Lecture 1 platelet and hemostasis

blood physiology

1. Blood Physiology Platelets and Hemostasis Dr. Dina Hamdy Merzeban Lecturer of Medical Physiology www.facebook.com/merzeban physiology http//:slideshare.net/ merzeban

2. Platelets (Thrombocytes) Structure Platelets do not have nuclei. Platelets are discoid-shaped packages of cytoplasm surround ed by a membrane.

3. the membrane contains : large amount of phospholipids [(platelet factor 3 (PF3 )] which plays an important role in clotting process. Receptors for collagen, V-W factor& fibrinogen. Glycoprotein coat

4. The platelet cytoplasm contains : actin and myosin (that can cause contraction of the platelet - important in clot retraction). 2 types of granules Dense granules (Ca++, ADP, serotonin) non- proteins. Alpha granules (factor XIII, PDGF, PAF) proteins.

5. Properties of platelets Platelets do not have nuclei Platelet count: 250,000–500,000 platelets/cmm. Synthesis: in bone marrow from megakaryocyte by budding, 30% stored in spleen. Life span of platelets: is 8 –12 days, average 10

6. Thrombocytopenia: is a condition in which the platelet count is greatly reduced (aplastic anemia) Thrombocytosis (platelets count is greatly increased as in (splenectomy)

7. Hemostasis Is the stoppage of bleeding

8. Steps of hemostasis Vascular spasm formation of a platelet plug, formation of a blood clot as a result of blood coagulation

9. Vascular spasm vasoconstriction is due to Local myogenic contraction. Nervous reflex due to pain (by sympathetic stimulation). Local humoral factors (ADP, serotonin & thromboxane A2).

10. Platelet plug formation Platelet adhesion Platelet activation platelet release reaction Platelet aggregation Platelet fusion: Platelet procagulant activity

11. Platelet plug formation platelet adhesion: adhesion of glycoprotein coat of platelets to the exposed sub endothelial collagen(VWF) platelet activation: enhanced by ADP& thrombin. Platelets swell, discharge their granules platelet release reaction

12. Platelet plug formation platelet aggregation: released ADP, TxA2 cause more platelet to aggregate to the site of vascular injury. --->more release reaction--->More ADP, TXA2 ---->more aggregation. N.B. prostacyclin antagonize effect of TXA2 as they induce vasodilatation and inhibit platelet aggregation.

13. Platelets plug formation

14. Platelet plug formation Platelet fusion: irreversible fusion of aggregated platelet to site of injury helped by ADP, thrombin and enzymes of platelet release. Platelet procagulant activity: helped by platelet factor 3 (PF3) .

15. B lood clot ting The clot begins to develop in 15 to 20 seconds if the trauma to the vascular wall has been sever e formation of blood clot by conversion of the soluble plasma protein fibrinogen to insoluble fibrin. Clot is formed of fibrin threads, blood cells and platelets. Coagulation factors are B globulins synthesized in liver classified into 3groups

16. 1) Fibrinogen group: Factor (I, V, VIII, XIII) they are activated by thrombin. They are consumed during coagulation so, they are not present in the serum. 2) Prothrombin group : (II, VII, IX, X) their synthesis in liver is VIT. K dependent. They are consumed during coagulation so, they are present in the serum. 3) Contact group: XI, XII. They are not consumed during coagulation so, they are present in the serum.

17. Clotting of Blood The blood contains about a dozen clotting factors. These factors are proteins that exist in the blood in an inactive state, but can be called into action when tissues or blood vessels are damaged. The activation of clotting factors occurs in a sequential manner. The first factor in the sequence activates the second factor, which activates the third factor and so on. This series of reactions is called the clotting cascade

18. Control of the Clotting Cascade Thrombin is the key to the clotting mechanism. If thrombin is present then clotting will proceeds, but if thrombin is absent then clotting will not occur. How then is thrombin controlled? Thrombin is derived from an inactive precursor called prothrombin. There are two pathways that lead to the conversion of prothrombin to thrombin; (1) the intrinsic pathway and (2) the extrinsic pathway.

19. Exrinsic path w ay Intrinsic pathway

20. Interactions between extrinsic& intrinsic system. Factor X is common pathway. clotting is initiated by both systems. Ca required for all steps except 1st two steps in intrinsic pathway. The extrinsic system (Factor VII) activates the intrinsic system (Factor IX). Once thrombin formed rapidly by extrinsic pathway it activates factor V (in the intrinsic pathway) which is the accelerator for prothrombin activator complex leads to more thrombin, more clot and vicious circle.

21. Fibrous organization or dissolution of the blood clot Once a blood clot has formed, it can follow one of two courses: (1) It can become invaded by fibroblasts, which subsequently form connective tissue all through the clot, or (2) it can dissolve

22. Anti- clotting mechanisms

23. Anti- clotting mechanisms   The tendency of blood to clot is balanced in vivo by limiting reactions that tend to prevent clotting inside the blood vessels and to break down any formed clots.

24. General limiting reactions. 1. Smooth vascular endothelium, thus there is no activation of platelets. 2. Presence of heparin, which is a naturally occurring anticoagulant. 3. Removal of activated factors by liver and their inactivation.  

25. B. Specific limiting reactions.   The interaction between thromoboxane A2 and prostacyclin: The interaction between the platelet-aggregating effect of thromboxane A2, and the antiaggregating effect of prostacyclin. This interaction causes a clot to form at the site of injury but keeps the vessel lumen free of clot. Note: aspirin increases prostacyclin  prevent platelet-aggregation , so given to prevent thrombosis.

26. B. Specific limiting reactions.   2. Antithrombin III: protease inhibitor block activity of factors IX, X, XI, XII.

27. B. Specific limiting reactions. 3. Fibrinolytic System

28. all endothelial cells except cerebral microcirculation produce thrombomodulin which is a thrombin binding protein. Thrombin thrombomodulin complex is anticoagulant which activate protein C. Active protein C (APC) and its cofactor protein S will: a) inactivate factor V, VIII b) inactivate an inhibitor of tissue plasminogen activator TPA. c) increase formation of plasmin which degrade fibrin.

29. Fibrinolytic System Plasmin ogen is converted to its active form plasmin, by the action of tissue-plasminogen activator (t-PA). Plasmin (fibrinolysin) is an enzyme that lyses fibrin and fibrinogen

30. It is also activated by urokinase or Streptokinse . Human t-PA is now produced by recombinant DNA techniques for clinical use in myocardial infarction and stroke.

31. Anticoagulants

32. In vitro anticoagulants used to prevent blood clotting outside the body. a) Precipitation of Ca++ by oxalate or deionization of Ca++ citrate. b) Unwettable silicon coated tube to prevent activation of platelet & XII. c) Heparin.

33. In vivo anticoagulants: Point of Comparison   Heparin   Dicumarol Origin: Mast cells and basophils. Plant Mode of action:   Facilitate action of Antithrombin III. Inhibits formation of vitamin K dependent clotting factors in the liver Site of action: In vivo & In vitro Only in vivo. Onset: Rapid onset. Slow onset Duration: Short duration Long duration Administration: Intravenous/ intramuscular Orally   Antidote: Protamine sulfate 1% Vitamin K.

34. Abnormalities of Hemostasis: There are three groups of abnormalities that can occur in hemostasis: A . Conditions that cause excessive bleeding B. Conditions that cause excessive intravascular clotting. C. Conditions that cause both excessive bleeding and intravascular clotting.

35. A- Conditions that cause excessive bleeding   1-Thrombocytopenic purpura:   This is due to deficiency of platelets, the symptoms appear when platelet count decreases below 50,000/mm". It is characterized by the presence many subcutaneous hemorrhages called petechiae and prolongation of bleeding time.  

36. A- Conditions that cause excessive bleeding 2-Vitamin K deficiency:  Vitamin K is a fat soluble vitamin synthesized by the intestinal bacterial flora. It is needed for the formation of factors, ll , VIl , IX and X by the liver. Deficiency of this vitamin leads to decrease in the formation of these coagulation factors and thus there is prolongation of coagulation time. Causes: Absence of intestinal bacterial flora which occurs in new born infants Treatment with antibiotics for long periods in adults. Obstruction of biliary ducts which leads to absence of bile needed for absorption of the vitamin.

37. A- Conditions that cause excessive bleeding   3- Hemophilia:   It is a congenital disease characterized by a tendency for severe bleeding after mild trauma. It is a sex linked recessive disease carried by females and manifested almost always in males. It causes prolongation of the whole blood coagulation time.

38. Conditions that cause excessive intravascular clotting (Thromboembolic Conditions):   Causes: Slow blood flow as occurs in leg veins due to long bed rest after operations, or with varicose veins In atherosclerosis due to roughness or vascular endothelium.

39. Conditions with both excessive bleeding and intravascular clotting Disseminated intravascular coagulation(DIC) : It is characterized by wide spread clotting accompanied with bleeding tendency due to consumption of many clotting factors. Causes: Retention of a dead fetus in the uterus for weeks Septicemia

40. Hemostatic Function Tests   1 . Blood count and blood film : The platelet count is reduced in thrombocytopenia. 2. Bleeding time: It is the time needed for bleeding to stop without clotting of the blood. The normal bleeding time is 1-3 minutes and it depends on platelet count and function. It is prolonged in thrombocytopenic purpura. 3 . Tests for blood coagulation:  

41. Hemostatic Function Tests 3 . Tests for blood coagulation: Whole blood coagulation time: It is the time needed for blood to clot. Normally, it is 3-10 minutes at 37 C. It is prolonged in both vitamin K deficiency, hemophilia, and liver diseases. The activated partial thromboplastin time (APTD): Normally it is 30-40 seconds and is prolonged in hemophilia. Prothrombin time: normal value is 15 seconds and is prolonged in vitamin K deficiency.  

42. Indications of transfusion To restore whole blood as in haemorrhage. To restore one element e.g. RBCs, WBC, platelets, plasma proteins, clotting factors. In erythroblastosis foetalis.

43. Precautions of transfusion Compatible by cross matching. Rh- person is transfused with Rh- blood. Free from blood born diseases. Free from contamination. Fresh & with high Hb%. Must be stored at 4 C for 21 days maximum

44. Complication of transfusion Agglutination Physical: excessive transfusion→ overloading → HF. Infective: e.g. infective hepatitis, AIDS or malaria. Mechanical: air or fat embolism.

45. Effects of incompatible blood transfusion (Agglutination) Blockade of capillaries:- Backache & joint pain Angina pain (if coronary arteries are occluded). Hemolysis→ liberation of:- Hemoglobin: • increase blood viscosity → HF • Jaundice • Renal failure: death Potassium: hyperkalemia → arrhythmia. Toxic substances e.g. histamine → VD & shock