Assessing Coagulation
The Coagulation System


The Platelet

  1. Vascular injury allows reflex vasoconstriction with exposure of the subendothelial matrix and reduced local blood flow.
  2. The starting point is the attachment of the soluble protein, von Willebrand factor, to the subendothelial matrix.
    Failure of this step may be due to
    1. Absence of von Willebrand factor - Autosomal Dominant and Recessive inheritance
    2. Malfunction of collagen - Scurvy

    Tests

    1. History and examination
    2. Ristocetin cofactor test for vWf
  3. Platelet adhesion. Normal von Willebrand factor, when bound to the subendothelium, exposes multiple intrinsic binding sites for the platelet specific membrane Glycoprotein Ib (GPIb).
    Failure of this step may be due to
    1. Occupancy of the GPIb receptor. Dextran, multiple myeloma and idiopathic thrombocytopaenia purpura can occupy the GPIb receptor.
    2. The number of platelets - Results of the bleeding time become abnormal when the platelet count falls below 100x109/L, however spontaneous small-vessel bleeding does not increase until the platelet count is less than 5x109/L.
    3. Delivery of the platelets - Platelets are normally concentrated in the peripheral blood stream. A functional dilution of platelets within the column of blood occurs when the packed cell volume is less than 20%
    4. Absence of the GPIb molecule. Proteolytic degradation during Cardiopulmonary bypass and storage of greater than 3 days causes loss of GPIb. The Bernard-Soulier Syndrome is a congenital lack of the protein.
  4. Platelet aggregation. The bound platelets disintegrate to initiate a platelet binding cascade.
    1. von Willebrand factor-GPIb binding stimulates platelet disintegration which exposes the Glycoprotein IIbIIIa binding site for further attachment of von Willebrand factor and fibrinogen.
    2. Dense granules in the platelets release ADP, enhancing platelet aggregation and disintegration, serotonin and thromboxane A2 which cause vasoconstriction and Calcium. Calcium is essential in the activation of the soluble proteins of the coagulation pathway.
    3. Alpha granules from the platelets release the coagulation proteins - fibrinogen, thrombospondin, fibronectin, factor V and factor VIII
    4. The phospholipid phosphatidylserine (Platelet Factor 3) is exposed on the surface of the disintegrating platelet, serving as an essential base for the initiation of soluble proteins in the coagulation pathway


    Failure of this step

    1. Insufficient number of platelets.
    2. Dysfunctional platelets.
      1. Prior activation occurs during cardiopulmonary bypass, storage, exposure to aspirin, uraemia and acute and chronic alcohol exposure.
      2. Congenitally impaired function - absence of Gp IIb-IIIa ("Glanzmann's thrombasthenia")


    Tests of the above axis

    1. History and examination
    2. Platelet count
    3. Platelet aggregometry
    4. Red blood cell count
    5. A suggested practical test of the integrity of the entire axis is a correctly performed
      bleeding time.

  5. The Revised Coagulation Pathway

    Tissue
    Injury
    red down arrow


    red down arrowXIIa??
    left red down arrow tissue factor right red down arrow
    XI XIa IX IXa X Xa
    rt red U arrow rt red U arrow red down arrow
    factor eight factor five
    Thrombin prothrombin

    XIII
    red down arrow
    cross-linked fibrin fibrin fibrinogen

    Key: Stimulates: red down arrow Inhibits: blue down arrow

  1. There is a rapid initiation of coagulation when "Tissue Factor" (a protein-phospholipid complex normally present on vascular cells and activated monocytes), is exposed to factor VII in the presence of calcium.
  2. The activated Tissue factor-VII complex activates factors IX and X
    1. Factor IXa enhances the production of Xa, especially in the presence of the co-enzyme VIIIa.
    2. Factor Xa converts Prothrombin to Thrombin (factor IIa). This is greatly facilitated by the presence of the coenzyme Va.
    3. Thrombin enhances its own generation by
      1. Activating the coenzymes V and VIII
      2. Activating factor XI, this generates more IXa and more XIa by positive feedback.
      3. Encouraging platelet aggregation and disintegration
  3. Thrombin cleaves fibrinogen yielding monomers of fibrin which then polymerises to form the fibrin clot. Factor XIII, activated by thrombin and Ca2+, stabilises this clot by forming covalent bonds between the fibrin molecules.

Thrombin's Positive Feedback!

Tissue
Injury
grey down arrow


grey down arrowXIIa??
left grey down arrow tissue factor right grey down arrow
XI XIa IX IXa X Xa
up red arrow rt grey U arrow rt grey U arrow grey down arrow
factor eight factor five
red arrow rt grey U arrow

Thrombin
rt grey U arrow

Regulation of Coagulation

Tissue
Injury
grey down arrow


grey down arrowXIIa??
left grey down arrow tissue factor right grey down arrow
XI XIa IX IXa X Xa EPI
blue arrow blue arrow
Antithrombin III,
heparan sulphate
blue arrow
 
factor eight
Thrombin
factor five



Proteins
C,S


Normal Fibrinolysis



left red down arrow
Tissue Injury

right red down arrow
XII XIIa          Release of..
         tPA, urokinase
red down arrow
prekallikrein Kallikrein   
red down arrow   red down arrow   red down arrow
plasminogen Plasmin
red down arrow
fibrin fibrin degradation products,
D-Dimer
Va, VIIIa, GPIb Inactivated


Regulation of Fibrinolysis



left grey down arrow
Tissue Injury

right grey down arrow
XII XIIa
grey down arrow tPA, urokinase
prekallikrein Kallikrein   
grey down arrow   grey down arrow   grey down arrow
blue arrow blue arrow blue arrow
Plasminogen Activator Inhibitor 1,
(tPA & UK also inhibited by alpha2 antiplasmin, alpha 2 macroglobulin)
plasminogen Plasmin
blue arrow Alpha-2 antiplasmin
(forms PAP complexes)



A Table of Synonyms

Factor

Synonym

Kind of Protein

Function

I

Fibrinogen

Structural

Linkage Strands

II

Prothrombin

Vit K Serine Proteinase

Activates I, V, VII, XIII, Prot C and Plts

V

Proaccelerin

Binding

Helps Xa activate II

VII

Stable Factor

Vit K Serine Proteinase

Activates Ix and X

VIII

Antihaemophilic

Binding

Helps IXa activate X

IX

Christmas Factor

Vit K Serine Proteinase

Activates X

X

Stuart-Prower Factor

Vit K Serine Proteinase

Activates II

XI

Thromboplastin antecedent

Serine Proteinase

Activates IX

XII

Hageman Factor

Serine Proteinase

Activates Kinin system

XIII

Fibrin stabiliser

Transglutaminase

Cross links fibrin

von Willebrand VIII related Ag

Binding

Binds plts and VIII

Extrinsic Pathway Inhibitor

Kunitz inhibitor
(2 headed serpin)

Inhibits TF-VIIa and Xa together

Antithrombin III

Serpin

Inhibits Serine Proteinases

Protein C

Vit K serine Proteinase

Inactivates Va VIIIa and PAI-1

Protein S

Vit K protein

Helps Protein C

Plasminogen

Serine Proteinase

Lyses fibrin

Alpha2 antiplasmin

Serpin

Inhibits plasmin

Prourokinase

Serine Proteinase

Activates plasminogen

Tissue plasminogen activator

Serine Proteinase

Activates plasminogen

Plasminogen activator inhibitor 1

Serpin

Inactivates tPA and urokinase

{Whatever happened to III, IV, VI? (heh)}

On to: Tests of the Coagulation System