{"id":6558,"date":"2026-06-03T14:09:56","date_gmt":"2026-06-03T18:09:56","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/pathophysiology\/?post_type=chapter&p=6558"},"modified":"2026-06-09T15:22:58","modified_gmt":"2026-06-09T19:22:58","slug":"section-7-hemostasis-blood-clotting","status":"web-only","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/pathophysiology\/chapter\/section-7-hemostasis-blood-clotting\/","title":{"raw":"Section 9:\u00a0 Hemostasis (Blood Clotting)","rendered":"Section 9:\u00a0 Hemostasis (Blood Clotting)"},"content":{"raw":"

Hemostasis<\/strong> is the rapid process (taking minutes) by which a damaged blood vessel is sealed to stop bleeding.\u00a0 From Greek\/Latin: hemo-<\/strong> (blood) + -stasis<\/strong> (motionless).\u00a0 Three major stages:<\/span><\/p>\r\n\r\n

Stage 1:\u00a0 Vascular Spasm<\/strong><\/span><\/h3>\r\n

Damaged endothelial cells<\/strong> (of blood vessel walls) release ADP, Tissue Factor<\/strong> and endothelin.\u00a0 <\/strong>Endothelin peptides are potent vasoconstrictors<\/strong> that trigger the smooth muscle layer of the blood vessel to contract, narrowing the vessel and minimizing blood loss.<\/span><\/p>\r\n\r\n

Stage 2:\u00a0 Platelet Plug Formation<\/strong><\/span><\/h3>\r\n

Platelets are attracted to the wound, activate, and adhere to exposed collagen<\/strong> of the damage blood vessel wall - forming an unstable plug<\/strong> within seconds to minutes.\u00a0 Prior to becoming activated, 30% of platelets are typically located in the spleen and 70% of platelets are circulating the blood stream.\u00a0 \u00a0Inactive platelets travel the bloodstream and have smooth surfaces; activated platelets become spiky<\/strong> and sticky.<\/strong>\u00a0 Activated platelets release:\u00a0 ADP<\/strong> (attracts more platelets), thromboxane A2<\/sub><\/strong> (vasoconstrictor; recruits more platelets), serotonin (vasoconstrictor), clotting factors<\/strong>, Ca2+<\/sup><\/strong>, prostaglandins,<\/strong> and PDGF<\/strong> (Platelet Derived Growth Factor).\u00a0 The recruitment and activation of more platelets is termed a positive feedback loop<\/strong> that ends when the wound is sealed.<\/p>\r\n\r\n

Stage 3:\u00a0 Coagulation<\/strong><\/span><\/h3>\r\n

Plasma protein fibrinogen<\/strong> (globular, water-soluble) is converted to fibrin<\/strong> (long, water-insoluble, rope-like proteins) that interweave through the platelet plug to stabilize it.\u00a0 Clot retraction<\/strong> then occurs as platelets contract, pulling torn vessel edges together.<\/p>\r\nFibrin<\/strong> production involves two pathways running simultaneously, both activating Prothrombin Activator (Factor X)<\/strong> in the process:\r\n\r\n\r\n\r\n\r\n
Extrinsic Pathway<\/strong><\/span><\/td>\r\nTissue Factor<\/strong> released from endothelial cells triggers a cascade (of clotting factors + calcium) activating enzyme Prothrombin<\/strong> Activator (Factor X)<\/strong> \u2192 converts inactive enzyme prothrombin<\/strong> \u2192 active thrombin<\/strong> \u2192 converts fibrinogen<\/strong> to fibrin.<\/strong>\r\n\r\nThis pathway is faster.<\/strong><\/td>\r\n<\/tr>\r\n
Intrinsic Pathway<\/strong><\/span><\/td>\r\nPlatelet Factor<\/strong> and calcium<\/strong> released from platelets trigger the activation of the intrinsic clotting factors cascade which also converges on Prothrombin Activator (Factor X)<\/strong> activation \u2192 converts prothrombin<\/strong> (inactive) \u2192 thrombin<\/strong> (active) \u2192 converts fibrinogen<\/strong> \u2192 fibrin.<\/strong>\r\n\r\nThis pathway is slower but also necessary<\/strong>.<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n[caption id=\"attachment_4217\" align=\"alignnone\" width=\"300\"]\"A<\/a> A basic diagram of the series of events that occur after a wound, starting with the attraction of platelets to the wound site, the cascade of reactions activating clotting factors, and ending with the conversion of fibrinogen into fibrin and scab formation. (Note: Not all 13 clotting factors are explicitly pointed out; blue clotting factors are inactive, green are active, and black is the activated fibrinogen)[\/caption]\r\n\r\n \r\n

Fibrinolysis and Healing<\/strong><\/span><\/h3>\r\n

Once healing begins, fibrin is dissolved through fibrinolysis.<\/strong>\u00a0 tPA (Tissue-Type Plasminogen Activator)<\/strong> converts plasminogen<\/strong> to active plasmin<\/strong> protease, which degrades fibrin.<\/strong>\u00a0 Macrophages<\/strong> remove cellular debris; platelets<\/strong> release PDGF<\/strong> to stimulate vessel regeneration; fibroblasts produce mitogens<\/strong> to stimulate mitosis (cell division) and tissue repair.<\/p>\r\n\r\n

Platelet Regulation<\/strong><\/span><\/h3>\r\n

Endothelial cells release prostacyclin<\/strong> (a prostaglandin family member) - acting as both a vasodilator and platelet inhibitor - to prevent excessive platelet aggregation.<\/p>\r\n\r\n

\r\n

Platelet Disorders<\/p>\r\n\r\n<\/header>\r\n

\r\n\r\nThrombocytopenia:<\/strong>\u00a0 Low platelet levels (nutritional deficiency, illness, or medication).\r\n\r\nSigns:\u00a0 prolonged bleeding, frequent nosebleeds, heavy menstruation, bruising, and petechiae (tiny red spots from capillary hemorrhages).\r\n\r\nHemophilia:<\/strong>\u00a0 Impaired blood clotting due to inherited mutations in clotting factors (most commonly Factor VIII or Factor IX, both on the X chromosome).\r\n\r\nThrombocytophilia (Thrombocytosis, Thrombocythemia):<\/strong>\u00a0 Abnormally high platelet counts, can lead to dangerous blood clots, increasing the risk of heart attacks and strokes.\r\n\r\n<\/div>\r\n<\/div>\r\n
\r\n

* Did you know?\u00a0 Facts About Hemostasis<\/strong><\/p>\r\n\r\n<\/header>\r\n

\r\n\r\nSurgeons may apply a topical collagen agen<\/strong>t to attract the patient's own platelets and stimulate natural hemostasis.\r\n\r\nApplying direct pressure<\/strong> to a wound is one of the most effective first-aid measures for slowing blood loss.\r\n\r\nSutures<\/strong> (stitches) close a wound mechanically, speed recovery, and reduce scar formation.\r\n\r\nVitamin K<\/strong> (found in green vegetables, grains, and organ meats - and produced by intestinal bacteria) is required for synthesis of multiple clotting factors.\u00a0 Deficiency impairs hemostasis.\r\n\r\n<\/div>\r\n<\/div>\r\n

<\/h3>","rendered":"

Hemostasis<\/strong> is the rapid process (taking minutes) by which a damaged blood vessel is sealed to stop bleeding.\u00a0 From Greek\/Latin: hemo-<\/strong> (blood) + -stasis<\/strong> (motionless).\u00a0 Three major stages:<\/span><\/p>\n

Stage 1:\u00a0 Vascular Spasm<\/strong><\/span><\/h3>\n

Damaged endothelial cells<\/strong> (of blood vessel walls) release ADP, Tissue Factor<\/strong> and endothelin.\u00a0 <\/strong>Endothelin peptides are potent vasoconstrictors<\/strong> that trigger the smooth muscle layer of the blood vessel to contract, narrowing the vessel and minimizing blood loss.<\/span><\/p>\n

Stage 2:\u00a0 Platelet Plug Formation<\/strong><\/span><\/h3>\n

Platelets are attracted to the wound, activate, and adhere to exposed collagen<\/strong> of the damage blood vessel wall – forming an unstable plug<\/strong> within seconds to minutes.\u00a0 Prior to becoming activated, 30% of platelets are typically located in the spleen and 70% of platelets are circulating the blood stream.\u00a0 \u00a0Inactive platelets travel the bloodstream and have smooth surfaces; activated platelets become spiky<\/strong> and sticky.<\/strong>\u00a0 Activated platelets release:\u00a0 ADP<\/strong> (attracts more platelets), thromboxane A2<\/sub><\/strong> (vasoconstrictor; recruits more platelets), serotonin (vasoconstrictor), clotting factors<\/strong>, Ca2+<\/sup><\/strong>, prostaglandins,<\/strong> and PDGF<\/strong> (Platelet Derived Growth Factor).\u00a0 The recruitment and activation of more platelets is termed a positive feedback loop<\/strong> that ends when the wound is sealed.<\/p>\n

Stage 3:\u00a0 Coagulation<\/strong><\/span><\/h3>\n

Plasma protein fibrinogen<\/strong> (globular, water-soluble) is converted to fibrin<\/strong> (long, water-insoluble, rope-like proteins) that interweave through the platelet plug to stabilize it.\u00a0 Clot retraction<\/strong> then occurs as platelets contract, pulling torn vessel edges together.<\/p>\n

Fibrin<\/strong> production involves two pathways running simultaneously, both activating Prothrombin Activator (Factor X)<\/strong> in the process:<\/p>\n\n\n\n\n
Extrinsic Pathway<\/strong><\/span><\/td>\nTissue Factor<\/strong> released from endothelial cells triggers a cascade (of clotting factors + calcium) activating enzyme Prothrombin<\/strong> Activator (Factor X)<\/strong> \u2192 converts inactive enzyme prothrombin<\/strong> \u2192 active thrombin<\/strong> \u2192 converts fibrinogen<\/strong> to fibrin.<\/strong><\/p>\n

This pathway is faster.<\/strong><\/td>\n<\/tr>\n

Intrinsic Pathway<\/strong><\/span><\/td>\nPlatelet Factor<\/strong> and calcium<\/strong> released from platelets trigger the activation of the intrinsic clotting factors cascade which also converges on Prothrombin Activator (Factor X)<\/strong> activation \u2192 converts prothrombin<\/strong> (inactive) \u2192 thrombin<\/strong> (active) \u2192 converts fibrinogen<\/strong> \u2192 fibrin.<\/strong><\/p>\n

This pathway is slower but also necessary<\/strong>.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

\"A<\/a>
A basic diagram of the series of events that occur after a wound, starting with the attraction of platelets to the wound site, the cascade of reactions activating clotting factors, and ending with the conversion of fibrinogen into fibrin and scab formation. (Note: Not all 13 clotting factors are explicitly pointed out; blue clotting factors are inactive, green are active, and black is the activated fibrinogen)<\/figcaption><\/figure>\n

 <\/p>\n

Fibrinolysis and Healing<\/strong><\/span><\/h3>\n

Once healing begins, fibrin is dissolved through fibrinolysis.<\/strong>\u00a0 tPA (Tissue-Type Plasminogen Activator)<\/strong> converts plasminogen<\/strong> to active plasmin<\/strong> protease, which degrades fibrin.<\/strong>\u00a0 Macrophages<\/strong> remove cellular debris; platelets<\/strong> release PDGF<\/strong> to stimulate vessel regeneration; fibroblasts produce mitogens<\/strong> to stimulate mitosis (cell division) and tissue repair.<\/p>\n

Platelet Regulation<\/strong><\/span><\/h3>\n

Endothelial cells release prostacyclin<\/strong> (a prostaglandin family member) – acting as both a vasodilator and platelet inhibitor – to prevent excessive platelet aggregation.<\/p>\n

\n
\n

Platelet Disorders<\/p>\n<\/header>\n

\n

Thrombocytopenia:<\/strong>\u00a0 Low platelet levels (nutritional deficiency, illness, or medication).<\/p>\n

Signs:\u00a0 prolonged bleeding, frequent nosebleeds, heavy menstruation, bruising, and petechiae (tiny red spots from capillary hemorrhages).<\/p>\n

Hemophilia:<\/strong>\u00a0 Impaired blood clotting due to inherited mutations in clotting factors (most commonly Factor VIII or Factor IX, both on the X chromosome).<\/p>\n

Thrombocytophilia (Thrombocytosis, Thrombocythemia):<\/strong>\u00a0 Abnormally high platelet counts, can lead to dangerous blood clots, increasing the risk of heart attacks and strokes.<\/p>\n<\/div>\n<\/div>\n

\n
\n

* Did you know?\u00a0 Facts About Hemostasis<\/strong><\/p>\n<\/header>\n

\n

Surgeons may apply a topical collagen agen<\/strong>t to attract the patient’s own platelets and stimulate natural hemostasis.<\/p>\n

Applying direct pressure<\/strong> to a wound is one of the most effective first-aid measures for slowing blood loss.<\/p>\n

Sutures<\/strong> (stitches) close a wound mechanically, speed recovery, and reduce scar formation.<\/p>\n

Vitamin K<\/strong> (found in green vegetables, grains, and organ meats – and produced by intestinal bacteria) is required for synthesis of multiple clotting factors.\u00a0 Deficiency impairs hemostasis.<\/p>\n<\/div>\n<\/div>\n

<\/h3>\n

Media Attributions<\/h2>