Primary Hemostasis

• Platelet plug formation, or primary hemostasis, is the initial step in preventing blood loss from an injured vessel.
• Hemostasis is the body's process of stopping blood loss from damaged blood vessels.
• Without hemostasis, even minor injuries could be life-threatening.
• During primary hemostasis, platelets clump together to form a plug at the injury site.
• Secondary hemostasis reinforces the platelet plug with a fibrin protein mesh.

Primary Hemostasis Steps

• Primary hemostasis is divided into five steps: endothelial injury, exposure, adhesion, activation, and aggregation.
• Endothelial Injury: Damage to the artery's innermost layer, the endothelium, occurs.
  • Nerves trigger reflexive contraction of smooth muscles near the injury, known as vascular spasm, reducing blood flow.
  • Injured endothelial cells decrease secretion of nitric oxide and prostaglandins and increase secretion of endothelin, causing smooth muscle contraction.
• Exposure: Collagen beneath the damaged endothelial cells is exposed, and damaged cells release Von Willebrand factor, which binds to the exposed collagen.
• Adhesion: Platelets circulating in the blood come into contact with the Von Willebrand factor bound to collagen.
  • Platelets have a surface protein called GP1B that allows them to bind to the Von Willebrand factor proteins.
• Activation: Platelets binding to Von Willebrand factor via GP1B become "activated", triggering several actions.
  • Platelets change shape, forming tentacle-like arms to grab other platelets.
  • Platelets release more Von Willebrand factor, serotonin (attracts more platelets), and calcium (used in secondary hemostasis).
  • Platelets secrete adenosine diphosphate (ADP) and thromboxane A2, activating other platelets, creating a positive feedback loop.
  • Prostaglandin and nitric oxide secreted by undamaged endothelial cells inhibit platelet activation, limiting the positive feedback loop to the injury site.
  • ADP and thromboxane A2 cause platelets to express a new surface protein called GPIIB/IIIA, marking full activation.
• Aggregation: ADP and thromboxane cause platelets to stick to collagen and free-floating platelets to express GPIIB/IIIA.
  • GPIIB/IIIA binds to fibrinogen (a circulating blood protein), linking platelets together, leading to rapid platelet aggregation at the injury site, forming a platelet plug.
  • Fibrinogen gets cleaved into fibrin during secondary hemostasis, forming a protein mesh to reinforce the platelet plug.

Summary of Hemostasis

• Primary hemostasis is the formation of a platelet plug to prevent blood loss from a damaged vessel.
• Secondary hemostasis involves the coagulation cascade forming a fibrin mesh over the platelet plug.
• Anticoagulation regulates clot formation, while clot retraction and fibrinolysis occur after hemostasis.

Anticoagulation

• Proteins prevent clots from growing too large and block blood flow to tissues supplied by the vessel. It also prevents clots from breaking off.
• Thrombin, or factor II, is a crucial clotting factor with multiple pro-coagulative functions.
  • It binds to platelet receptors, causing them to activate.
  • It activates cofactors factor V (common pathway), and factor VIII (intrinsic pathway).
  • It proteolytically cleaves fibrinogen (factor I) into fibrin (factor Ia).
  • Thrombin cleaves stabilizing factor (factor XIII) into factor XIIIa, which forms cross-links between fibrin chains.
• Protein C, with cofactor protein S, interacts with thrombomodulin on endothelial cells, which means undamaged cells limit coagulation to injury site.
  • Complex of protein C, protein S, and thrombin-thrombomodulin cleaves and inactivates active factor V and VIII.
• Antithrombin III (antithrombin) binds thrombin and factor X, inactivating them.
  • It also inhibits factors VII, IX, XI, and XII with less affinity.
  • Heparin binds to antithrombin, increasing its affinity for target proteins.
• Healthy endothelial cells release nitric oxide and prostacyclin, reducing thromboxane A2 production in activated platelets, preventing GPIIB/IIIA expression and platelet aggregation.

Clot Retraction

• Actin and myosin proteins in activated platelets contract lamellipodia (structures that increase surface area), binding to the fibrin mesh and endothelial lining.
• Integrin αIIbβ3 receptors on platelets bind to fibrin, activating actin and myosin, contracting the lamellipodia and tightening the fibrin mesh.
• Clot retraction pulls wound edges closer, allowing endothelial cells to divide and repair the tissue.

Fibrinolysis

• Fibrinolysis is the process of dissolving the blood clot through the degradation of the fibrin mesh.
• Circulating plasminogen (produced by the liver) is converted by tissue plasminogen activator (tPA) into plasmin (active form).
• Healthy endothelial cells release tPA when exposed to coagulation factors (mainly factor X and thrombin).
• Endothelial cells also release plasminogen activator inhibitor 1 and antiplasmin, sequestering plasminogen and plasmin, respectively.
• Plasmin acts as a protease, cutting fibrin into smaller pieces, dissolving the clot by releasing trapped red blood cells and platelets.
• Other proteins activate plasmin, including coagulation factors IXa, XIIa, kallikrein, and protein C.
• TPA is clinically used as a "clot buster" to dissolve blood clots.
• Clot retraction stabilizes the clot and pulls wound edges together.
• Fibrinolysis is dependent on tPA, which converts plasminogen into plasmin, an enzyme that cleaves fibrin and dissolves clots.

Vitamin K Role

• Vitamin K is essential for blood coagulation by aiding the conversion of certain coagulation factors into their mature forms.
• Without vitamin K, the body would be unable to control clot formation.
• Hemostasis is divided into two phases: primary (platelet plug formation) and secondary (fibrin mesh reinforcement).
• Coagulation factors are activated via proteolysis.
• There are twelve coagulation factors numbered factors I-XIII (no factor VI).
• Most factors are produced by the liver, and vitamin K is required to produce factors II, VII, IX, and X.
• Vitamin K is abundant in green leafy foods and synthesized by bacteria in the gastrointestinal tract.
• It is a fat-soluble vitamin (A, D, E, K), meaning that it can be stored in fat cells instead of being excreted by the kidneys.
• Vitamin K quinone, the dietary form, is converted to vitamin K hydroquinone by quinone reductase using NADPH.
• Vitamin K hydroquinone donates electrons to gamma glutamyl carboxylase, converting non-functional coagulation factors II, VII, IX, and X into functional forms by adding a carboxyl group onto glutamic acid residues on the proteins.
• Quinone reductase converts vitamin K epoxide back into vitamin K quinone.
• Warfarin blocks epoxide reductase, preventing vitamin K recycling and inhibiting activation of factors II, VII, IX, and X.
• Intrinsic pathway: factor XII comes into contact with activated platelets or collagen, activates factor XI, which activates factor IX which activates factor X.
• Factor X starts the common pathway where it activates factor II, which activates factor I that builds the fibrin mesh
• Extrinsic pathway: exposed tissue factor activates factor VII, which activates factor X and starts the common pathway.
• Vitamin K is required for the function of the extrinsic, intrinsic, and common pathways because it produces factors II, VII, IX, and X.

Primary hemostasis is the first stage of blood vessel repair. It involves a series of steps to form a platelet plug at the injury site. These steps include endothelial injury, exposure, adhesion, activation, and aggregation, all essential for stopping blood loss.