The role of the hematocrit in bleeding.

Low hematocrit is an often neglected cause in the pathogenesis of a prolonged bleeding time in an anemic patient. There has been ample evidence in the literature, indicating a relationship between hematocrit and the bleeding time; and that the transfusion of RBCs may correct the prolonged bleeding time often observed in anemic patients. It is unclear how a low hematocrit causes a prolongation in the bleeding time; however, two hypotheses have been put forward. First, in small blood vessels, blood flow is such that the RBCs cause the physical dispersion of platelets towards the subendothelial surface, thus promoting interaction with the blood vessel wall. Secondly, following injury to a small blood vessel, RBCs activate platelets by releasing small amounts of adenosine diphosphate (ADP) into the microvasculature following the hemolysis that often occurs during hemostasis. The fact that the hematocrit influences the bleeding time may be of clinical importance in the treatment of anemic patients, particularly those presenting a bleeding tendency.