Haemostasis Flashcards
What is the normal response to injury to the endothelail cell lining?
vessel constriction -> formation of unstable platelet plug (platelet adhesion and aggregation) -> stabilisation of plug with fibrin (blood coagulation) -> dissolution of clot and vessel repair
What are the functions of the endothelium?
Maintain barrier between blood and tissue factor.
Synthesis – PGI2, thrombomodulin, vWF, plasminogen activators
How are platelets created?
stem cell precursors -> megakaryocytes -> megakaryocytes mature and granulate -> megakaryocytes produce ~4000 platelets.
What is the circulating lifespan of a platelet?
around 10 days
What are some ultrastructure features of a platelet?
Dense granules alpha-granules thrombin receptors surface glycoproteins (Glp1a, Glp1b, Glp2b/3a) phospholipid membrane microtubules and actomysin
What are the roles of platelets?
haemostastis and thrombosis cancer atherosclerosis infection inflammation
What happens in platelet adhesion?
- vessel damage exposes collagen
- vWF binds to collagen and undergoes structural change into linear shape exposing binding sides
- circulating platelets bind to Glp1a on vWF
- platelets can directly bind to collagen via GPV1 and a2b1 (but only under low shear stress - not arteies/capillaries)
- interaction of platelts and collagen -> platelet activation -> conformation change and degranulation -> release of ADP and thromboxane
What happens in platelet aggregation?
- activated platelets (a2bB3) recruit additional platelets
- a2bB3 also binds fibrinogen -> platelet plug
What happens during blood coagulation?
tissue damage causes tissue factor to bind to factor 7a
factor 7a binds to cell surface (using gla domain)
F12->12a
F11-11a
7a catalyses F9->9a
F10->10a (9a and 7a and Ca2+)
*7a cleaves 9 and 10 - removes activation peptide-> 9a and 10a
factor 10a converts prothrombin (F2)-> thrombin (very inefficient)
thrombin cleaves F8->8a (8a is cofactor for 9a)
F8a/9a complex activates much more factor 10a
F5a (also activated by thrombin) forms complex with 10a produces much more thrombin
fibrinogen -> fibrin
What are Gla domains?
vitamin k dependent proteins
contain 9-11 gamma-carboxylic acid residues
bind 6/7 Ca2+ ions -> structural transition which allows it to bind to negatively charged phospholipid cell/ platelet cell
Where are clotting factors, fibrinolytic factors and inhibitors synthesised?
Liver - Most coagulation proteins.
Endothelial cells - vWF
Megakaryocytes - vWF, Factor V
What happens once a clot has formed?
Tissue factor pathway inhibitor (TFPI) binds/inactivates TF-F7a complex (but very low concentration of TFPI)
Protein C pathway
- activated by thrombin-thrombomodulin (on endothelial cells) complex
- chops up F5a and 8a (co factors) -> no more clotting
antithrombin inhibits F9a, 10a and 2a (thrombin)
how do warfarin and heparin work?
Warfarin
- In the liver, vitamin K mediates production of factors II, VII, IX and X which have extra carboxyl groups on
- The extra carboxyl groups (gamma-carboxyglutamic acid) allow the clotting factors to adhere to platelet membrane PLs
- Warfarin inhibits vitamin K epoxide reductase and thus reduces creation of secreted clotting factors - reduces the clotting factors that bind to platelet PLs (phospholipids)
- Thus warfarin inhibits the platelet surface interactions as the clotting factors cannot bind to the surface with Ca2+
- Warfarin = long-term anticoagulation following venous thrombosis AND treatment of atrial fibrillation
Heparin
- binds t antithrombin and accelerates its actions by 3000x
What laboratory tests can be used to test blood coagulation?
APTT – Activated Partial Thromboplastin Time
- Initiates coagulation through F12 and detects abnormalities in INTRINSIC and COMMON pathways.
PT – Prothrombin Time.
- Initiates coagulation through tissue factor and detects abnormalities in EXTRINSIC and COMMON pathways.
TCT/TT – Thrombin Clotting Time.
- Add thrombin -> shows abnormalities in fibrinogen to fibrin conversions.
