Haemostasis

Prof. K. Sivapalan

June 2013 Haemostasis 2

Thrombocytes• 2 – 4 μm in diameter.• Half life – 4 days.• 300,000 / μL.• Break off from

megakaryocytes. • Colony stimulating

factors and thrombopoietin- liver and kidney.

• 60 – 70 % in circulating blood - balance in spleen.

June 2013 Haemostasis 3

Properties of Platelets

• A ring of microtubules in periphery.• Extensively invaginated membrane.• Membrane contains receptors for:

– Collagen, von Willebrand factor and fibrin.• Dense granules in cytoplasm:

– Serotonin, ADP, other nuclear tides.• α – granules in cytoplasm :

– Clotting factors and platelet-derived growth factor [PDGF – stimulates wound healing and mitogen for vascular smooth muscle.]

June 2013 Haemostasis 4

Platelet activation.

• Binds to exposed collagen and von Willebrand factor (when damage to blood vessel). This is platelet adhesion.

• This activates platelets [ADP]• Platelets change shape, put out

psudopodia and release granules and causes platelet aggregation.

June 2013 Haemostasis 5

Changes in the platelet.• Platelet activation results in change in shape, putting out

psseudopodia, release of granules and aehesion to other platelets.

• Platelet Activating Factor secreted by neutrophils and monocytes stimulates G protein which activates phospholipase C to form diacylglycerol. This also causes release of granules.

• Increased cytoplasmic calcium and diacylglycerol activate Phospholipase A2. This causes release of arachidonic acid from membrane phospholipids which is converted into Thromboxan A2

• Thromboxan and other substances released cause vasoconstriction, platelet aggregation, clot formation

• Aspirin prevents the above reaction and alters the balance between thromboxan and prostacycline and prevents clotting in low doses.

June 2013 Haemostasis 6

Effects of platelet aggregation.

• Repair of the blood vessels.

• Block damaged capillaries.

• Vasoconstriction.• Clotting.• [Test for platelet

function: bleeding time.]

June 2013 Haemostasis 7

Thrombocytopenia.

• Results in capillary bleeding [purpura].

• Caused by-– Marrow disorders.– Alcohol, cytotoxic drugs,

viral infections.– Hereditary.– Immunologically mediated

destruction.– Increased consumption of

platelets.

June 2013 Haemostasis 8

Thrombocytosis.

Causes:• Spleanectomy.• Postoperatively, delivery.• Haemorrhage or haemolysis.• Extreme exercise.Risk:• Thrombotic diseases- deep vein

thrombosis.

June 2013 Haemostasis 9

Hemostasis.

• Vascular spasm: local myogenic, serotonin – lasts for about 20 – 30 minutes.

• Platelet plug.• Clotting of blood.• Organization by fibrous tissue.

June 2013 Haemostasis 10

Coagulation of blood – clotting.

• Fibrinogen → Fibrin.• Polymerization of

fibrin with branching.• Loose mesh of

interlacing strands.• Formation of covalent

bonds → dense, tight aggrigate.

June 2013 Haemostasis 11

Important reactions.

Fibrinogen Fibrin.

Thrombin

Prothrombin

Factor x (activated)

Intrinsic system. 2 – 5 minutes.

Extrinsic system. 15 – 30 Seconds.

Platelet Factor, Ca++,

Activated Factor V.

Stabilization.

Factor XIII

Activated XIII

Clot retraction. [platelets] 30 – 60 min.

June 2013 Haemostasis 12

Coagulation cascade.

Contact with wettable, negatively charged surface – Intrinsic system.

Prekallikerin Kallikerin.

HMW Kilinogen

XII XIIa.

XI XIIa

IX IXa

Tissue factor – Extrinsic system.

[Tissue Thromboplastin- TPL+TFI]

VIIa VII

Ca++

TPL+TFI

X Xa

Activated VIII,

Platelet factor (PL),

Ca++

Ca++, PL, TPL

June 2013 Haemostasis 13

Role of liver in clotting.

• Synthesizes:– Fibrinogen.– Prothrombin.– Other clotting factors.

• Needs:– Vitamin K.

• Removes activated clotting factors.

June 2013 Haemostasis 14

Propagation of clot formation.

• Clot formation can be initiated at any vessel by damage to endothelium by platelet plug.

• Activated clotting factors on the surface of the clot can cause further clotting.

• Platelet plug can form on the surface of the clot which can initiate further clotting

• Rapid flow wash off the factors which are diluted and removed in liver.

June 2013 Haemostasis 15

Clotting and Anti clotting mechanisms.

• Clotting and anti clotting mechanisms are balanced under normal circumstances.

• It is essential to maintain blood in liquid form but prevent loss if the vessels are damaged.

June 2013 Haemostasis 16

Anti clotting mechanisms.

• Anti thrombin III [circulating protease] inactivates activated factors IX,X,XI and XII. Heparin facilitates it.

• Prostacycline of Endothelium antagonizes thromboxane A2 of platelets. (aggregation)

• Endothelium has thrombomodulin. It’s reaction with thrombin leads to fibrinolysis, inactivation of factor V and VIII.

June 2013 Haemostasis 17

Fibrinolytic system.Thrombomodulin in endothelium.

Binds to Thrombin

Prtotein C. Activated protein C.[APC]

Inhibitor of tissue plasminogen activator inhibited.

PlasminogenPlasmin.

Lyses of fibrin.

VIIIa Inactive VIIIa

Va Inactive Va

Tissue plasminogen activator

June 2013 Haemostasis 18

Abnormalities of clotting.

Defective clotting:• Abnormalities of platelet function.• Congenital deficiency of clotting factors-

– Hemophelia A – factor VIII [ X linked].– Hemophelia B – factor IX.

• Von Willebrand factor deficiency.• Vitamin K deficiency and Liver diseases.Enhanced clotting: Increased platelets. Absent Protein C.Intravascular Clotting is thrombosis and if carried in blood it

is embolism. Both can obstruct blood vessels and cause ischemia to organs.

June 2013 Haemostasis 19

Anticoagulants.

• Heparin promotes antithrombin III which inactivates factors IX, X, XI and XII.

Warfarin inhibits Vitamin K.Streptokinase activates plasminogen and

disolves fibrin. [snake, bacteria]Aspirin reduces thromboxan A2 formation.