1. Blood Clotting In the absence of blood vessel damage, platelets are repelled from each other and from the endothelium of blood vessels. When a blood vessel is injured and the endothelium is broken, glycoproteins in the platelet’s plasma membrane are now able to bind to the exposed collagen fibers. Endothelial cells produce von Willebrand’s factor, which binds to both collagen and the platelets. When platelets stick to collagen, they degranulate as the secretory granules release their products. These granules contain ADP, serotonin, and a prostaglandin called thromboxane A 2. 1

2. The ADP and thromboxane A 2 released from activated platelets recruits new platelets to the vicinity and makes them “sticky,” so that they adhere to those stuck on the collagen. This process repeated many times by the adhering platelets until platelet plug form in the damaged vessel. The activated platelets also help to activate plasma clotting factors, leading to the conversion of a soluble plasma protein known as fibrinogen into an insoluble fibrous protein, fibrin. 2

3. (a) Platelet aggregation is prevented in an intact endothelium because it separates the blood from collagen, a potential platelet activator. Also, the endothelium secretes nitric oxide (NO) and prostaglandin I 2 (PGI 2 ), which inhibit platelet aggregation. An enzyme called CD39 breaks down ADP in the blood, which would otherwise promote platelet aggregation. 3

4. (b) When the endothelium is broken, platelets adhere to collagen and to von Willebrand’s factor (VWF), which helps anchor the platelets that are activated by this process and by the secretion of ADP and thromboxane A 2 ( Tx A 2 ), a prostaglandin. 4

5. (c) A platelet plug is formed and reinforced with fibrin proteins. 5

6. Clotting Factors: Formation of Fibrin The platelet plug is strengthened by a meshwork of insoluble protein fibers known as fibrin. Colorized scanning electron micrograph of a blood clot. The threads of fibrin have been colored green, the erythrocytes are shown red, and the platelets have been colored purple. Blood clots contain platelets and fibrin, and trapped red blood cells. Fluid squeezed from the clot as it retracts is called serum, which is plasma without fibrinogen, the soluble precursor of fibrin. 6

7. 7 The conversion of fibrinogen into fibrin may occur via either of two pathways:- 1- The intrinsic pathway The intrinsic pathway is initiated by the exposure of plasma to a negatively charged surface, such as that provided by collagen at the site of a wound or by the glass of a test tube. This contact pathway activates a plasma protein called factor XII. Active factor XII in turn activates another clotting factor, which activates yet another. The plasma clotting factors are numbered in order of their discovery, which does not reflect the actual sequence of reactions. The next steps in the sequence require the presence of Ca 2+ and phospholipids, the latter provided by platelets.

8. 8 These steps result in the conversion of an inactive prothrombin, into the active enzyme thrombin. Thrombin converts the soluble protein fibrinogen into fibrin monomers. These monomers are joined together to produce the insoluble fibrin polymers that form a meshwork supporting the platelet plug. 2- The extrinsic pathway This pathway of clot formation is initiated by tissue factor (or tissue thromboplastin, also known as factor III), a membrane protein found in many different tissues. This pathway generates thrombin and fibrin more rapidly than the intrinsic contact pathway Thrombin, once formed, activates factors VIII and V in a positive feedback fashion.

9. 9 The clotting pathways. (1) The extrinsic clotting pathway is initiated by the release of tissue factor. (2) The intrinsic clotting pathway is initiated by the activation of factor XII by contact with collagen or glass. (3) Extrinsic and intrinsic clotting pathways converge when they activate factor X, eventually leading to the formation of fibrin.

10. 10 Dissolution of Clots As the damaged blood vessel wall is repaired, activated factor XII promotes the conversion of an inactive molecule in plasma into the active form called kallikrein. Kallikrein, in turn, catalyzes the conversion of inactive plasminogen into the active molecule plasmin. Plasmin is an enzyme that digests fibrin into “split products,” thus promoting dissolution of the clot.

11. 11 Anticoagulants Clotting of blood in test tubes can be prevented by the addition of sodium citrate or ethylenediaminetetraacetic acid (EDTA), both of which chelate calcium. Heparin is also anticoagulant. Heparin activates antithrombin III, a plasma protein that combines with and inactivates thrombin.

12. 12 Some Acquired and Inherited Clotting Disorders and a Listing of Anticoagulant Drugs.