The lungs are a pair of essential respiratory
organs situated in the chest cavity one on each side of the heart
and surrounded by a double membrane, the pleura. They are
protected laterally by ribs, interiorly by the sternum
and posteriorly by the vertebral column. They are separated
from the abdomen by the transverse muscular partition called thediaphragm.
The lungs are smooth, spongy and pink in color. The right lung is
divided into three lobes, while the left lung is divided into two
lobes: it is smaller so as to accommodate the heart in between.
The lung is mainly formed by bronchioles and alveoli.
Each bronchus after entering the lung divides into smaller and smaller
bronchioles each about a millimeter or less in diameter, like a
tree branching into thinner branches forming slender twigs. The
ultimate branches of the bronchi called terminal bronchioles
leading to respiratory bronchioles, end in, clusters of alveolar
sacs each bearing a number of alveoli looking like a bunch
of grape-like structures. The branches of the pulmonary artery and
pulmonary vein, which divide and re-divide along with the branches
of the bronchi, ultimately form a capillary network around the alveoli.
Thus the lungs are essentially made up of a complex branching system
of bronchi ending in alveoli, bound together by connective
tissue and containing a network of blood capillaries.
The alveoli are lined throughout by flat
epithelial cells to facilitate exchange of gases. They are surrounded
by a rich and fine network of blood capillaries. There are about
750 millions of these
Figure 17.4 (A) Structure of lungs (B) Part of
the lung enlarged
