**2. The physiology of respiration and pneumothorax**

The pressure in the pleural space is determined by the difference between the lung elastic recoil and volume changes of the semi-rigid chest wall. The rib cage moves in three dimensions; the girdle handle movement of the ribs increases the anteroposterior and the lateral dimensions of the chest, whereas the piston-pump movement of the diaphragm leads to an increase in the vertical dimension of the chest cavity. The chest and diaphragm movements create a physiological negative pressure within the pleural space that forces the lung to change shape and volume with the respiratory cycle, resulting in inflation and deflation. Neutralising this negative pressure in the pleural space leads to lung collapse, as the elastic structure of the lung favours its collapse (recoil). Pneumothorax or air in the pleural space invariably leads to lung collapse. A thin film of fluid exists between the parietal and visceral pleurae to lubricate the sliding of these two structures, roughly 15 mls in a 70 kg adult person. The fluid is a microvascular filtrate produced by the parietal pleura and is cleared also by the parietal pleural lymphatics, a process similar to that in any other body organ.
