Comprehensive review of the hemostatic process as well as the coagulation cascade

1. HEMOSTASIS AND BLOOD
COAGULATION
ALLAN SIGANGA
MBCHB III

2.  Normal hemostasis comprises a series of regulated processes that
maintain blood in a fluid , clot free state in normal vessels while
rapidly forming a localized hemostatic plug at the site of vascular
injury.
 Achieved through the following mechanisms:
1. Transient arteriolar vasoconstriction
2. Platelet adherence, activation and aggregation
3. Activation of thrombin leading to subsequent formation of a blood clot
4. Fibrinolysis
 primary hemostasis - It takes place when there are injuries to small
vessels during which the affected vessels contract to seal off the
wound and platelets are mobilized, aggregate, and adhere to
components of the subendothelium of the vasculature. Platelet
adhesion requires the presence of various factors such as von
Willebrand factor (vWF) and platelet receptors (IIb/IIIa and Ib/IX).
Additional platelets are attracted to the site of injury by the release of
platelet granular contents, such as adenosine diphosphate (ADP)

3.  Secondary hemostasis - involves the response of the coagulation
system to vessel injury. It is required to control bleeding from large
wounds and is a continuation of the primary hemostatic mechanisms.
 Whereas the outcome of primary hemostasis is the formation of the
platelet plug, the outcome of secondary hemostasis is the formation of
a thrombus.

5. ANTITHROMBOTIC
PROPERTIES OF NORMAL
EPITHELIUM Platelets - intact endothelium prevents platelets from engaging the highly
thrombogenic subendothelial ECM.
 Prostacyclins ie PGI2 and nitric oxide produced by endothelium impedes
their adhesion, potent vasodilators , inhibit aggregation
 Endothelial cells also produce Adenosine diphosphatase which degrades
ADP further inhibiting platelet aggregation.
 Coagulation factors – the inhibitory effects are mediated by factors
expressed on endothelial surfaces particularly heparin like molecules,
thrombomodulin, and tissue factor pathway inhibitor.
 Heparin like molecules act indirectly by enhancing the inactivation of
thrombin by antithrombin III.
 Thrombomodulin binds to thrombin thereby modifying the substrate
specificity of thrombin so that it cleaves and activates protein C instead of
fibrinogen.
 TFPI directly inhibits tissue factor –factor VIIa complex and factor Xa.
 Fibrinolysis - endothelial cells synthesize tissue type plasminogen factor
that cleaves plasminogen to plasmin which in turn cleaves fibrin to degrade
thrombi.

6. PROTHROMBOTIC PROPERTIES OF
INJURED/ACTIVATED ENDOTHELIUM
 Activation of platelets - endothelial injury brings platelets into
contact with Subendothelial ECM which includes among its
constituents von Willebrand factor . vWF is held fast to the ECM
through interactions with collagen as well as Gp1b found on the
surface of platelets allowing it to as glue that binds platelets tightly to
denuded vessel walls.
 Activation of clotting factors – in response to TNF / IL-1 or
endotoxin endothelial cells produce tissue factor the major in vivo
activator of coagulation and downregulate thrombomodulin. Also
bind coagulation factors IXa and Xa augmenting the catalytic
activities.
 Antifibrinolytic effects – activated endothelial cells secrete
plasminogen activator inhibitors which limit fibrinolysis.

8. PLATELETS
 Platelet function depends on several integrin family glycoprotein
receptors, a contractile cytoskeleton and 2 types of cytoplasmic
granules:
 Alpha granules – express the adhesion molecule P-selectin on their
membranes and contain fibrinogen, fibronectin, factors V & VIII ,
platelet factor 4 , platelet derived growth factor , transforming
growth factor beta.
 Dense bodies (delta granules) - ADP, ATP , ionized calcium ,
histamine, serotonin , epinephrine.
PLATELET ADHESION
Depends on vWF and Gp1b. Under stress( i.e in flowing blood) vWF
undergoes a conformational change allowing it to bind to collagen in
the ECM and to platelet Gp1b.

9. PLATELET ACTIVATION
 Platelet adhesion leas to an irreversible shape change and secretion
of both granule types ( platelet activation)
 Calcium and ADP released are important as Calcium is required by
several coagulation factors and ADP is a potent activator of resting
platelets.
 Activated platelets synthesize thromboxane A2 (TxA2) a PG that
activates additional nearby platelets as well play an important role in
platelet aggregation.
 Irreversible shape change includes change from smooth discs to
spheres with numerous long, spiky membrane extensions as well as
subtle changes in their plasma membranes.
 Enhances subsequent aggregation and increase surface area
available for interaction with coagulation factors .
 Subtle membrane changes include an increase in the surface
expression of negatively charged phospholipds providing binding
sites for both calcium and coagulation factors and a conformation
change in platelet GpIIb /IIIa that permits it to bind to fibrinogen.

10. PLATELET AGGREGATION
 Prompted by bridging interactions between fibrinogen and GpIIb/IIIa
receptors on adjacent platelets. [ Glanzmann thrombasthenia]
 Concurrent activation of the coagulation cascade generates thrombin
which stabilizs the platelet plug through two mechanisms:
1. Thrombin activates a platelet surface receptor (protease activated
receptor[PAR]) which in concert with ADP and TxA2 further
enhances platelet aggregation. Platelet contraction follows creating
the secondary hemostatic plug.
2. Thrombin activates fibrinogen to fibrin

12. COAGULATION CASCADE

13.  Critical labs assess the cascade using two standard assays:
1. Prothrombin time – screens for factors VII,X,II,V and fibrinogen.
(extrinsic pathway) .
2. Partial thromboplastin time – screens for the activity of factors
XII,XI, IX, VIII,X,V,II and fibrinogen
 once activated coagulation cascade must be tightly restricted to the
site of injury.
 Besides restricting factor activation to sites of exposed phospholipids ,
clotting is controlled by three general categories of anticoagulants:
1. Antithrombin III – inhibit the activity of thrombin and factors
Ixa,Xa,XIa and XIIa . Activated by binding to heparin like molecules ie
the adminsitration of heparin to limit thrombosis.
2. Protein c & S - vitamin k dependent proteins that act in a complex to
proteolytically inactivate cofactors Va, VIIIa
3. tissue factor pathway inhibitor – inactivates factor Xa and tissue
factor-factor VIIa complexes.