Overview
This lecture covers the processes involved in hemostasis (blood clotting), including clotting stages and key platelet disorders: thrombocytopenia and thrombocytosis.
Hemostasis Process
- Hemostasis is the process of forming a blood clot to prevent blood loss after vessel injury.
- It involves injured vessel walls, circulating platelets (thrombocytes), and plasma coagulation proteins.
- Inadequate hemostasis leads to excessive bleeding; excessive hemostasis can cause unwanted clotting (thrombosis) and sometimes paradoxical bleeding.
- Platelets normally range between 150,000-400,000 per mm³.
Stages of Hemostasis
- Primary Hemostasis: Vessel constriction and platelet aggregation at the injury site to form a loose platelet plug.
- Secondary Hemostasis: Coagulation cascade converts the platelet plug into a stable fibrin clot using intrinsic or extrinsic pathways.
- Intrinsic pathway is triggered by exposure of blood to altered endothelium; extrinsic pathway is triggered by trauma to the vessel wall.
- Both pathways lead to prothrombin being converted to thrombin, which changes fibrinogen to fibrin.
- Final Stage (Fibrinolysis): Clot retraction and removal occur as the vessel heals, with plasmin dissolving the fibrin clot.
Platelet Disorders
Thrombocytopenia
- Defined as a platelet count below 150,000/mm³ (especially below 50,000/mm³).
- Causes prolonged bleeding, petechiae (pinpoint skin spots), and purpura (skin discoloration that doesn't blanch).
- Can result from bone marrow suppression (chemo, alcohol), immune issues, infections, sequestration in the spleen, or dilution from blood loss.
- Treatment focuses on addressing the underlying cause.
Thrombocytosis
- Defined as a platelet count above 400,000/mm³.
- Increases risk of abnormal clotting and sometimes bleeding.
- Causes include transitory factors (stress, intense exercise), primary bone marrow diseases (polycythemia vera, leukemia), and secondary responses (hemorrhage, spleen issues).
- Treatment is mainly for primary conditions; secondary/transitory types usually need only cause management.
Key Terms & Definitions
- Hemostasis — The process of stopping bleeding via blood clotting.
- Platelets (Thrombocytes) — Cell fragments essential for blood clot formation.
- Thrombocytopenia — Low platelet count, increasing bleeding risk.
- Thrombocytosis — High platelet count, increasing risk for thrombosis.
- Coagulation Cascade — Series of steps leading to fibrin clot formation.
- Fibrinolysis — Breakdown and removal of a blood clot.
- Petechiae — Pinpoint bleeding spots under the skin.
- Purpura — Red or purple skin discolorations from bleeding underneath.
Action Items / Next Steps
- Review causes, signs, and treatments for thrombocytopenia and thrombocytosis.
- Understand the stages of hemostasis and the coagulation cascade.
- Prepare for exam questions on platelet functions and related disorders.