HEMOSTASIS

1. HEMOSTASIS • Prevention of blood loss • achieved by several mechanisms: • (1) vascular constriction • (2) formation of a platelet plug, • (3) formation of a blood clot-blood coagulation • (4) eventual fibrous tissue growth into the blood clot to close the hole permanently

2. 1. Vascular Constriction • Blood vessel cut/ruptured- the trauma causes smooth muscle in the wall to contract- -reduces flow of blood from ruptured vessel. Mechanisms are: • (1) local myogenic spasm • (2) local factors from the traumatized tissues and platelets, & • (3) nervous reflexes- initiated by pain nerve impulses or other sensory impulses from traumatized vessel or nearby tissues. • However, more vasoconstriction results from local myogenic contraction of the blood vessels initiated by direct damage to the vascular wall • In smaller vessels, the platelets thromboxane A2 responsible • The greater the degree trauma, the greater the vascular spasm. • Spasm can last for minutes or hours, allowing for platelet plugging and blood coagulation to take place.

3. 2. Platelet Plug formationm • If blood vessel cut is very small- sealed by a platelet plug, rather than by a blood clot. • extremely important for closing minute ruptures in very small blood vessels that occur many thousands of times daily. • In thrombocytopenia - thousands of small hemorrhagic areas under the skin/ internal tissues, but not in the normal persons

4. 3. Blood Coagulation in Ruptured Vessel • Fibrin now enmeshes blood cells to form the CLOT • Initially proposed by Morawitz in 1958 as: • Begins 15 to 20 secs if vascular wall trauma is severe, and in 1 to 2 minutes if the trauma is minor. • Initiated by substances from traumatized vascular wall, platelets & blood proteins adhering to traumatized vascular wall • Thromboplastin formed: converts Prothrombin to Throbmbin • Thrombin now converts Fibrinogen to fibrin in presence of Ca+

5. Then Clot retraction • Within 3 to 6 minutes after rupture of a vessel, if the vessel opening is not too large, the entire opening or broken end of the vessel is filled with clot. • After 20 minutes to an hour, the clot retracts; this closes the vessel still further • Platelets also play an important role in this clot retraction

6. 4. Fibrous tissue growth into the blood clot • After clotting, next is one of two courses: (1) Clot become invaded by fibroblasts, which then form connective tissue all through the clot, or (2) it can dissolve • Usual course for clot in a small hole is invasion by fibroblasts (promoted partially by growth factor from platelets), continues to complete organization of the clot into fibrous tissue within about 1 to 2 weeks. • But when excess blood leaked into tissues and tissue clots have occurred where they are not needed, special substances within the clot become activated to dissolve the clot

7. Mechanism of Blood Coagulation • More than 50 important substs that cause or affect blood coagulation found in blood and in tissues • called procoagulants (promote coagulation) & others anticoagulants (inhibit coagulation) • Coagulatation depends on the balance between these • Normally, anticoagulants predominate, thus blood does not coagulate while circulating in blood vessels. • But if vessel is ruptured, procoagulants from the area of tissue damage become "activated" and override the anticoagulants, and then a clot does develop. • Major aim-conversion of prothrombin to thrombin & then polymerization of fibrinogen to fibrin fibers

9. Coagulation: 3 essential steps: (1) formation of prothrombin activator (a complex of activated substances) by a complex cascade of chemical reactions in response to rupture of vessel/ damage to the blood itself, • Two pathways for this reaction- Intrinsic and extrinsic (2) The prothrombin activator catalyzes conversion of prothrombin into thrombin. (3) The thrombin acts as an enzyme to convert fibrinogen into fibrin fibers that enmesh platelets, blood cells, and plasma to form the clot.

10. 1. Formation of Prothrombin Activator Generally considered to be formed in 2 ways, though the two constantly interact in reality (1) the extrinsic pathway- begins with trauma to the vascular wall and surrounding tissues & (2) the intrinsic pathwaythat begins in blood itself • In both pathways, a series of different plasma proteins called blood-clotting factors play major roles. • Most are inactive forms of proteolytic enzymes, if converted to active forms, enzymatic actions cause successive, cascading reactions of clotting process.

11. A. Extrinsic Pathway Traumatized vascular wall /traumatized extravascular tissues that come in contact with blood ↓ Release of tissue factor or tissue thromboplastin (from traumatized tissue) composed especially of phospholipids from the membranes of the tissue plus a proteolytic enzyme (lipoprotein complex)↓ Activation Factor VII - VIIa ↓ ca+ Activation of Factor X - Xa ↓Ca+ , PF3, Va ( factor V activated by thrombin) Prothrombin activator (+ Ca+, splits prothrombin to thrombin) At first, Factor V in prothrombin activator complex inactive, but once thrombin forms- activation & acceleration

12. Intrinsic Pathway Trauma to blood/exposure to collagen from traumatized vessel wall ↓ XII →XIIa & release of platelet phospholipids(PF 3) ↓ HMW kininogen, accelerated by prekallikrein. XI → XIa ↓ca+ IX → IXa. ↓ VIIIa, Ca+,PF3 (if either Factor VIII or platelets are deficient, this step is deficient- eg hemophilia) ↓ X → Xa X Action of Xa to form prothrombin activator is the same as in extrinsic pathway.

13. Interaction Between the Extrinsic and Intrinsic Pathways • After blood vessels rupture, clotting occurs by both pathways simultaneously. • Tissue factor initiates extrinsic pathway, whereas contact of Factor XII and platelets with collagen in the vascular wall initiates the intrinsic pathway. • Difference is that extrinsic pathway can be explosive; once initiated, speed is limited only by the amount of tissue factor released from traumatized tissues & quantities of Factors X, VII, and V in the blood. With severe tissue trauma, clotting can occur in as little as 15 secs. • Intrinsic much slower, requiring 1 to 6 mins to cause clotting

14. Prothrombin activator as determinant • Usually formation of prothrombin activator is rate limiting step, because subsequent reactions normally occur rapidly to form the clot • Thus, in the final prothrombin activator complex, Xa causes splitting of prothrombin to form thrombin; activated Factor V greatly accelerates this and platelet phospholipids act as a vehicle that further accelerates the process. • Note positive feedback effect of thrombin through Factor V

15. Prothrombin to Thrombin • Prothrombin activator, in the presence of sufficient Ca++, causes conversion of prothrombin to thrombin • Prothrombin: Unstable plasma protein, formed by liver, mol wt of 68,700; splits easily into smaller compounds, one of which is thrombin, (mol.wt of 33,700) almost 1/2. • Vitamin K required for normal formation of prothrombin & a few other clotting factors. • Therefore, either lack of vitamin K or liver disease prevents normal prothrombin formation thus decreases the prothrombin level so low that a bleeding tendency results.

16. Fibrinogen to Fibrin • Thrombin (protein enzyme) acts on fibrinogen to remove 4 low-mol-weight peptides from each molecule forming one mol of fibrin monomer that has automatic capability to polymerize with each other to form fibrin fibers. • Polymerize within secs into long fibrin fibers that constitute reticulum of the clot. • In early stages, monomer mols held together by weak noncovalent hydrogen bonding, and are not cross-linked with one another; but thrombin activates fibrin-stabilizing factor present in normal plasma globulins but also released from platelets entrapped in clot.. • covalent bonds between more of the fibrin monomer molecules -adding tremendously to strength of the fibrin meshwork. • Platelets play an role because much of the prothrombin1st attaches to prothrombin receptors on the platelets already bound to the damaged tissue

17. Fibrinogen. • High-mol-weight protein (MW = 340,000) formed in the liver (reduced by liver disease ) • Because of its large size, little leaks from blood vessels into the interstitial fluids, and thus interstitial fluids ordinarily do not coagulate. • Yet, when the permeability of the capillaries becomes pathologically increased, fibrinogen then leak into the tissue fluids in sufficient quantities to allow clotting of these fluids in much the same way that plasma and whole blood can clot.

18. Blood Clot,Clot Retraction-Serum: • Clot-meshwork of fibrin fibers entrapping blood cells, platelets & plasma • Fibrin fibers also adhere to damaged surfaces of vessels; thus blood clot is adherent to vascular opening & thereby prevents further blood loss • Within few minutes after clot is formed, it begins to contract & usually expresses most of the fluid from the clot within 20 to 60 minutes. • expressed fluid called serum because all its fibrinogen and other clotting factors have been removed; thus serum differs from plasma (cant clot) • Platelets are necessary for clot retraction to occur. Therefore, failure of clot retraction is an indication that the number of platelets in the circulating blood might be low. • Also, platelets contribute directly to clot contraction by activating platelet thrombosthenin, actin, and myosin mols (all contractile proteins) and cause strong contraction of the platelet spicules attached to the fibrin-compress the fibrin meshwork into a smaller mass. • Contraction activated & accelerated by thrombin & by ca+ released from mitochondria, endoplasmic reticulum, and Golgi apparatus of platelets.

19. Vicious Circle of Clot Formation • Once a critical amount of thrombin is formed, a vicious circle develops -more blood clotting and more thrombin formed; & clot grows until blood leakage ceases • Thus clot initiates a vicious circle (positive feedback) • Mainly due to the proteolytic action of thrombin on many blood-clotting factors in addition to fibrinogen. • Thrombin has a direct proteolytic effect on prothrombin itself, tending to convert this into still more thrombin • Also acts on some of the blood-clotting factors responsible for formation of prothrombin activator- • Egs - acceleration of the actions of Factors VIII, IX, X, XI, and XII and aggregation of platelets

20. Lysis of Blood Clots-Plasmin • The plasma proteins contain plasminogen (or profibrinolysin) that, when activated to plasmin (or fibrinolysin) digests fibrin fibers and some other protein coagulants such as fibrinogen, Factors V, VIII, XII & prothrombin • Therefore, whenever plasmin is formed, it can cause lysis of a clot by destroying many of the clotting factors, thereby sometimes even causing hypocoagulability of the blood.

21. Activation of Plasminogen to Form Plasmin • a large amount of plasminogen is trapped in the clot along with other plasma proteins • The injured tissues and vascular endothelium very slowly release a powerful activator called tissue plasminogen activator (t-PA) • t-PA after few days converts plasminogen to plasmin, which in turn removes the remaining unnecessary blood clot. • In fact, many small blood vessels in which blood flow has been blocked by clots are reopened by this mechanism

22. Role of Ca+ in Intrinsic & Extrinsic Pathways • Except for the first 2 steps in intrinsic pathway, required for promotion or acceleration of all the blood-clotting reactions • Thus, in t absence of ca+, blood clotting by either pathway does not occur. In-vivo, conc seldom falls low enough to significantly affect clotting • But in-vitro, blood can be prevented from clotting by reducing the ca+ concentration either by deionizing ( react with substances such as citrate ion) or by precipitating it with substances like oxalate ion