Protective functions of Protective functions of bloodblood

Hemostasis systemHemostasis system

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HemostatisHemostatis HemostasisHemostasis or stoppage of bleeding (stasis = halting). or stoppage of bleeding (stasis = halting). No hemostasis No hemostasis No sealing No sealing bleed to death from bleed to death from

minor woundsminor wounds The hemostasis response is The hemostasis response is

fast fast localized and localized and carefully controlledcarefully controlled

Involves many Involves many blood coagulation factorsblood coagulation factors normally normally present in plasma as well as some substances that are present in plasma as well as some substances that are released by platelets and injured tissue cells.released by platelets and injured tissue cells.

During hemostasis, following steps occur: During hemostasis, following steps occur: – Vascular spasms, Vascular spasms, – Platelet plug formation, Platelet plug formation, – Coagulation, or blood clotting.Coagulation, or blood clotting.– Growth of fibrous tissue in clot to close the hole in vessel.Growth of fibrous tissue in clot to close the hole in vessel.

Blood loss at the site is permanently prevented when Blood loss at the site is permanently prevented when fibrous tissue grows into the clot and seals the hole in fibrous tissue grows into the clot and seals the hole in the blood vessel.the blood vessel.

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CoagulationCoagulation or blood clotting or blood clotting– Complicated process, Liquid Blood Complicated process, Liquid Blood becomes gel, becomes gel, – Over 50 Substances are involvedOver 50 Substances are involved– Factors that enhance clot formation are called Factors that enhance clot formation are called

clotting factorsclotting factors or or procoagulantsprocoagulants..– Factors that inhibit clotting are called Factors that inhibit clotting are called anticoagulants.anticoagulants.– Balance between these two groups of factors. Balance between these two groups of factors.

Normally, anticoagulants dominate and clotting is Normally, anticoagulants dominate and clotting is prevented; but when a vessel is ruptured, prevented; but when a vessel is ruptured, procoagulant activity in that area increases procoagulant activity in that area increases dramatically and clot formation begins. dramatically and clot formation begins.

– The procoagulants are numbered I to XIII according The procoagulants are numbered I to XIII according to the order of their discovery; hence the numerical to the order of their discovery; hence the numerical order does not reflect the reaction sequence.order does not reflect the reaction sequence.

– Most of these factors are plasma proteins made by Most of these factors are plasma proteins made by the liver that circulate in an inactive form in blood the liver that circulate in an inactive form in blood until mobilized.until mobilized.

CoagulationCoagulation

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Three Phases of Coagulation: Three Phases of Coagulation: A complex substance called prothrombin activator A complex substance called prothrombin activator

is formed. is formed. Prothrombin activator converts prothrombin (a Prothrombin activator converts prothrombin (a

plasma protein) into thrombin, (an enzyme). plasma protein) into thrombin, (an enzyme). Thrombin catalyzes the joining of fibrinogen Thrombin catalyzes the joining of fibrinogen

molecules present in plasma to a fibrin mesh.molecules present in plasma to a fibrin mesh.

Role of Vitamin K in coagulation.Role of Vitamin K in coagulation.Vitamin K Vitamin K not directlynot directly involved in involved in coagulation, this fat-soluble vitamin is coagulation, this fat-soluble vitamin is required for the synthesis of four of the required for the synthesis of four of the procoagulants made by the liver i.e (II, procoagulants made by the liver i.e (II, VII, IX and X).VII, IX and X).

3-Coagulation3-Coagulation

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Clotting may be initiated by either the Clotting may be initiated by either the Intrinsic Pathway Intrinsic Pathway Extrinsic pathway Extrinsic pathway

Both pathways are usually triggered by the tissue-Both pathways are usually triggered by the tissue-damaging events. Clotting of blood outside the body (such damaging events. Clotting of blood outside the body (such as in a test tube) is initiated only by the intrinsic as in a test tube) is initiated only by the intrinsic mechanism.  mechanism.  

    – Critical components in both mechanisms are negatively Critical components in both mechanisms are negatively

charged membranes, particularly those on platelets charged membranes, particularly those on platelets that contain phosphatidylserine (platelets that contain phosphatidylserine (platelets phospholipids), also known as phospholipids), also known as PFPF33 (platelet factor 3).(platelet factor 3).

– Many intermediates of both pathways can be activated Many intermediates of both pathways can be activated only in the presence of only in the presence of PFPF33. .

Phase 1- Formation of prothrombin Phase 1- Formation of prothrombin ActivatorActivator

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Intrinsic PathwayIntrinsic PathwayIn the slower intrinsic pathway, all factors needed for clotting In the slower intrinsic pathway, all factors needed for clotting are present in (intrinsic to) the blood. are present in (intrinsic to) the blood.

Extrinsic PathwayExtrinsic PathwayBy contrast, when blood is exposed to an additional factor in By contrast, when blood is exposed to an additional factor in tissues underneath the damaged endothelium called tissue tissues underneath the damaged endothelium called tissue factor (TF), factor III, or factor (TF), factor III, or tissue thromboplastintissue thromboplastin, the “shortcut” , the “shortcut” extrinsic mechanism, which bypasses several steps of the extrinsic mechanism, which bypasses several steps of the intrinsic pathway, is triggered.intrinsic pathway, is triggered.

Role of calciumRole of calcium– Each pathway requires ionic calcium and involves the activation Each pathway requires ionic calcium and involves the activation

of a series of procoagulants, each functioning as an enzyme to of a series of procoagulants, each functioning as an enzyme to activate the next procoagulant in the sequence. activate the next procoagulant in the sequence.

– The intermediate steps of each pathway cascade toward a The intermediate steps of each pathway cascade toward a common intermediate, factor X. common intermediate, factor X.

– Activated factor X complexes with calcium ions, PF3, and factor Activated factor X complexes with calcium ions, PF3, and factor V to form prothrombin activator. V to form prothrombin activator.

– Slowest step of the blood clotting process, but once formed, the Slowest step of the blood clotting process, but once formed, the clot forms in 10 to 15 seconds.clot forms in 10 to 15 seconds.

Phase 1- Formation of prothrombin Phase 1- Formation of prothrombin ActivatorActivator

1111

Intrinsic PathwayBlood Trauma or

contact with collagen

XII(Hageman)

Activated XII (XIIa)

HMW Kininogen, Prekellikerein

XI(PTA)

Activated XI (XIa)

IX(PTC)

Activated IX (IXa)

Ca++

X(SPF)

Activated X (Xa)

Ca++

VIII (AHF-A)

VIIIaThrombin

Ca++

ThrombinV

Prothrombin Activator

Va

Prothrombin Thrombin

or PF3

Ca++

(1)

(2)

(3)

(4)

(5)

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Extrinsic Pathway

Tissue trauma

VII(Proconvertin)

Activated VII (VIIa)

X(SPF)

Activated X (Xa)

Ca++

Ca++

ThrombinV

Prothrombin Activator

Va

Prothrombin Thrombin

or PF3

Ca++

(1)

(2)

(3)

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Phase 2: Common Pathway to Phase 2: Common Pathway to ThrombinThrombin

Prothrombin activator catalyzes Prothrombin activator catalyzes the transformation of the plasma the transformation of the plasma protein prothrombin to the active protein prothrombin to the active enzyme enzyme thrombinthrombin..

Prothrombin Activator complex

Prothrombin Thrombin

Extrinsic pathway

Intrinsic pathway

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Phase 3: Common Pathway to the Fibrin Phase 3: Common Pathway to the Fibrin MeshMesh

1.1. Thrombin catalyzes the polymerization of Thrombin catalyzes the polymerization of fibrinogenfibrinogen (another plasma protein made by the (another plasma protein made by the liver). liver).

2.2. Thrombin is a protein Thrombin is a protein enzyme enzyme with weak with weak proteolytic capabilities. It acts on fibrinogen to proteolytic capabilities. It acts on fibrinogen to remove four low-molecular weight peptides from remove four low-molecular weight peptides from each molecule of fibrinogen, forming one each molecule of fibrinogen, forming one molecule of molecule of fibrin monomer. fibrin monomer.

3.3. Fibrin monomers has the automatic capability to Fibrin monomers has the automatic capability to polymerize with other fibrin monomer molecules polymerize with other fibrin monomer molecules to form fibrin fibersto form fibrin fibers. .

4.4. Many fibrin monomer molecules polymerize within Many fibrin monomer molecules polymerize within seconds into seconds into long fibrin fibers.long fibrin fibers.

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5.5. During early polymerization, During early polymerization, fibrin fibers are held together fibrin fibers are held together by weak non covalent by weak non covalent hydrogen bonding, No cross-hydrogen bonding, No cross-linkage with one another.linkage with one another.

6.6. fibrin-stabilizing factor fibrin-stabilizing factor causes the cross linkage of causes the cross linkage of fibrin fibers fibrin fibers (Released from (Released from platelets entrapped in the platelets entrapped in the clot). clot).

7.7. Activated by thrombin Activated by thrombin

8.8. This activated substance This activated substance operates as an enzyme to operates as an enzyme to form form covalent bonds covalent bonds between between fibrin monomer molecules, as fibrin monomer molecules, as well as multiple cross well as multiple cross linkages between adjacent linkages between adjacent fibrin fibers.fibrin fibers.

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1.1. Within 30 to 60 minutes, the clot is stabilized further Within 30 to 60 minutes, the clot is stabilized further by a platelet-induced process called clot retraction. by a platelet-induced process called clot retraction.

2.2. Platelets contain contractile proteins (actin and Platelets contain contractile proteins (actin and myosin), and they contract in much the same manner myosin), and they contract in much the same manner as muscle cells. as muscle cells.

3.3. As the platelets contract, they pull on the surrounding As the platelets contract, they pull on the surrounding fibrin strands, squeezing serum (plasma minus the fibrin strands, squeezing serum (plasma minus the clotting proteins) from the mass, compacting the clot clotting proteins) from the mass, compacting the clot and drawing the ruptured edges of the blood vessel and drawing the ruptured edges of the blood vessel more closely together. more closely together.

4.4. Even as clot retraction is occurring, vessel healing is Even as clot retraction is occurring, vessel healing is taking place. taking place.

5.5. Platelet-derived growth factor (PDGF) released by Platelet-derived growth factor (PDGF) released by platelet degranulation stimulates smooth muscle cells platelet degranulation stimulates smooth muscle cells and fibroblasts to divide and rebuild the wall.and fibroblasts to divide and rebuild the wall.

6.6. As fibroblasts form a connective tissue patch in the As fibroblasts form a connective tissue patch in the injured area, endothelial cells, stimulated by vascular injured area, endothelial cells, stimulated by vascular endothelial growth factor (VEGF), multiply and restore endothelial growth factor (VEGF), multiply and restore the endothelial lining.the endothelial lining.

4-Clot Retraction and Repair4-Clot Retraction and Repair

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A process called A process called fibrinolysisfibrinolysis removes removes unneeded clots when healing has occurred. unneeded clots when healing has occurred.

Because small clots are formed continually Because small clots are formed continually in vessels, this cleanup is important. Without in vessels, this cleanup is important. Without fibrinolysis, blood vessels would gradually fibrinolysis, blood vessels would gradually become completely blocked.become completely blocked.

The critical natural “clot buster” is a fibrin-The critical natural “clot buster” is a fibrin-digesting enzyme called digesting enzyme called plasminplasmin, which is , which is produced when the plasma protein produced when the plasma protein plasminogen is activated. plasminogen is activated.

FibrinolysisFibrinolysis

Large amounts of plasminogen are Large amounts of plasminogen are incorporated into a forming clot, where it incorporated into a forming clot, where it remains inactive until appropriate signals remains inactive until appropriate signals reach it.reach it.

The presence of a clot in and around the blood vessel The presence of a clot in and around the blood vessel causes the endothelial cells to secrete causes the endothelial cells to secrete

– tissue plasminogen activator (tPA). tissue plasminogen activator (tPA).

Along with thatAlong with that– Activated factor XII and Activated factor XII and – thrombin thrombin

released during clotting also serve as plasminogen released during clotting also serve as plasminogen activators. As a result, most plasmin activity is confined activators. As a result, most plasmin activity is confined to the clot, and any plasmin that strays into the plasma to the clot, and any plasmin that strays into the plasma is quickly destroyed by circulating enzymes. is quickly destroyed by circulating enzymes.

Fibrinolysis begins within two days and Fibrinolysis begins within two days and continues slowly over several days until the continues slowly over several days until the clot is finally dissolved.clot is finally dissolved.

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Normally, two homeostatic mechanisms prevent clots from becoming Normally, two homeostatic mechanisms prevent clots from becoming unnecessarily large: unnecessarily large:

swift removal of clotting factors, and swift removal of clotting factors, and inhibition of activated clotting factors. inhibition of activated clotting factors.

Limiting the Activity of ThrombinLimiting the Activity of Thrombin– As a clot forms, almost all of the thrombin produced is bound onto the fibrin As a clot forms, almost all of the thrombin produced is bound onto the fibrin

threads. threads.

– This is an important safeguard because thrombin also exerts positive This is an important safeguard because thrombin also exerts positive feedback effects on the coagulation process prior to the common pathway. feedback effects on the coagulation process prior to the common pathway.

It speed up the production of prothrombin activator by acting through factor V,It speed up the production of prothrombin activator by acting through factor V, It also accelerates the earliest steps of the intrinsic pathway by activating It also accelerates the earliest steps of the intrinsic pathway by activating

platelets.platelets.

– Thus, fibrin effectively acts as an anticoagulant to prevent enlargement of Thus, fibrin effectively acts as an anticoagulant to prevent enlargement of the clot and prevents thrombin from acting elsewhere. the clot and prevents thrombin from acting elsewhere.

– Thrombin not bound to fibrin is quickly inactivated by Thrombin not bound to fibrin is quickly inactivated by antithrombin IIIantithrombin III, a , a protein present in plasma. It inactivates the protease activity of thrombin protein present in plasma. It inactivates the protease activity of thrombin and factors IXa, Xa, XIa and XIIa by forming complexes with them.and factors IXa, Xa, XIa and XIIa by forming complexes with them.

– HeparinHeparin, the natural anticoagulant contained in basophil and mast cell , the natural anticoagulant contained in basophil and mast cell granules, inhibits thrombin by enhancing the activity of antithrombin III. granules, inhibits thrombin by enhancing the activity of antithrombin III.

Factors Limiting Normal Clot GrowthFactors Limiting Normal Clot Growth

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Blood TestingBlood Testing