The pleural space, derived from the intraembryonic coelom, is limited by a serous membrane including the mesothelium formed by cells possessing not only the characteristic features of epithelial cells but also the potential of secretory cells (cytokines and growth factor). Blood supply to visceral pleurae differs depending on the species while the lymphatic circulation is directly connected to the pleural space via pores in the parietal pleura. Pleural physiology and movement of pleural fluid are directly related to the particular structures of the pleura.