The significance of coagulative and thrombotic changes after haemorrhage and injury

Abstract

Haemorrhagic shock may progress under certain circumstances by the events described in the diagram shown in Figure 1. Normally, arterial blood pressure is adequate: arterioles are open, though with adequate tone; arteriovenous shunts are closed; and only about 20% of the capillaries are open at any one time. Flow is so good in the functioning capillaries that the cells are satisfied with about 20% perfusion time. When mast cells lining and adjacent to a closed capillary become slightly anoxic, they secrete histamine which causes the capillary to open. The histamine is destroyed in the blood quickly and does not circulate. When the mast cells are satisfied, they halt histamine secretion and the capillary closes. In this way the capillaries take turns in being perfused. Capillary flow is rapid and the fall inpH alongthecapillaryissmall, perhaps from 7·4 to 7·3. Lactic acid is continuously being excreted in minimal amounts by the cells and carried away by the capillary flow; normally this is so good that lactic acid does not accumulate. The sequence of events in haemorrhagic shock is as follows.