Introduction to Hemostasis and Platelet Function

Hemostasis

A series of cellular and humoral systems balancing blood flow within circulation; the cessation of bleeding.

Vasoconstriction

Blood vessels constrict to reduce blood flow.

Platelet Activation

Platelets adhere, aggregate, and secrete granules to form a platelet plug.

Coagulation Enzyme Activation

Plasma enzymes interact to form an insoluble fibrin clot.

Fibrinolysis

Plasmin digests fibrin to remove clots and prevent blockage.

Thrombosis

Formation of a clot in a blood vessel; a thrombus.

Emboli

Foreign objects like clots or air bubbles causing blockages.

Coagulation

Sequential interaction of plasma enzymes, cofactors, and related substances forming a fibrin clot.

Fibrinogen

Plasma protein converted to fibrin (active form), which acts as a 'glue' to stop bleeding.

Platelets

Smallest blood elements, essential for stopping bleeding and forming plugs.

Coagulation Factors

Enzymes (procoagulants) responsible for blood clotting.

Primary Hemostasis

Process: Platelet plug formation.

Secondary Hemostasis

Process: Fibrin clot formation via enzymatic reactions.

Key Mediators of Primary Hemostasis

Blood vessels, platelets.

Key Mediators of Secondary Hemostasis

Coagulation factors.

End Result of Primary Hemostasis

Platelet plug.

End Result of Secondary Hemostasis

Insoluble fibrin clot.

Response of Primary Hemostasis

Initial, rapid, and short-lived.

Response of Secondary Hemostasis

Delayed but long-term.

General Process of Hemostasis

Involves vasoconstriction, platelet plug formation, coagulation cascade, clot formation, and fibrinolysis.

Extravascular Components

Tissues surrounding vessels provide back pressure and trap escaping blood.

Vascular Components

Blood vessels regulate flow and maintain balance with anti-thrombotic, procoagulant, and fibrinolytic properties.

Intravascular Components

Platelets and coagulation factors within the blood actively participate in hemostasis.

Platelet Activation and Aggregation

Involves adhesion, aggregation, and release of substances promoting hemostasis.

Coagulation Cascade

Involves primary hemostasis (temporary platelet plug) and secondary hemostasis (stabilization with fibrin).

Key Factors in Coagulation Cascade

Tissue Factor activates the extrinsic pathway; Thrombin converts fibrinogen to fibrin; Factor XIII stabilizes the fibrin clot.

Fibrinolysis Mechanism

Plasminogen is activated to plasmin, which digests fibrin.

Significance of Fibrinolysis

Prevents clot persistence, which could cause blockages like strokes.

Thrombus Formation

Clot formed at the injury site.

Antithrombotic Actions

Include Tissue Plasminogen Activator (t-PA) initiating fibrinolysis and Thrombomodulin blocking the coagulation cascade.

Tunica Intima

Inner layer of blood vessels composed of endothelial cells that produce anti-thrombotic substances and anticoagulants.

Tunica Media

Middle layer of blood vessels composed of smooth muscle that regulates vasoconstriction.

Tunica Adventitia

Outer layer of blood vessels that provides structural support and is composed of fibroblasts that produce collagen and tissue factor.

Platelets

Cell fragments that aggregate to form a temporary plug and release granules like ADP, serotonin, and thromboxane A2.

Coagulation Factors

Proteins that act sequentially to stabilize the platelet plug by forming a fibrin clot.

Fibrinolytic System

System that breaks down the fibrin clot once healing is complete.

Normal Hemostatic Balance

Dynamic equilibrium between procoagulants and anticoagulants that prevents unnecessary clotting.

Anticoagulants

Substances that prevent unnecessary clotting, such as antithrombin and Protein C/S.

Procoagulants

Substances that facilitate clot formation when needed, such as tissue factor and coagulation factors.

Endothelial Role

In resting state, produces antithrombotic substances; in activated state, releases tissue factor and von Willebrand factor to initiate clotting.

Vasoconstriction

Immediate narrowing of blood vessels to minimize blood loss, enhanced by chemical mediators from platelets.

Platelet Activation

Process involving adhesion, shape change, aggregation, and release reaction of platelets.

Platelet Plug Formation

Formation of a temporary seal at the site of vascular injury by aggregated platelets.

Fibrin Clot Formation

Process where the coagulation cascade generates thrombin, converting fibrinogen to fibrin, which reinforces the platelet plug.

Fibrin Stabilization

Cross-linking of fibrin by Factor XIIIa to stabilize the clot.

Clot Dissolution (Fibrinolysis)

Process where plasminogen is converted to plasmin, which degrades fibrin into soluble fragments, restoring normal blood flow.

Bleeding Disorders

Conditions resulting from deficiencies in platelets, coagulation factors, or fibrinolysis inhibitors.

Thrombotic Disorders

Conditions characterized by excessive clot formation, such as deep vein thrombosis or stroke.

Therapeutic Interventions

Treatments including anticoagulants, platelet inhibitors, and fibrinolytics to manage clotting disorders.

Primary Hemostasis

The process of forming a temporary plug to seal injuries in blood vessels, primarily involving platelets.

Platelet Characteristics

Platelets are discoid-shaped, 2-4 µm in diameter, with a mean platelet volume (MPV) of 8-10 fL.

Normal Count of Platelets

Normal platelet count ranges from 150-400 × 10^9/L, with some sources stating up to 450 × 10^9/L.

Lifespan of Platelets

Platelets have a lifespan of 8-11 days, with 9-12 days in circulation.

Megakaryocytopoiesis

The process of platelet production occurring in the bone marrow by large, multinucleated cells called megakaryocytes.

Thrombopoietin (TPO)

A hormone primarily produced by the liver that stimulates platelet production, with its plasma concentration inversely proportional to platelet and megakaryocyte mass.

Megakaryoblast

Early stage of megakaryocyte development.

Promegakaryocyte

Intermediate stage.

Megakaryocyte

Large, multinucleated cell responsible for platelet production.

Metamegakaryocyte

Final stage before platelet release.

Endomitosis

Megakaryocytes undergo endomitosis (mitosis without telophase), resulting in a multilobulated nucleus.

Thrombocytopoiesis

Platelets are formed from the megakaryocyte cytoplasm. Proplatelet processes extend into the venous sinuses of the bone marrow, where platelets are shed.

Platelet Release

A single megakaryocyte can shed 2000-4000 platelets.

Thrombopoietin (TPO)

Produced by the liver, inversely proportional to megakaryocyte and platelet mass.

Meg-CSF

Works synergistically with TPO to induce differentiation.

IL-3

Works synergistically with TPO to induce differentiation.

BFU-Meg

Least mature progenitor, participates in normal mitosis.

CFU-Meg

Intermediate stage.

LD-CFU-Meg

Initiates endomitosis (DNA replication without cytokinesis).

RUNX1 Transcription Factor

Mediates transition from mitosis to endomitosis.

NF-E2

Promotes DNA replication, leading to increased ploidy (e.g., 8N → 16N → 32N).

128N Ploidy

Associated with hematologic disease.

MK-I (Megakaryoblast)

Size: 14-18 μm. Features: Round nucleus, high nucleus-to-cytoplasm ratio (3:1), basophilic cytoplasm, no visible granules.

MK-II (Promegakaryocyte)

Size: 15-40 μm. Features: Indented nucleus, abundant cytoplasm, few azurophilic granules.

MK-III (Megakaryocyte)

Size: 30-50 μm. Features: Lobulated nucleus, basophilic chromatin, visible DMS.

Demarcation System (DMS)

Serves as 'platelet territories,' defining individual platelets.

Reticulated Platelets ('Stress Platelets')

Larger than normal platelets, diameter > 6 μm. MPV (Mean Platelet Volume): 12-14 fL.

RUNX1

Mediates transition to endomitosis, inhibiting the Rho/ROCK pathway.

NF-E2

Promotes DNA replication and increased ploidy.

FOG-1 and GATA-1

Regulate megakaryocyte differentiation and maturation.

MK-I (Megakaryoblast)

14-18 μm, round nucleus, not visible granules, no shedding, high nucleus-to-cytoplasm ratio (3:1), basophilic cytoplasm.

MK-II (Promegakaryocyte)

15-40 μm, indented nucleus, few azurophilic granules, developing abundant cytoplasm, no shedding, visible and defined granules.

MK-III (Megakaryocyte)

30-50 μm, lobulated nucleus, basophilic chromatin, azurophilic, alpha, and dense granules.

Peripheral Zone (Outer Zone)

Facilitates platelet attachment to injury site and aggregation for plug formation, maintains negative surface charge to prevent unwanted platelet activation.

Glycocalyx

Adhesive layer made of mucopolysaccharides, contains glycoproteins that interact with thrombin, von Willebrand factor (vWF), and fibrinogen.

Gp Ib/IX/V

Receptors for vWF, essential for adhesion.

Gp IIb/IIIa

Binding sites for fibrinogen, necessary for aggregation.

Phospholipid Bilayer

Selectively permeable membrane structure of the plasma membrane.

Phosphatidylinositol

Precursor for arachidonic acid (thromboxane A2 production).

Phosphatidylserine

Flips to outer leaflet during activation, enhancing platelet activation.

Cholesterol

Maintains membrane fluidity.

Structural Zone (Sol-Gel Zone)

Provides structural support, shape change, and contractility.

Microfilaments

Components include actin, which is contractile and anchors glycoproteins and proteoglycans, and myosin, which facilitates contraction.

Thrombosthenin

Actomyosin complex responsible for clot retraction.

Organelle Zone

Facilitates secretion of cellular contents.

Alpha Granules

Store proteins like platelet-derived growth factor (PDGF), fibronectin, vWF, fibrinogen, factor V, etc., and are the most numerous granules (50-80 per platelet).

Dense Granules

Contain small molecules (calcium, ADP, ATP, serotonin, etc.) critical for aggregation, with 2-7 per platelet.

Mitochondria

Site for ATP synthesis and glycogen metabolism.

Surface-Connected Canalicular System (SCCS)

Invagination of the plasma membrane, enhancing platelet interaction with the environment, and route for endocytosis and secretion of alpha granules.

Dense Tubular System (DTS)

Condensed rough endoplasmic reticulum remnant that sequesters calcium and contains enzymes (phospholipase A2, cyclooxygenase, thromboxane synthase), controlling platelet activation and prostaglandin synthesis.

Alpha Granules

Most numerous granules in platelets, with 50-80 granules per platelet, filled with proteins including coagulation proteins like Factor V, Factor VIII, fibrinogen, and vWF.