Diagram 9.3: The respiratory system
The lungs fill most of the chest or thoracic cavity, which is completely separated from the abdominal cavity by the diaphragm. The lungs and the spaces in which they lie (called the pleural cavities) are covered with membranes called the pleura. There is a thin film of fluid between the two membranes. This lubricates them as they move over each other during breathing movements.
The pleural cavities are completely airtight with no connection with the outside and if they are punctured by accident (a broken rib will often do this), air rushes in and the lung collapses. Separating the two lungs is a region of tissue that contains the oesophagus, trachea, aorta, vena cava and lymph nodes. This is called the mediastinum. In humans and sheep it separates the cavity completely so that puncturing one pleural cavity leads to the collapse of only one lung. In dogs, however, this separation is incomplete so a puncture results in a complete collapse of both lungs.