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How does air get into the chest cavity?-When the chest cavity is surgically opened to expose the lung, air enters it because normally there is a partial vacuum between the lung and the pleura which is broken when the pleura is opened. In normal respiration the pressure within the pleura is about 10 cm. of water during expiration and 4 cm. of water during inspiration. These pressures are below that of normal atmospheric pressure. This subatmospheric pressure is often referred to as "negative pressure.' Because only a portion of the air is removed upon expiration, a partial vacuum results. In other words, the air in the chest cavity is at such a low pressure vacuum so that it does not force all the air from the lung. This system is broken when the pleura surrounding the lung is opened (Figure 1).