THE WINDPIPE
The windpipe, or trachea, is a tube about four and a
half inches long and one inch in diameter. It extends from the bottom of the
larynx through the neck and into the chest cavity. At its lower end it divides
into two tubes, the right and left bronchi.
The bronchi divide and subdivide
many times into smaller branches that penetrate deep into the lungs. The
esophagus, which carries food to the stomach, is immediately behind the
trachea. Rings of cartilage hold the trachea and bronchi open between breaths.
The windpipe wall is lined with mucous membrane, and there are many hairslike
cilia fanning upward toward the throat, moving dust particles that have been
caught in the sticky membrane, thus preventing them from reaching the lungs.
Respiratory infections such as colds and sore throats may sometimes extend down
into the trachea and bronchi; they are then called tracheitis and bronchitis.
Inflammation of the walls of these passages causes
harsh breathing and deep cough.
THE LUNGS
The two human lungs weigh about two and a
half pounds. They have an area forty to fifty times greater than the total
surface area of the body's skin-equivalent, some investigators have noted, to
the area of a tennis court. Within a lung, the bronchi divide and subdivide,
becoming smaller and smaller, until the branches reach a very fine size at
which they are called bronchioles.
Each bronchiole ends in a microscopic air
sac, called an alveolus. It has been estimated that human lungs contain more
than 750 million alveoli. Filled with air, these tiny sacs give the lungs their
characteristic appearance of large sponges.
A vast network
of capillaries penetrates the lungs. Tiny capillaries contact each alveolus.
Air in an alveolus is separated from the blood by two thin membranes-the wall
of the alveolus and the equally thin wall of a capillary. These thin walls
permit ready exchange of gases between blood and air. The lungs ate covered by
a double membrane. One, the pleural membrane, lies over the lungs; the other
lines the chest cavity. Separating the two is a thin layer of fluid which,
during breathing, prevents the two membranes from rubbing against each other.
Inflammation of the pleura can cause roughness and irritation, the condition
called pleurisy. When it is present, the physician, with an ear against the
chest, can hear the membranes rubbing each other with each breathing motion.
