STRUCTURE OF THE LUNGS & THE ROUTE THE AIR
TAKES
Lungs are found in the thoracic cavity. They are protected by the rib cage. The left
lung is slightly smaller than the right due to the location of the heart. Each lung is
covered by serous membranes called the pulmonary pleura. They are arranged like a
double skin bag in which the outer membrane, called the parietal membrane, lines the
thoracic cavity and the inner membrane, called the visceral membrane, lines the lung.
In between the two membranes is the pleural cavity, which contains the pleural fluid.
The function of this lubricating fluid is to reduce friction between the two membranes
during the mechanics of breathing.
On its journey to the lungs, air drawn into the body passes through many structures:
Nasal Passages
 Air enters through the mouth and nose.
 Is warmed, filtered and moistened by the nasal lining.
Pharynx & Larynx
 Air proceeds through these organs and is further warmed, filtered and moistened.
 In swallowing, the larynx is drawn upwards and forwards against the base of the
epiglottis, thus preventing the entry of food.
Trachea
 It is composed of 18 rings of cartilage, which are lined by a mucous membrane
and ciliated cells providing protection against dust.
 It directs air into the right and left primary bronchi.
Bronchi & Bronchioles
 The bronchus further sub divides into lobar bronchi.
 Further sub divisions of these air ways form bronchioles which in turn branch into
the smaller terminal or respiratory bronchioles.
 The bronchioles enable the air to pass into the alveoli to ensure pulmonary
diffusion.
Alveoli
 They are responsible for gaseous exchange.
 Each alveolus has an extensive capillary network to ensure efficient diffusion.
 The alveolar walls are extremely thin and are composed of epithelial cells which
are lined by a thin film of water, essential for dissolving oxygen from the inspired
air.