Blood in the pleural cavity usually remains liquid for some considerable time, particularly if it is a pure hemothorax. Hemo-pneumothoraces tend to undergo coagulation, and blood cannot be removed with a needle or drain. Large collections result in collapsed lung, and thrombus organization may lead to fibrothorax. Where feasible, significant collections should be removed as soon as possible. Blood does not usually form a complete coagulum in the pleural space. It is thought that physical agitation of the chest wall impairs the maturation of clots, and because of this liquefaction occurs. Relatively healthy lung collapses with bleeding, and the concept that a collection of blood can seal a point of hemorrhage is not supported by the evidence. Bleeding from the pulmonary circulation is at a lower pressure than the systemic circulation and tends to stop more rapidly. With systemic bleeding the local intrapleural pressure has to approximate to systemic blood pressure before bleeding will stop. Therefore clot formation is essential if the entire pleural space is not to be filled. Rapid hemorrhage and hematological abnormalities may prevent clot formation. Hemorrhage then only stops when the systemic blood pressure has fallen as a result of hypovolemia and hypotension, and the pleural cavity contains almost the entire circulating volume. While the right pleural space is 5 to 10 per cent larger than the left, large right hemothoraces have more severe effects on venous return.

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