CASCADE THEORY of CoagulationBy: Raissa T. Guldam BSMT - IV
Overview of Hemostasis• Hemostasis is a dynamic process whereby blood coagulation is initiated and terminated in a rapid and tightly regulated fashion (Nathan, Orkin, Ginsburg, & Look, 2003).• Hemostasis is regulated by 3 basic components—namely, the vascular wall, platelets, and the coagulation cascade.• Primary hemostasis – constriction of the damaged blood vessels and formation of platelet plug• Secondary hemostasis – coagulation factors present in the blood interact, forming a fibrin meshwork (clot)
Overview of Blood Coagulation V aso co n- strictio n P latelet P latelet A ctivatio n P lu gV essel P latelet C lot A gg reg ationIn ju ry T hro m bin T issu e F acto r C o agu latio n C ascad e
Coagulation Mechanism• Most of the substances necessary for coagulation are present in an INERT form and must be converted to an ACTIVATED state.• As one ENZYME is formed, it then becomes available to convert the next ZYMOGEN to its activated enzyme.• The activated form of a factor is indicated by the letter “a” to the right of the Roman Numeral (activated factor X = Xa).
Coagulation Cascade• The classic theory of coagulation was described by Paul Morawitz in 1905.• This model described each clotting factor as a proenzyme that could be converted to an active enzyme.• The “cascade” and “waterfall” models suggested that the clotting sequences were divided into 2 pathways.
• Coagulation could be initiated via an “intrinsic pathway,” so named because all the components were present in blood, or by an “extrinsic pathway,” in which the subendothelial cell membrane protein, tissue factor (TF), was required in addition to circulating components.• The initiation of either pathway resulted in activation of FX and the eventual generation of a fibrin clot through a common pathway.
The “Cascade” Theory• Blood coagulation is a series of biochemical reactions.• Transforms circulating substances into insoluble gel.• Converts soluble fibrinogen into fibrin.• Process requires plasma proteins, phospholipids and calcium.• A fibrin clot is the end product of coagulation.
Coagulation Cascade Intrinsic pathway XII XI Extrinsic pathway IXAPTT VII VIII X PT Prothrombin thrombin V, Ca, P/L (II) fibrinogen fibrin XIII STABILIZED FIBRIN
• It is thought of as occurring in 4 phases: 1. Contact phase 2. Activation of Factor X 3. Conversion of Prothrombin to Thrombin 4. Formation of a Fibrin Clot
A. Contact Phase1. Small amounts of factor XII automatically activate when coming in contact with negatively charged surfaces (collagen) Negatively charged XII XIIa (small amounts) surface2. Prekallikrein is converted to kallikrein by small amount of XIIa XIIa Prekallikrein Kallikrein CLICK
3. The kallikrein formed, together with HMWK, activates more factor XII. Kallikrein XII XIIa (larger amounts) HMWK4. Factor XI is activated by factor XIIa. XIIa XI XIa
B.1. Activation of Factor X via the Intrinsic Pathway5. Factor IX is activated by factor XIa in the presence of calcium ions. XIa IX IXa Ca++6. Conversion of factor X to factor Xa most probably takes place on the surface of the platelet and is catalyzed by a complex composed of factor IXa, factor VIII, calcium ions and phospholipid (from platelet).
IXa - VIII X Xa Pl – Ca++* Factor VIII acts as a cofactor
B.2. Activation of Factor X via the Extrinsic System• Tissue factor is released and act as a cofactor in initiating coagulation (since all cells have this except those in the blood)1. Factor VII binds to tissue factor and to calcium ions to activate factor X. VII – Tissue factor X Xa Pl – Ca++
2. This complex (factor VII, tissue factor, and calcium ions) is also capable of activating small amounts of factor IX. VII – Tissue factor IX IXa (small amounts) Pl – Ca++*As factor Xa is formed, it in turn will activate factor VII.*Factor IXa will also activate factor VII but not as efficiently as factor Xa.* Factor VIIa is much more active and will speed up the activation of factor X.
Summary• The INTRINSIC coagulation mechanism requires a contact phase.• This is followed by the activation of factors IX and X.• The EXTRINSIC mechanism’s primary action is the activation of factor X.• From this point on, the two systems combine into what is termed the COMMON PATHWAY.
C. Conversion of Prothrombin to Thrombin• This is the beginning of the COMMON PATHWAY.1. Calcium ions are bound to prothrombin, which adheres to platelet surfaces along with factor Xa and factor V (cofactor). This complex attacks the prothrombin molecule, forming fragment 2 and fragment 1.2. Xa - V Fragment 2 + Prothrombin Pl – Ca++ Fragment 1.2
2. In the presence of this same complex, thrombin is formed from fragment 2. Xa - V Fragment 2 Thrombin Pl – Ca++* The resultant thrombin is capable of activating factor V, thereby increasing its activity.
D. Formation of the Fibrin Clot1. Fibrin formation occurs by the action of thrombin on fibrinogen.Fibrinogen Thrombin Fibrinopeptide A & B + Fibrin monomer2. The fibrin monomer will polymerize end to end and laterally to form fibrin polymers (fibrin strands) which are soluble in 5 M urea. Fibrin polymer Fibrin monomer (soluble in 5 M urea)
3. Factor XIII, activated by thrombin and calcium ions, converts the fibrin polymer to a covalent cross- linked fibrin clot. This stable fibrin clot is insoluble in 5 M urea. Fibrin polymer XIIIa Stable fibrin clot Ca++ (insoluble in 5 M urea)4. In the presence of activated platelets the formed clot will retract. Activated Stable fibrin clot platelets Retracted clot
Intrinsic PathwayXII XIIa PTT XI XIa Partial Thromboplastin Time IX IXa VIIIa+Ca+Pl X Xa Va+Ca+Pl II IIa Fibrinogen Fibrin
Extrin sicp a th w a y In ju red = C alciu m & P L co m p lex C ells V II * = activ e serin e p ro tease unk. T F X * V IIa *X a Va V fib rin o g en p ro th ro m b in * th ro m b in F ib rin m o n o m er X III X IIIa Com m on p a th w a y CLOT F ib rin p o ly m er
Intrinsic Vs. Extrinsic Intrinsic Pathway Extrinsic Pathway Contact Activation Pathway Tissue factor pathway Begins with the exposure of a Starts when blood is in contactforeign surface like collagen as with tissue factor (TF) a result of endothelium damageXII, Prekallikrein, HMWK, XI, IX, TF and VII and cofactor VIII Both pathways lead to the COMMON PATHWAY which consists of factor X, cofactor V, Pf3, Ca++, prothrombin and fibrinogen. The final product of coagulation is cross-linked fibrin, produced in response to factor XIII, Ca++ and thrombin.
• The Prothrombin Time or PT is a laboratory screening test used to detect coagulation disorders.• It measures the activity factors of the EXTRINSIC PATHWAY including factors II, V, VII, X and I (fibrinogen).• The extrinsic factors not measured in the PT test are Factors III (Thromboplastin) and IV (Calcium).• The PT is also used into monitor oral anticoagulant therapy such as warfarin.
Feedback Mechanisms• Fibrin formation at the site of small wounds takes place in minutes.• Feedback mechanisms increase the velocity of the cascade reactions.• Positive feedback loop – increases the reaction rate• Negative feedback loop – dampens the reaction rate