When airways are damaged, air can escape from them and be trapped in the surrounding tissues in the neck (subcutaneous emphysema) and mediastinum (pneumomediastinum); if it builds up to high enough pressures there, it can compress the airways.[2] Massive air leaks from a ruptured airway can also compromise the circulation by preventing blood from returning to the heart from the head and lower body; this causes a potentially deadly reduction in the amount of blood the heart is able to pump out.[7] Blood and other fluids can build up in the airways, and the injury can interfere with the patency of the airway and interfere with its continuity.[2] However, even if the trachea is completely transected, the tissues surrounding it may hold it together enough for adequate air exchange to occur, at least at first.[4]