Each lung is enveloped in its own double-walled sac called the pleurae. The inner
wall, the visceral pleura, adheres to the outer surface of the lung. The outer
wall, the parietal pleura, is an extension of the visceral pleura that doubles
back on itself at the hilum (hilus) and runs along the surfaces of the rib cage,
diaphragm, and mediastinum. Both pleurae are serous membranes, which secrete
a thin layer of watery pleural fluid into the pleural cavity that separates them.
An open space does not normally exist in the pleural cavity, because the pleural
fluid loosely attaches the two membranes. During breathing movements, this slippery
seal allows the two membranes to freely slide past one another. The pleurae also
form a barrier that helps protect the lungs from infections that can occur elsewhere in the thoracic cavity.
