2. Circulatory System: Hemostasis Flashcards
What is hemostasis?
The reduction and stoppage of blood loss from a damaged blood vessel
What are the 3 mechanisms of hemostasis?
- Vascular spasm
- Platelet plug formation
- Blood clotting (coagulation)
What happens during vascular spasm?
Damaged vessels constrict to reduce blood flow
Vessels are made of smooth muscle tissues.
Vascular spasms last about 30 minutes
What are the steps of platelet plug formation?
- Platelets adhere to exposed collagen from damaged vessel
- Release reaction: platelets extend processes (becom active) and release substances (e.g. vasoconstrictors, clotting factors)
- Aggregation: platelets stick together to form a plug
What happens during coagulation
Fibrinogen converts to fibrin to form a mesh that traps cells and platelets, forming a clot
Conversion to fibrin occurs in 3 phases
Fibrinogen (inactive form of fibrin) can be found in plasma proteins
What are the 2 coagulation pathways?
- Extrinsic pathway: rapid, triggered by tissue damage which leads to tissue factor (thromboplastin) to leak
- Intrinsic pathway: slower, triggered by damage within blood vessel
Thromboplastin should not be found in high levels in an injured area.
For the intrinsic pathway, when platelets become active, they release clotting factors that aren’t usually found in blood, so the intrinsic pathway is triggered
What is the end product of the extrinsic or intrinsic pathway?
The pathways create prothrombinase (prothrombin activator)
Both pathways create factor X, factor X in combination with Calcim ion is what makes the prothrombinase
What is prothrombinase used for
Prothrombinase is used to activate prothrombin to create thrombin
What is thrombin used for?
Thrombin is used to activate fibrinogen and create fibrin
Fibrin in combination with Calcium ions is what makes the fibrin mesh
What are the 3 phases that turn fibrinogen to fibrin to create the fibrin mesh?
Phase 1: prothrombinase forms by Tissue factor (thromboplastin) from outside the blood vessel (extrinsic pathway) or Chemicals from activated platelets (intrinsic pathway)
Phase 2: using prothrombinase (prothrombin activator) to convert prothrombin into its active form; thrombin
Phase 3: using thrombin to convert fibrinogen into its active form: fibrin
Fibrin in combination with calcium ions is what makes the fibrin mesh
How does the body prevent blood clotting from starting in the absence of injury?
Blood clotting factors circulate in an inactive form → become activated upon injury
• Presence of anticoagulants (inhibitors of blood clotting)
• Anti-thrombin → blocks the activity of clotting factors
• Heparin → boosts the activity of anti-thrombin
Is blood clotting a positive or negative feedback loop?
Blood clotting is under positive feedback. Once blood clotting has begun, a positive feedback starts: formation of thrombin
stimulates formation of more thrombin
Blood clotting has rapid initation and progression of blood clotting
Explain the localization of blood clotting
• Active clotting factors work only at the site of injury
• Elsewhere, they are rapidly diluted by flowing blood and inactivated by anti-coagulants
• As the blood clot forms, all the active thrombin is bound to the fibrin clot
What happen during clot dissolution (fibrinolysis)?
The inactive enzyme, plasminogen, is incorporated into blood clots as they are formed
• Various factors including tissue plasminogen activator (tPA) in the blood clot eventually activate plasminogen to form plasmin, which breaks down the blood clot
What are the 3 main hemostasis disorders?
Thrombus: Blood clot in an uninjured vessel which usually decreases blood flow
2. Embolus: Dislodged blood clot or other material (eg. thrombus, air bubble, globules of fat) that is flowing freely in a blood vessel
• A thrombus or an embolus that blocks passage of blood to a tissue can lead to death of that tissue (e.g., heart attacks, stroke)
3. Hemophilia: A deficiency in the ability to form blood clots due to a deficiency in the ability to produce specific clotting factors
Most common type is hemophilia A due to deficiency of factor VIII
What are the substances/drugs that affect blood clotting?
- Vitamin K: stimulates liver to produce thrombin and other clotting factors
- Anticoagulants: prevent clots (eg. Heparin, warfarin, aspirin)
- Thrombolytic agents: dissolve clots (eg. tPA, streptokinase)
Bacteria in large intestine can make Vitamin K
