The gas exchange system in humans and other mammals consisits of two major parts, a conduction system, for the conduction of inspired and expired gases, and an interface for the exchange of gases between air and blood. The conducting system begins with the nasal passages and continues as the trachea. The trachea divides to form the left and right bronchi which supply the lungs. The bronchi divide into numerous smaller bronchioles which eventually lead into the alveoli, where gas exchange occurs.
The inside of the thoracic cavity is lined with a pleural membrane. Each lung is also surrounded by a separate pleural membrane. The space between the two pleural membranes is fillied with a thin film of fluid(known as pleural fluid), which prvents friction between them during breathing.