration: 150,000 to 450,000/μl.
Platelets have functional characteristics of whole cells despite lacking nuclei and reproduction ability.
Page 9: Cytoplasmic Components of Platelets
Platelet cytoplasm contains:
Contractile proteins (actin, myosin, thrombosthenin).
Endoplasmic reticulum and Golgi apparatus residues for enzyme synthesis and calcium storage.
Mitochondria and systems for ATP and ADP production.
Page 10: Additional National Components of Platelets
Include: 4. Enzymes for prostaglandin synthesis (local hormone reactions). 5. Fibrin-stabilizing factor related to blood coagulation. 6. Growth factors for cellular growth aiding in vascular wall repair.
Page 11: Platelet Granule Contents
Alpha Granules:
von Willebrand factor, thrombospondin, platelet-derived growth factor (PDGF).
Platelet activating factor (PAF), fibrinogen, coagulation factors V and XI.
Dense Granules:
Serotonin, ADP, calcium, ATP.
Page 12: Platelet Membrane Surface Characteristics
Glycoproteins on the surface facilitate adhesion to injury sites and prevent adherence to healthy endothelium.
However, glycoprotein receptors help in adhesion and aggregation of platelets
Key receptors include:
Collagen, ADP, von Willebrand factor (vWF), and fibrinogen.
Page 13: Role of Platelet Surface Receptors
Various receptor functions:
Gp Ib-IX-V: mediates vWF-dependent adhesion.
Gp IIb/IIIa: binds fibrinogen for aggregation; receptor for vWF and fibronectin.
Gp Ia-IIa: for collagen adhesion.
Page 14: Platelet Properties
The membrane contains phospholipids functioning in the clotting process.
Platelet lifespan: 8 to 12 days; removed from circulation primarily by the spleen's macrophages.
Page 15: Platelet Unique Properties
Three critical properties:
Adhesion: to exposed vessel wall collagen.
Aggregation: sticking to one another, facilitated by fibrinogen and Gp IIb-IIIa.
Mechanisms promoting aggregation include thrombin, ADP, and PAF.
Page 16: Activation and Release of Platelets
Activated platelets change shape and become spherical, leading to aggregation via thrombin and ADP.
Shape change due to cytoskeletal reorganization and contraction of actomyosin filament.
Page 17: Chemical Factors in Platelet Function
Exposure to collagen initializes platelet activation and plug formation.
Several factors mediate adhesion and aggregation (e.g., thromboxane A2, ADP).
Page 18: Mechanisms for Platelet Plug Formation
Mechanisms for sealing vascular openings rely on:
Rapid adhesion upon contacting collagen and vWF.
Shape change and release of activating factors that promote aggregation.
Page 19: Mechanism Overview for Platelet Plug Formation
Platelets change in morphology and become sticky, allowing further aggregation leading to plug formation.
Page 20: Formation of Fibrin Threads
After initial platelet plug formation, blood coagulation leads to the development of fibrin threads that reinforce the plug.
Page 21: Role of Platelets in Blood Coagulation
At the site of injury, activated platelets attract more platelets, forming a temporary, loose plug until fibrin solidifies it.
Page 22: Endothelial Regulation
Endothelial cells prevent premature platelet activation by releasing nitric oxide and prostacyclin, increasing cGMP and cAMP levels.
Injury exposes collagen and vWF, facilitating platelet adherence and activation.
Page 23: Importance of Platelet Mechanism for Closing Vascular Holes
Platelets can fuse with endothelial cells to seal minute ruptures vertically.
Page 24: Blood Clot Formation
Blood clot formation commences quickly after vessel injury, initiated by various activators releasing from the injured site, leading to coagulation.
Page 25: Clot Follow-up
After approximately 20-60 minutes, the clot retracts, consolidating the healing area.
Page 26: Fibrous Organization of Blood Clots
Clots can either:
Be invaded by fibroblasts forming connective tissue.
Dissolve over time via enzymatic action ( plasmin).
Page 27: Growth Factor Role in Clot Organization
Growth factors from platelets promote fibroblast invasion to facilitate tissue repair.
Page 28: Blood Coagulation Mechanism General Overview
More than 50 substances affect coagulation balances: procoagulants favoring clotting and anticoagulants inhibiting it.
Page 29: Clotting Factors
Factors involved in coagulation include prothrombin, fibrinogen and multiple others (see comprehensive table).
Page 30: Steps in Blood Coagulation
Formation of prothrombin activator following vascular damage.
Prothrombin is converted into thrombin.
Thrombin converts fibrinogen into fibrin forming the actual clot.
Page 31: Prothrombin to Thrombin Conversion
Requires prothrombin activator and calcium.
Page 32: Thrombin Action on Fibrinogen
Thrombin facilitates rapid polymerization of fibrinogen into fibrin, leading to clot formation.
Page 33: Prothrombin Details
Prothrombin is continuously synthesized by the liver; a deficiency results in bleeding tendency.
Page 34: Vitamin K Dependency
Essential for prothrombin activation and several clotting factors; deficiency leads to bleeding risks.
Page 35: Fibrinogen Role in Clot Formation
High molecular weight protein essential in coagulation; can leak into tissues under certain conditions.
Page 36: Thrombin on Fibrinogen
Converts fibrinogen to fibrin, initiating clot formation; early fibrin networks are weak.
Page 37: Fibrin Stabilizing Factor Activation
Fibrin stabilizing factor incorporates into forming clots, enhancing strength through cross-linked bonds.
Page 38: Composition of Blood Clot
A meshwork of fibrin fibers containing blood cells and platelets stabilizing the structure against leaks.
Page 39: Clot Retraction and Serum Expression
Clot retracts, expressing fluid called serum, which lacks clotting factors. Platelets play a critical role in this process.
Page 40: Platelet Contributions to Clot Stability
Platelets allow further cross-linking of fibrin fibers and enhance clot strength.
Page 41: Positive Feedback Mechanism of Clotting
Initial clot formation enhances subsequent clotting through thrombin's actions on other factors.
Page 42: Initiation of Coagulation
Involves trauma to tissue or blood, leading to prothrombin activator formation.
Page 43: Pathways for Prothrombin Activator Formation
Clotting occurs via intrinsic or extrinsic pathways, constantly interacting.
Page 44: Extrinsic Pathway Initiation
Starts with a tissue injury, releasing tissue factor.
Page 45: Steps in the Extrinsic Pathway
Tissue factor release.
Activation of factor X via factor VII and tissue factor interaction.
Page 46: Role of Factor X in Coagulation
Activated factor X creates prothrombin activator; thrombin is then generated.
Page 47: Positive Feedback Impact of Thrombin
Initiated clotting rapidly accelerates rate of clot formation and factor activation.
Page 48: Intrinsic Pathway Initiation
Triggered through blood damage and collagen exposure; cascades result in prothrombin activator formation.
Page 49: Early Intrinsic Pathway Steps
Activates factor XII and platelet phospholipids after trauma.
Page 50: Activation of Factor XI
Factor XI activated by factor XII, moving forward in the cascade.
Page 51: Further Intrinsic Pathway Actions
Factor IX activated for subsequent generation of factor X by factors VIII and platelet phospholipids.
Page 52: Common Pathway of Clotting Mechanisms
Similar actions as in extrinsic pathway lead to prothrombin activation and then thrombin production.
Page 53: Calcium Ions in Coagulation
Essential for all reactive steps in blood clotting cascades beyond the initial factors.
Page 54: Simultaneous Activation of Pathways
Extrinsic pathways can lead to rapid clotting, while intrinsic pathways are slower but equally important.
Page 55: Endothelial Surface Factors
Significance: Smooth surfaces prevent clotting; glycocalyx and thrombomodulin play crucial roles in anticoagulation.
Page 56: Anticoagulant Functions of Endothelial Cells
Maintains blood flow and prevents clotting through various mechanisms, including factor regulation.
Page 57: Thrombomodulin Role in Clot Control
Thrombomodulin binds thrombin, inactivating factors for anticoagulation regulation.
Page 58: Damaged Endothelium and Clotting Activation
Loss of endothelial integrity accelerates intrinsic pathway activation resulting in clot formation.
Page 59: Endothelial Production of Inhibitory Factors
Post-injury reduction in release factors like prostacyclin and NO leads to platelet aggregation.
Page 60: Antithrombin III and Fibrin Action
Vital in removing thrombin during clotting; includes factors that enhance anticoagulation and stability.
Page 61: Fibrin Fiber Adsorption of Thrombin
Adsorbed thrombin prevents excessive clotting; regulates thrombin activity after clot formation.
Page 62: Heparin's Role in Blood Anticoagulation
Heparin functions as a powerful anticoagulant, enhancing the activity of antithrombin III significantly.
Page 63: Heparin Mechanism Interaction
Interaction with antithrombin further promotes rapid removal of activated coagulation factors.
Page 64: Role of Mast Cells in Heparin Production
Heparin is produced by various cells, preventing thrombus formation especially in the lungs and liver.
Page 65: Plasmin's Role in Clot Dissolution
Plasminogen is converted to plasmin in tissue healing, enabling clot lysis.
Page 66: Mechanism of Plasmin Activation
Plasminogen trapped in clots when activated, enabling gradual tissue recovery and vessel reopening.
Page 67: Summary of Fibrinolytic Pathway
Fibrinolysis occurs post-clotting, crucial for maintaining blood flow in microvessels.
Page 68: Fibrin Degradation Products
Breakdown of fibrin leads to smaller fragments measurable in blood work for clotting assessment.
Page 69: Protein C and Fibrinolysis Regulation
Activated protein C plays a crucial role in regulating fibrinolysis and maintaining hemostatic balance.
Page 70: Blood Coagulation Conditions
Multiple deficiencies can lead to excessive bleeding, particularly relevant in vitamin K and specific coagulation factors.
Page 71: Conditions Causing Excessive Bleeding
Common causes: vitamin K deficiency, hemophilia, and thrombocytopenia, leading to bleeding disorders.
Page 72: Vitamin K Deficiency Overview
Associated with liver disease and can lead to elevated bleeding risks due to reduced coagulation factor production.
Page 73: Hemophilia Overview
Genetic condition affecting males, most commonly due to factor VIII deficiency leading to clotting impairment.
Page 74: Thrombocytopenia Defined
Results from low platelet counts; important in managing small vessel bleedings and total body hemorrhage risks.
Page 75: Diagnosing Thrombocytopenia
Low platelet count correlates with risk levels and affects surgery outcomes.
Page 76: Blood Coagulation Proficiency Tests
Measurement techniques for bleeding time, clotting time, and INR standards to assess coagulation status.
Page 77: Bleeding and Clotting Time Tests
Essential tests in identifying disorders related to coagulation factor deficiencies.
Page 78: Prothrombin Time Measurement
Prothrombin time indicates factors status in clotting; measured using specific laboratory procedures.
Page 79: International Normalized Ratio (INR)
Standardizes prothrombin time results for consistent assessment regardless of testing methods.
Page 80: Functionality of Other Clotting Factor Tests
Key tests assess concentrations of other clotting factors, assuring proper hemostatic responses.
Page 81: Primary Hemostasis Mechanisms
Initial responses to injury involve primary platelet plug formation crucial for vascular integrity.
Page 82: Secondary Hemostasis Formation
Continues the coagulation response, completing vascular repair mechanisms and stabilizing the initial plug.
Page 83: Thrombin Action and Fibrin Formation
Functions in secondary hemostasis include thrombin actions on fibrinogen generating fibrin for clot stabilization.
Page 84: Summary of Hemostasis Mechanisms
Overall processes involve a delicate balance of factors ensuring efficient clot structure and prevention of bleeding.