Blood coagulation and haemostasis: A review

Abstract

In the event of haemorrhage, various factors may operate singly or in combination to arrest the bleeding. Although the extravascular tissues and the vasculature itself must play a role, by far the most important factors are the platelets and the blood coagulation system. Much of the area of platelet physiology still presents perplexing problems. However, it is an established fact that platelets aggregate to form a platelet plug at the site of a ruptured vessel; this is the first step in haemostasis and indeed is thought to be the predominant factor in capillary haemostasis. After provisional control of the bleeding has been achieved by the platelet plug, fibrin is deposited, and this effectively seals the break in vessel continuity and prevents further bleeding (Fig. 1). The deposition of fibrin is brought about by the complex interaction of the twelve coagulation factors. While the initial steps in the coagulation mechanism are still in some respects conjectural, the basic principle has not changed since Schmidt and Morawitz outlined the so-called classical theory of blood coagulation, which is simply the conversion of prothrombin to thrombin by the enzymatic action of an activator (called by most workers thromboplastin). The thrombin then activates fibrinogen to form the fibrin, which is the major constituent of the clot. Whether a substance "thromboplastin" really exists is still debated, but, although no such substance has been isolated, most workers accept its existence in theory at least. Two pathways of the coagulation process have been delineated: the extrinsic and the intrinsic. In the intrinsic pathway, all the known coagulation factors, with the exception of factor VII, take part in the eventual formation of fibrin. Factors VIII, IX, XI, and XII are not present in the extrinsic system (Figs. 2, 3, 4). After a clot has formed, it later undergoes two distinct changes: retraction and lysis. Though several factors, physical as well as chemical, are involved in clot retraction, the most important appears to be an adequate number of normally functioning platelets. On the other hand, lysis is a function of another enzymatic system opposed to coagulation, i.e. the fibrinolytic system. © 1967 Canadian Anesthesiologists.