Hemothorax is a collection of blood in the space between the chest wall and the lung (the pleural cavity). People with a large amount of blood may feel short of breath, and breathing may be shallow and rapid. A massive amount of blood may cause blood pressure to become dangerously low (shock. Blood pressure is usually low... read more ). The skin may be cool, sweaty, and bluish. Hemothorax can be minor and heal without any problem. Sometimes, if the bleeding is severe and treatment is delayed, it may be life-threatening.