About Lung Diseases
Lungs and Respiration
Function. During a normal day, we
breathe nearly 25,000 times. The more than
10,000 liters of air we inhale is mostly
oxygen and nitrogen. In addition, there
are small amounts of other gases, floating
bacteria and viruses. It also contains the
products of tobacco smoke, automobile
exhaust, and other pollutants from the
atmosphere in varying amounts. The lungs
have both "respiratory" and
"nonrespiratory" functions. The
respiratory function of the lungs is "gas
exchange." This is the term for the
transfer of oxygen from the air into the
blood and the removal of carbon dioxide
from the blood. The nonrespiratory
functions of the lungs are mechanical,
biochemical, and physiological.
The lungs provide the first line of
defense against airborne irritants and
bacterial, viral, and other infectious
agents. They also remove volatile
substances and particles of matter
generated within the body. The lungs
control the flow of water, ions, and large
proteins across its various cellular
structures. Together with the liver, they
remove various products of the body's
metabolic reactions. The lungs also
manufacture a variety of essential
hormones and other chemicals that have
precise biological roles.
Structure. The lungs are shaped like
cones and textured like a fine-grained
sponge that can be inflated with air. They
sit within the thoracic cage where they
stretch from the trachea (windpipe) to
below the heart. About 10 percent of the
lung is solid tissue, the remainder is
filled with air and blood. This unique
structure of the lung is delicate enough
for gas exchange and yet strong enough to
ma in-tam its shape and enable it to
perform the many functions vital for
keeping us healthy. Two "plumbing"
systems, the airways for ventilation
(exchange of air between the lungs and the
atmosphere) and the circulatory system for
perfusion (blood flow), are coordinated by
special muscles and nerves.
This arrangement enables the lung to
perform its primary function of rapidly
exchanging oxygen from inhaled air with
the carbon dioxide from the blood. Air
enters the body through the nose or the
mouth, and travels down the throat and
trachea into the chest through a pair of
air tubes called bronchi (plural for
bronchus). The bronchi divide and
subdivide into successive generations of
narrower and shorter branching tubes of
unequal length and diameter. The final
destination for inhaled air is the network
of about 3 million air sacs, called
alveoli, located at the ends of the lungs'
air passages.
Between the trachea and alveoli, the
lungs look like an inverted tree. The
first (main) branching of the trachea
leads to the left and right lungs. The two
lungs fill most of the chest cavity.
Between the lungs are located the heart,
the major blood vessels, the trachea, the
esophagus (tube leading from the throat to
the stomach), and lymph nodes. The thorax
(chest wall) surrounds and supports the
lungs. Movement of the air into the lungs
is controlled by the respiratory muscles
of the thorax. These muscles, collectively
called the ventilatory apparatus, include
the diaphragm (the muscle that separates
the chest and abdominal contents) and the
muscles that move the ribs. When the
respiratory muscles contract, the chest
enlarges like a bellows sucking in air
(inhalation). As air fills the lungs, they
expand automatically. The lungs return to
their original (resting) size when we
exhale.
The performance of the ventilatory
apparatus is coordinated by specific nerve
sites, called respiratory centers, located
in the brain and the neck. The respiratory
centers respond to changes in oxygen,
carbon dioxide, and acid levels in the
blood. Normal concentrations of these
chemicals in arterial blood are maintained
by changing the breathing rate. The right
lung is slightly larger than the left lung
and is divided into three sections or
lobes; the left lung has only two lobes.
Each lobe is subdivided into two to five
bronchopulmonary segments. The segments
are further subdivided into lobules served
by smaller branches of the bronchi.
The outside of the lung and the inside
of the chest cavity are lined by a single
continuous membrane called the pleura. The
portion of the pleura surrounding the
lungs is called the visceral pleura, while
the portion lining the chest cavity is
called the parietal pleura. The potential
space between the lungs and the inside of
the chest cavity is called the pleural
space or pleural cavity. The pleural space
is moistened with a fluid that lubricates
the pleurae as they slide back and forth
on each other during ventilation.
Normally the pleural space contains
only a small amount of fluid and is free
of any gas, blood, or other matter. Blood
vessels, bronchi, and nerves come together
at the entrance of the lung called the
hilum. Bronchopulmonary lymph nodes,
important for the drainage of the lungs,
are located here. The extensive nervous
system of the lungs extends from the hilum
to almost all of the lungs' structural
units.
The Conducting Airways
The first 16 subdivisions of the
bronchi ending in terminal bronchioles are
called the conducting airways. Terminal
bronchioles are the smallest airways
without alveoli. They further divide into
respiratory bronchioles, ending in
alveolar ducts. Respiratory bronchioles
have occasional alveoli budding from their
walls, while alveolar ducts are completely
lined with alveoli. The last seven
branchings of the bronchioles where gas
exchange occurs are called the respiratory
zone. The terminal respiratory unit of the
lung from the respiratory bronchiole to
the alveolus is called the acinus.
Gas exchange between inhaled air and
blood takes place in the alveoli. Blood is
brought to the alveoli through a fine
network of pulmonary capillaries where it
is spread in a thin film. The barrier
separating the air and blood is extremely
thin, 50 times thinner than a sheet of
tissue paper. A large surface area (80
square meters, as large as a tennis court)
is available for gas exchange. In the
resting state, it takes just about a
minute for the total blood volume of the
body (about 5 liters) to pass through the
lungs. It takes a red cell a fraction of a
second to pass through the capillary
network. Gas exchange occurs almost
instantaneously during this short
period.
Infection results when there is
alteration in normal host defense
mechanisms or diminution in the general
immune status of an individual, or when an
immunocompetent individual is exposed to a
virulent organism which overwhelms the
host defenses.
"The Lungs in Health, and Disease"
National Institute of Health, National
Heart, Lung, and Blood Institute.
Lung Disease
- Every year, close to 361,000 Americans die of lung disease. Lung disease is America's number three killer, responsible for one in seven deaths.
- Lung disease is not only a killer, most lung disease is chronic. More than 25 million Americans are now living with chronic lung disease.
- 84.8 million Americans -- children and adults -- have some form of chronic respirators disease. 25.6 million of these suffer from such lung diseases as emphysema, asthma and chronic bronchitis. Center for Diseases Control and Prevention.
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