g. Bronchioles. As the bronchi enter the lungs at the hilum (depression or pit)
of each lung, the bronchi divide into innumerable branches of progressively diminishing
size, until divisions of very narrow diameter are reached, and these are termed
bronchioles. The bronchi and the bronchioles are lined by a mucous membrane of
ciliated, columnar epithelium that is continuous with the lining of the trachea. The entire
length of the bronchial tree is richly supplied with elastic fibers.
h. Alveolar Ducts and Sacs. Bronchioles run into smaller branches, which are
known as alveolar ducts. These lead to small spaces called alveolar sacs.
i. Alveoli. The walls of the alveolar sacs are lined with tiny "cubicles," called
alveoli. The walls of the alveoli are thin enough to allow the easy passage of gases.
The alveoli are in contact with a plexus of capillaries. The alveolar sacs, which are in
contact through the bronchial tree with the atmosphere, are continually filled with air. It
has been estimated that each lung contains seven million alveoli.
(1) Lobes. The lungs are two in number (right and left) and are contained in
the thoracic cavity. The lungs are coneshaped organs with their apexes pointed
toward the base of the neck. The base of the lung conforms to the convex surface of
the diaphragm. The lungs overlap the greater portion of the heart. The right lung is
divided into three lobes by two fissures. The left lung is divided into two lobes by one
fissure. The apex of the heart continually pushing against the left lung has eliminated a
lobe during embryological development. On the surface of each lung is a depression
called the hilum. At the hilum, the structures that comprise the root of the lung enter
and leave the lung. These structures are a bronchus, the pulmonary artery, two
pulmonary veins, bronchial arteries and veins, lymphatics, nerves, and lymph nodes.
The lungs are bounded superiorly by the upper portion of the thorax, inferiorly by the
diaphragm. They are also bounded laterally, anteriorly, and posteriorly by the ribs and
intercostal muscles, sternum, and vertebral column.
(2) Pleura. Each lung is covered by a membrane, called the pleura,
composed of a single layer of endothelial cells lying upon a delicate connective tissue
membrane. The lung may be pictured as having been pushed into the side (not into the
interior) of a closed membranous sac. The lung is thus covered by two layers of the
pleura. Between the pleural layers is found the pleural cavity. The two layers of pleura
are in contact with each other except for a thin film of serous fluid known as pleural fluid.
This fluid allows the lung to inflate and deflate without any friction. Inflammation of the
pleural membrane is called pleurisy.
(a) Visceral. The pleural layer adherent to the lung is called the
(b) Parietal. The pleural layer lining the thoracic cavity is called the