Lung

Lungs are responsible for breathing, which provides oxygen to the blood and in exchange rid the blood of carbon dioxide. The inhaled air passes through a tree of respiratory tubes: first through the trachea that branch into two bronchi, each supplying one of the two lungs In the lung, each bronchus (plur. bronchi) branches into narrower bronchi that eventually terminate in a bronchiole.

The lung can be histologically divided into these main components: bronchi, bronchioles, respiratory zone, and blood vessels. These are intermixed and surrounded by a visceral pleura, a membrane that protects the lung from friction that occurs when the lung moves during breathing. Parietal pleura is surounding the whole lung cavity and the pleural space is the small space between the two different protective pleura layers.

The bronchus histologically consists of the following five layers:

• The mucosa, lining the inside of the bronchus. The mucosa in turn can be divided into respiratory epithelium, the most superficial layer, and lamina propria, the tissue right beneath the respiratory epithelium
• The muscularis mucosa, a smooth muscle layer under the mucosa.
• The submucosa, a connective tissue layer with seromucous glands.
• The cartilaginous layer, a layer of cartilage plates located beneath the submucosa.
• The adventitia, the deepest layer separating the bronchus from surrounding tissues.