Differential diagnosis and treatment of excessive bleeding rest upon an understanding of normal hemostatic mechanisms. Although all of the intricacies of this complicated physiological process are not fully understood, enough information is available to form a theory. The simple diagram (Fig. 1) presents the main facets
of normal hemostasis. The steps probably occur simultaneously but it is convenient to consider them in four sections: vascular, tissue, platelet and plasma.
Steps in Normal Hemostasis
—Intact blood vessels are obviously necessary to contain the circulating blood. In addition, blood vessels, particularly those with muscular walls, respond to mechanical (injury) or chemical stimuli (see serotonin, below) with constriction, thus reducing blood flow to an injured area.
—Normal tissues play a multiple role. First, all of the many circulating factors important in maintenance of fluidity as well as formation of the blood clot are formed in various tissue sites (Table 1). Second,