Robbins Ch. 4 - Hemostasis and the Clotting Cascade Flashcards
What is hemostasis?
Process by which blood clots form at injury sites
Precisely orchestrated cascade involving platelets, clotting factors, and endothelium that occurs at the site of a vascular injury and culminates in the formation of a blood clot
clot stops bleeding and loss of fluids
A 27 yo male prisoner is shanked in the arm with a long screwdriver. What is the first step in hemostasis/formation of clot/plugging hole?
Arteriolar vasoconstriction
What 2 things mediates the first step in the clotting process?
First step is vasoconstriction
Transient, mediated by reflex neurogenic mechanisms
augmented by localized secretion of things like endothelin, an endothelium-derived vasoconstrictor
After vasoconstriction, what happens next in hemostasis to control bleeding?
Formation of platelet plug
How is the platelet plug created?
Endothelium is breached/shanked
Endothelium will now show the previously hidden von Willebrand Factor (vWF) and collagen
vWF and collagen adhere and activate platelets
activated platelets go from cute, clumsy blebs to spiky tinker toys to increase surface area.
Platelet granules released.
Additional platelets are recruited, aggregate to form primary hemostatic plug
Congratulations, a primary hemostatic plug has been formed. Snitches will indeed only get stitches today. What is the next step in hemostasis?
Secondary hemostasis: deposition of fibrin
What is the process for secondary hemostasis?
Tissue factor is also revealed when endothelium is breached
- procoagulant glycoprotein, expressed in subendothelial vessel wall (in smooth mm cells, fibroblasts)
Tissue factor binds and activates Factor VII
Clotting cascade happens
Thrombin formed
Thrombin cleaves fibrinogen into fibrin
Fibrin has been created in secondary hemostasis. How does this help with hemostasis?
Fibrin will form an insoluble meshwork
- also a potent activator of platelets, creating more platelet aggregation at injury site
Secondary hemostasis will consolidate plug created in primary hemostasis
Primary and secondary hemostasis has occurred, and a robust clot/plug has been formed. What happens now?
Clot stabilization and resorption
Polymerized fibrin and platelet aggregates undergo contraction to form a solid, permanent plug that prevents further hemorrhage
t-PA works here
What is t-PA?
tissue plasminogen activator
A blood protein that limits clotting to site of injury
What general role do platelets play in hemostasis?
Forms initial primary plug
- seals vascular defects/injury/site of shanking
- makes surface that binds and concentrates activated coagulation factors
What is the cellular origin and rough morphology of platelets?
Platelets are disc-shaped bags of proteins, with no nucleus
Shed from megakaryocytes in bone marrow, released into blood stream
What general components do platelets need in order to function?
- several glycoprotein receptors
- contractile cytoskeleton
- 2 types of cytoplasmic granules
Platelets have 2 types of granules - alpha and gamma. What is in alpha granules? (7)
Alpha granules
adhesion molecule P-selectin on membranes
also contain coagulation proteins - fibrinogen, Factor V, vWF
wound healing proteins - fibronectin, platelet factor 4, PDGF, TGF-þ
What are the contents of gamma granules in platelets? (5)
Gamma/dense granules:
ADP
ATP
Ca++
5HT - serotonin
epinephrine
After a traumatic injury, platelets come into contact with vWF and collagen. What series of reactions happen next to form a platelet plug?
Platelet adhesion
Platelets changing shape
Secretion/release of granule contents
Platelet activation
Platelet aggregation
How is platelet adhesion mediated?
Interactions with vWF
vWF will act as a ‘bridge’ between platelet surface glycoprotein receptor Ib (GpIb) and exposed collagen
Platelets need to change shape in order to form a plug and create hemostasis. What other changes accompany this process?
Platelets get spiky in order to increase surface area
With this you get:
- alterations in glycoprotein IIb/IIIa, increases affinity for fibrinogen
- translocation of negatively charged phospholipids to platelet surface
Platelets need to put negatively charged phospholipids on its surface during adhesion. Why? What phospholipids?
Often will phosphatidylserine on platelet surface
Negative phospholipids will bind Ca++ and serve as nucleation sites for assembly of coagulation factor complexes
Platelet activation often includes what 2 events?
Release of granules
Change in shape
What triggers platelet activation?
Thrombin cleaving PAR (protease-activated receptor, a GPCR)
Release of ADP
What is recruitment?
Platelet activation and release of ADP, which induces more platelets to activate and join the plug.
After platelet activation has happened, what occurs?
Platelet aggregation
Conformational change in GIIb/IIIa allows for fibrinogen to bind
- fibrinogen will form bridges between adjacent platelets, which leads to aggregation
Platelet aggregation also occurs with thrombin activation. What does thrombin activation contribute to?
Converts fibrinogen into insoluble fibrin
- cements platelets in place and creates definitive secondary hemostasic plug
In addition to fibrin and platelets, what else should you expect to see in a hemostatic plug?
Trapped RBCs and leukocytes
- leukocytes get stuck on p-selectin on activated platelets
Each step of the clotting cascade involves what 3 general actors?
Enzyme - activated coagulation factor
Substrate - inactive proenzyme form of a coagulation factor
Cofactor - reaction accelerator
What coagulation factors need calcium as a cofactor? What other cofactor do these need?
Factors II, VII, IX and X
Also will need Vitamin K
Antagonized by coumadin
What are the steps of the intrinsic pathway?
Negative charged surface
XII → XIIa
XIIa, XI → XIa
XIa, IX → IXa + VIIIa
IXa + VIIIa → X
X → Xa + Va
prothrombin → thrombin
fibrinogen → fibrin
What is the quick and dirty run down of the steps of the intrinsic pathway?
XII
XI
IX
VIII
X and V
thrombin and fibrin
What are the steps of the extrinsic pathway?
TF and VII → TF + VIIa
X → Xa + Va
Prothrombin to thrombin
fibrinogen to fibrin
You have learned about the intrinsic and extrinsic pathway for clotting in vitro, what is the cascade for clotting in vivo?
VII + TF → VIIaTF, works on
IX → IXaVIIIa, works on
X → XaVa, works on
prothrombin → thrombin
fibrinogen → fibrin
What are the inhibitory elements of the clotting cascade in vivo?
Thrombin will inhibit severel places upstream:
XaVa
IxaVIIIa
conversion of Xi to XIa
XIa will inhibit activation of IX
What are the platelet inhibitory effects of endothelium?
Serves as a barrier between platelets and vWF
Releases:
Prostacyclin - PGI2
NO
Adenosine disphospatase - breaks down ADP
Binds and alters activity of thrombin (potent platelet activator)
What are the anticoagulant effects of endothelium?
Normal epithelium shields coagulation factors from tissue factors
Expresses factors that actively oppose coagulation:
thrombomodulin
endothelial protein C receptor
heparin-like molecules
tissue-factor pathway inhibitor
What does thrombomodulin and endothelial protein C receptor do?
Bind thrombin and protein C in a complex on the endothelial surface
- thrombin will lose ability to activate coagulation factors and platelets
- thrombin will activate protein C
What is protein C? How does it do, and how is it activated?
It’s a vitamin-K dependent protease that requires a cofactor - protein S
Protein C/Protein S is a potent inhibitor of Factors Va and VIIIa
What do heparin-like molecules present on the surface of endothelium do?
Bind and activates thrombin III
What does heparin III do?
Heparin III inhibits thrombin and factors IXa, Xa, XIa, and XIIa
- most of the intrinsic pathway
How does tissue factor pathway inhibitor work?
Requires protein S as a cofactor
Binds and inhibits tissue factor and factor VIIa complexes
Endothelium also has some fibrinolytic effects. How are these mediated?
Normal epithelials make t-PA, a key component of the fibrinolytic pathway