1. Hemostasis
Mike Clark, M.D.

2. Hemostasis A series of reactions for stoppage of bleeding
During hemostasis, three phases occur in rapid sequence
Vascular spasms – immediate vasoconstriction in response to injury
Platelet plug formation
Coagulation (blood clotting)

3. Vascular Spasm
The first step is immediate constriction of damaged vessels caused by (1) disturbance of myogenic tone and (2) vasoconstrictive paracrine secretions released by the endothelium – like endothelin and thromboxane. Vasoconstriction temporarily decreases blood flow and pressure within the vessel. When you put pressure on a bleeding wound, you also decrease flow within the damaged vessel.

4. Platelet Plug Formation
Platelets do not stick to each other or to blood vessels
Upon damage to blood vessel endothelium platelets:
With the help of von Willebrand factor (VWF) adhere to collagen
Are stimulated by thromboxane A2
Stick to exposed collagen fibers and form a platelet plug
Release serotonin and ADP, which attract still more platelets
The platelet plug is limited to the immediate area of injury by prostacyclin

5. Coagulation
A set of reactions in which blood is transformed from a liquid to a gel
Coagulation follows intrinsic (new name contact activation pathway) and extrinsic pathways (new name tissue factor pathway) and a common pathway
The final three steps of this series of reactions are:
Prothrombin activator is formed
Prothrombin is converted into thrombin
Thrombin catalyzes the joining of fibrinogen into a fibrin mesh

6. Extrinsic Intrinsic
Pathway Pathway
Common Pathway
The Actual Resultant Clot

8. Classes of Clotting Factors
Thrombin sensitive – Factors I, V, VIII and XIII
Vitamin K Dependent – II, VII, IX, X

9. Coagulation versus Anticoagulation
It is important that we do not form inappropriate blood clots. Inappropriate blood clots can lead to sudden death. If a clot (thrombus) forms in the circulation, it could break free and travel (embolus) and block the circulation in vital organs. However, if we get cut we do not want to bleed to death. So we need hemostasis to occur quickly. NOTE: A thrombus is a sitting clot and an embolus is a moving clot.

10. Coagulation versus Anticoagulation
So that we do not form inappropriate blood clots, we have anticoagulants in the blood. This along with other mechanisms to deter hemostasis, is in place prevent inappropriate blood clotting. When we do get cut, we wish for clotting to occur quickly, thus clotting occurs using a physiologic enzymatically converted cascade that involves positive feedback. Clotting Occurs Quickly!

11. Proenzyme A Inactive Clotting Factor A Something Activates it
Activated Enzyme A Active Clotting Factor A
Inactive Enzyme B Inactive Clotting Factor B
Active Enzyme B Active Clotting Factor B
Inactive Enzyme C Inactive Clotting Factor C
Active Enzyme C Active Clotting Factor C
NOTE: As an inactive enzyme is activated, it becomes an enzyme for the subsequent reaction. This is the enzyme cascade action.

12. Proenzyme A Inactive Clotting Factor A Something Activates it
Activated Enzyme A Active Clotting Factor A
Inactive Enzyme B Inactive Clotting Factor B
Active Enzyme B Active Clotting Factor B
Inactive Enzyme C Inactive Clotting Factor C
Active Enzyme C Active Clotting Factor C
NOTE: In some cases, a subsequent enzyme can retrospectively act as an enzyme for a previous reaction. This is positive feedback; the accelerates the clotting cascade allowing for quick clotting.

13. Coagulation
Figure 17.13a

14. Coenzymes and Cofactors
Calcium (Ca++) is absolutely necessary in all clotting reactions, except two reactions. It is acting as a cofactor in the enzymatic reactions.
Certain substances, such as tissue factor and tissue phospholipid will act as coenzymes.
NOTE: If a coenzyme or cofactor is required in an enzymatic reaction, the reaction will not occur if the substances are not present.

15. Detailed Events of Coagulation
Figure 17.13b

16. Coagulation Phase 1: Two Pathways to Prothrombin Activator
May be initiated by either the intrinsic or extrinsic pathway
Triggered by tissue-damaging events
Involves a series of procoagulants
Each pathway cascades toward factor X
Once factor X has been activated, it complexes with calcium ions, PF3, and factor V to form prothrombin activator

17. Coagulation Phase 2: Pathway to Thrombin
Prothrombin activator catalyzes the transformation of prothrombin to the active enzyme thrombin

18. Coagulation Phase 3: Common Pathways to the Fibrin Mesh
Thrombin catalyzes the polymerization of fibrinogen into fibrin
Insoluble fibrin strands form the structural basis of a clot
Fibrin causes plasma to become a gel-like trap
Fibrin in the presence of calcium ions activates factor XIII that:
Cross-links fibrin
Strengthens and stabilizes the clot

19. Extrinsic Pathway (1)
In order to activate the extrinsic pathway a hole must be poked all the way through the blood vessel wall. When that occurs a chemical called “tissue factor” leaks in from the tissue fluids outside of the blood vessel. That chemical along with some circulating calcium in the blood stream activate an enzyme in the bloodstream that converts inactive clotting factor 7 to an active factor 7.

20. Extrinsic Pathway (2)
Activated Factor 7 complexes with circulating calcium and acts as an enzyme to convert inactive Factor 10 to active factor 10.
Activated Factor 10 complexes with a already active Factor 5 along with a platelet phospholipid and circulating calcium to from “Prothrombin Activator.” Prothrombin Activator starts the “common pathway.”

21. Intrinsic Pathway (1)
The intrinsic pathway will initiate if the lining endothelial blood vessel cells are disturbed or blood touches a wettable surface, like metal or glass. Foreign bodies and pathogens in the circulation can activate the intrinsic pathway. It is the intrinsic clotting pathway that causes blood to clot in a glass test tube.

22. Intrinsic Pathway (2)
The blood vessel wall injury or foreign bodies activate an inactive Clotting Factor 12 to an active one. Active clotting factor 12, then acts as an enzyme to activate inactive clotting factor 11. These two activations (12,11) do not require calcium. Factor 11 plus circulating calcium activate an inactive factor 9 to an active 9. Remember, I said that the clotting factor numbers are not in order, because the numbers were given in time of clotting factor discovery and not the process.

23. Intrinsic Pathway (3)
Active Clotting Factor 9, complexes with an already active Clotting Factor 8 plus some calcium to act as an enzyme to activate an inactive clotting factor 10 to an active one.
As in the Extrinsic pathway, once Factor 10 is activated, it will assist in forming “Prothrombin Activator”, thus starting the Common Pathway.

24. Common Pathway
Activated Factor 10 plus an already active clotting factor 5, plus platelet phospholipid and calcium will form “Prothrombin Activator.”
Prothrombin activator acts to convert Clotting Factor 2 (Prothrombin) to Thrombin.
Thrombin plus calcium act to convert clotting factor 1 (Fibrinogen) to Fibrin. Fibrin is the actual stringy clot substance.

25. Tightening and Strengthening the Clot
Once the clot forms, it needs to tighten to give a good plug over the blood vessel damage.
Two things help this: 1. platelet contraction and 2. the action of Clotting Factor 13, termed fibrin stabilizing factor.
Fibrin stabilizing factor gives better covalent bonding within the clot.

26. Factor XIII action

27. Clot Retraction and Repair
Clot retraction – stabilization of the clot by squeezing serum from the fibrin strands
Repair
Platelet-derived growth factor (PDGF) stimulates rebuilding of blood vessel wall
Fibroblasts form a connective tissue patch
Stimulated by vascular endothelial growth factor (VEGF), endothelial cells multiply and restore the endothelial lining

28. Factors Limiting Clot Growth or Formation
Two homeostatic mechanisms prevent clots from becoming large
Swift removal of clotting factors
Inhibition of activated clotting factors
Protein C – glycoprotein produced in the liver and is the major inhibitor of coagulation. It degrades factors V and VIII. It needs Protein S to work.
Protein S – Produced in liver and acts as a cofactor for Protein C.
Thrombomodulin – binds to thrombin and activates and activates Protein C
Antithrombin III – produced in liver and is major inhibitor of Thrombin -

29. Heparin Produced by mast cells, basophils and endothelial cells.
Complexes with Antithrombin III to inhibit Thrombin – but also inhibits XII, XI, X, IX

30. Plasminogen

31. Inhibition of Clotting Factors
Thrombin not absorbed to fibrin is inactivated by antithrombin III
Heparin, another anticoagulant, also inhibits thrombin activity

32. Inhibition of Clotting Factors
Fibrin acts as an anticoagulant by binding thrombin and preventing its:
Positive feedback effects of coagulation
Ability to speed up the production of prothrombin activator via factor V
Acceleration of the intrinsic pathway by activating platelets

33. Factors Preventing Undesirable Clotting
Unnecessary clotting is prevented by endothelial lining the blood vessels
Platelet adhesion is prevented by:
The smooth endothelial lining of blood vessels
Heparin and PGI2 secreted by endothelial cells
Vitamin E quinone, a potent anticoagulant

34. Commercial Anticoagulants
Heparin – acts as an inhibitor of the intrinsic pathway – measured by the PTT
Coumadin- acts an a vitamin K antagonist. (Factors 2, 7, 9,10 inhibited) Extrinsic Pathway- PT
NOTE: Factor 7 has the shortest serum half-life
NEW ONE SEEN OF TV – XARELTO
It blocks factor X (10).

35. Hemostasis Disorders: Thromboembolytic Conditions
Thrombus – a clot that develops and persists in an unbroken blood vessel
Thrombi can block circulation, resulting in tissue death
Coronary thrombosis – thrombus in blood vessel of the heart

36. Hemostasis Disorders: Thromboembolytic Conditions
Embolus – a thrombus freely floating in the blood stream
Pulmonary emboli can impair the ability of the body to obtain oxygen
Cerebral emboli can cause strokes

37. Hemostasis Disorders: Thromboembolytic Conditions
Embolus – a thrombus freely floating in the blood stream
Pulmonary emboli can impair the ability of the body to obtain oxygen
Cerebral emboli can cause strokes

38. Hemostasis Disorders
Disseminated Intravascular Coagulation (DIC): widespread clotting in intact blood vessels
Residual blood cannot clot
Blockage of blood flow and severe bleeding follows
Most common as:
A complication of pregnancy
A result of septicemia or incompatible blood transfusions

39. Hemostasis Disorders: Bleeding Disorders
Thrombocytopenia – condition where the number of circulating platelets is deficient
Patients show petechiae due to spontaneous, widespread hemorrhage
Caused by suppression or destruction of bone marrow (e.g., malignancy, radiation)
Platelet counts less than 50,000/mm3 is diagnostic for this condition
Treated with whole blood transfusions

40. Hemostasis Disorders: Bleeding Disorders
Inability to synthesize procoagulants by the liver results in severe bleeding disorders
Causes can range from vitamin K deficiency to hepatitis and cirrhosis
Inability to absorb fat can lead to vitamin K deficiencies as it is a fat-soluble substance and is absorbed along with fat
Liver disease can also prevent the liver from producing bile, which is required for fat and vitamin K absorption

41. Hemostasis Disorders: Bleeding Disorders
Hemophilias – hereditary bleeding disorders caused by lack of clotting factors
Hemophilia A – most common type (83% of all cases) due to a deficiency of factor VIII
Hemophilia B – due to a deficiency of factor IX
Hemophilia C – mild type, due to a deficiency of factor XI

42. Hemostasis Disorders: Bleeding Disorders
Symptoms include prolonged bleeding and painful and disabled joints
Treatment is with blood transfusions and the injection of missing factors
PT (Prothrombin Time) measures extrinsic pathway
PTT (Partial Thromboplastin Time) measures intrinsic pathway
INR (International Normalized Ratio)

43. Diagnostic Blood Tests
Laboratory examination of blood can assess an individual’s state of health
Microscopic examination:
Variations in size and shape of RBCs – predictions of anemias
Type and number of WBCs – diagnostic of various diseases
Chemical analysis can provide a comprehensive picture of one’s general health status in relation to normal values