Introduction

The hemostatic system is a complex interaction between the vasculature, cellular components and plasma proteins that interact to maintain hemostasis in the healthy body. Further, this complex interaction occurs across a variety of flow states, under variable pressure conditions. The hemostatic system on a wider scale also interplays with other physiological systems, including those that facilitate immune and inflammatory responses, angiogenesis and wound repair.

There is currently no mechanism for assessing or testing the hemostatic system in its true physiological state. All laboratory tests isolate specific components of the system, and assess those components under artificial constructs that hopefully give the clinician valuable information about the way the patient will behave. In this context, we divide the hemostatic system into primary, secondary and tertiary hemostasis to better define the interdependent mechanisms that combine to maintain overall hemostasis. Primary hemostasis describes the cellular interaction of platelets and the endothelium, and the initiation of the platelet plug that is localized to the point of injury at the vessel wall. Secondary hemostasis describes the activation of the coagulation system that is initiated, amplified and prolonged in a sequence of activations of coagulation proteins, and regulated by a series of positive and negative feedback mechanisms. Tertiary hemostasis is a description of the fibrinolytic system which regulates the breakdown of blood clots as healing vessels regain vascular integrity. In reality none of these components act in isolation. Nor do these processes act in sequential timeframes as the names would suggest. However, such terminology is useful in allowing an incredibly complex and interwoven system to be considered in a way that helps us to understand pathophysiology of diseases, explain clinical presentations and direct our currently available therapies.