The
respiratory system is responsible for
gas exchange between the
blood and the external environment. Air passes from the environment through a moist passage lined entirely with
mucosa down to the
lungs where the exchange occurs.
Air passes through the
nostrils (nares) into the
nasal cavity. Inside the
nasal cavity the
conchae, ridges of the
ethmoid bone, cause
turbulence in the incoming air, forcing the air to contact the
mucosa. The sticky
mucous of the
mucosa traps many of the particles (dust,
bacteria etc.) in the incoming air, preventing them from entering the
lungs. The air is also warmed and humidified in the
nasal cavity.
After exiting the
nasal cavity the air passes through the
pharynx into the
trachea. The
trachea is lined with
ciliated columnar epithelium, which traps and sweeps dust up and out of the respiratory passages. The
trachea is also surrounded by
cartilaginous rings, which prevent the
trachea from closing due to
low lung
pressure during breathing.
The
trachea splits into the left and right
bronchi; each
bronchus leading to one
lung. The
bronchi split into many
bronchioles, which in turn lead to clusters of
alveoli (air sacs). The epithelium of the
alveoli are the site of
gas exchange between the air and the
blood.
The
pleura is the
serous membrane surrounding the
lungs. The
visceral pleura clings tightly to the surface of the lungs. The
parietal pleura clings to the inner walls of the chest and the
diaphragm. In between the two layers of
pleura is the
pleural fluid; a sticky fluid
secreted by the
pleura that keeps the two layers bonded together. Because of this arrangement, the lungs expand and contract with the movements of the chest wall and the
diaphragm.
Figure 12.1.1 By Lord Akryl (http://cancer.gov) [Public domain], via Wikimedia Commons 
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1 - Anatomy and Physiology Basics