2-1
Blood has the biomechanical ability to change from a liquid into a fibrin clot, further evolve into a mature
clot, and finally dissolve back into a liquid. These physiological processes encompass not just coagulation
but also other aspects of overall hemostasis including platelet activation, clot retraction, and lysis. Any
weak link in this coagulation and hemostasis sequential process can result in bleeding complications or
elevated risk of thrombosis.
Hemostasis Basics
The coagulation process addresses the reactions occurring in blood or plasma that precede and initiate
the formation of of fibrin clot.
The Coagulation Cascade
Traditionally, this process has been explained with a cascade hypothesis. Over years of research
this coagulation hypothesis has been revised and expanded, however it still provides a foundation
for most coagulation testing, including prothrombin time (PT), activated partial thromboplastin time
(aPTT), thrombin time (TT), and activated clotting time (ACT).
The coagulation cascade hypothesis defines three pathways leading to initial fibrin formation: the
intrinsic, extrinsic, and common pathways. The intrinsic and extrinsic pathways merge into the
common pathway when factor X becomes activated Xa.
Chapter 2:
Hemostasis Fundamentals
XIIa
XIa
Contact Activator
XII
Intrinsic Pathway
IXa
VIII
X
IX
XI
Tissue Factor
Extrinsic Pathway
VII
X
Common Pathway
Xa
V
Prothrombin
Thrombin
Fibrinogen
Fibrin
