The pleura is host to many diseases, some systemic, others spreading from the underlying lung. Lung carcinoma metastases are the commonest lesions seen by pathologists. The most frequent cause of pleural problems is an effusion, secondary to congestive cardiac failure. This chapter will consider normal pleural physiology, as well as benign and malignant processes.
The pleural space, separating the lung and chest wall, contains a small amount of liquid that lubricates the pleural surfaces during breathing (see Chapter 1). The volume of fluid in the pleural space is small, in the range of 0.2–0.5 ml. Normally, the protein and cellular content is low, with no inflammatory cells. The thin layer of liquid between the pleural surfaces has the important function of providing mechanical coupling between the chest wall and lung. For this connection to be effective, the volume of pleural liquid required for lubrication must be kept to a minimum. In addition, the colloid osmotic pressure must be low to keep this volume small. This fluid ensures instantaneous transmission of perpendicular forces between the two structures, and allows their sliding movement in response to shearing forces.