Hemostasis Flashcards
What are the factors in hemostasis?
-vascular constriction, platelet aggregation, clot formation, thrombin, clotting cascade, fibroblast invasion & ct. formation, clot dissolution, prevention of inappropriate clotting,
Hemostasis
the arrest of bleeding
Bleeding requires ____
break in vessel wall & pressure gradient
What are differences in bleeding types?
- injury to microcirculation vs. large a.
- injury to large a. vs. v
What processes act to stop blood flow from damaged vessel?
- Vasoconstriction
- Platelet aggregation (plug)
- Blood clotting
- Fibrous tissue growth/clot dissolution
Mechanism of platelet plug fromation
- Break occurs
- ADP & thromboxane A2 released
- ADP/thromboxane A2 recruits more platelets & thromboxane A2/ADP
- Platelet aggregate to form plug
- ADP also interacts with intact endothelium to inhibit platelet aggregation there by stimulating the release of prostacyclin & NO.
Thromboxane A2 is a
vasoconstrictor
What limits platelet plug to the damaged area?
aka what inhibits platelet aggregation
Prostacyclin & NO release from normal endothelium
Major roles of aggregated platelet plug
- Seals break in vessel wall
- Actin-myosin complexes in platelets constrict to compact plug
- Additional vasoconstrictor release
- Releases chemicals that contribute to clotting cascade
vessel walls break in
capillaries all the time b/c they’re so fragile.
Clot formation is
the most powerful hemostatic mechanism
first defense against vessel break
platelet plug
second defense against platelet plug
clotting
Clot formation function
strengthen and support platelet plug.
Clotted blood
meshwork of fibrin that traps blood cells and serum
what activated clot formation
collagen fibers in the break.
What has to happen for clot formation to occur?
fibrinogen (solumbe) must convert to fibrin (insoluble compound)
Step 1 in clot formation
fibrinogen converts to fibrin with the help of Ca and thrombin
Fibrin’s job
creates clot’s threadlike structure
Prothrombin becomes thrombin by…
interacting with Ca and prothrombin activator
What is absolutely needed for clot formation to even begin?
Calcium!
Thrombin does what things?
Test Q
- Enhances prothrombin activation
- Enhances platelet aggregation (which leads to prothrombin release/activation)
- Activates Factor XIII which stabilized fibrin meshwork
- Stimulates conversion of fibrinogen to fibrin
Clotting cascade’s 2 pathways
extrinsic and intrinsic
Extrinsic clotting cascade pathway
clotting of blood that escaped into tissues.
How does the extrinsic pathway begin?
traumatized tissue releases tissue thromboplastin which DIRECTLY activates factor X
Intrinsic clotting cascade pathway
Factor XII activated by contact with collagen or foreign object
Clot retration
after clot formation, trapped platelets contract, pulling damaged vessel walls closer
Where do extrinsic and intrinsic clotting cascade pathways converge?
on Factor 10!
Which clotting cascade pathway is faster?
extrinsic!
What initiates the intrinsic clotting cascade pathway?
contact with inactive Factor XII (Hageman factor) which activates factor XII
Fates of clot formation (2)
- Invasion by fibroblasts (causes scaring)
2. Dissolution of the clot
Which clot formation fate is typical?
Invasion by fibroblasts
What happens during an invasion by fibroblasts?
-Fibroblasts from connective tissue through the clot
When does fibroblast invasion occur?
within hours after clot formation
How long until there is complete organization into fibrous tissue?
within 1-2 weeks of clot formation
Where does clot dissolution occur?
where clots are not needed
How does clot dissolution occur?
The proteolytic enzyme plasmin digests fibrin fibers & other clotting factors
Factor XII is involved in…
- Clot formation
2. Clot dissolution
How does factor XII cause clot dissolution?
Plasmin is trapped in clot during formation and factor XII helps activate it
What stimulates plasminogen to become plasmin?
factor XII and tissue plasminogen activator (tPA)
Plasmin
fibrinolytic enzyme that dissolves clots
Tissue plasminogen activator (tPA)
stimulates plasmin formation and plasminogen activation
How is inappropriate clot formation prevented?
Tissue plasminogen activator (tPA) continuously converts plasminogen to plasmin so fibrin in blood doesn’t turn blood into a solid/gel.
ongoing low level of fibrin formation is countered by low level of fibrinolytic activity
Conditions of excessive bleeding
- Hemophilia
- Platelet deficiency (thrombocytopenia)
- Vitamin K deficiency
Hemophilia
- caused by deficiency in a clotting cascade factor
- common in males
- mostly a genetic inability to make factor VIII
platelet deficiency (thrombocytopenia)
leads to hundreds of small, confined hemorrhagic areas as blood leaks from sm. vessels before coagulation can occur.
how long does it normally take platelet coagulation to occur?
1-2 min to start clotting and 6-10 min to complete it
Vitamin K deficiency
- leads to formation of structurally incomplete clotting factors which causes bad clotting.
- -Warfarin (caumadin) = Vit. K antagonist
