Hemothorax Definition: A hemothorax is when there is blood inside of the thorax, the lung cavity. Most cases of hemothorax are associated with trauma or blunt trauma to the chest. A hemothorax can be associated with a pneumothorax, but it is not required. Therapy: It is essential to determine it the blood in the thoracic cavity is coming from a ruptured vessel that may require surgery. Thus, physicians will try to determine the cause of the hemothorax as soon as possible. In some cases, the bleeding is only temporary and it will clot and stop on its own. In these cases, the physician will observe the patient through time. If there is ever any problem with breathing, this needs to be addressed immediately.