During inspiration,air is pumped into the pleural cavity through a valvular opening in the visceral pleura and underlying injured lung,leading to build up of positive pressure within the hemithorax.Lung collapses first,and as air continuously collects in the pleural cavity,mediastinum shifts towards the oppposite side,fuher decreasing the volume of the functioning lung.Fuher increase in the pleural pressure,reduces the venous return,atrial filling,and ventricular filling and so cardiac output and cardiac function.It causes sudden death & emergency treatment is required. Reference:Bailey & Love's sho practise of surgery,25th edition,page no:1116,1117.