of lung cancer in the United States. Over time
breathing exposes the respiratory tract to numer-
ous insults from substances such as environmental
pollution, viruses, BACTERIA, pollens, dust, and
other materials. These exposures may injure or
damage the lungs, resulting in conditions such as
ASTHMA, PNEUMONITIS, bronchitis, and pneumonia,
as well as infections such as TUBERCULOSIS and
INFLUENZA. Such pulmonary conditions can con-
tribute to deteriorating lung functions, particularly
in people who have not maintained adequateAER-
OBIC FITNESS.
Maintaining a steady level of AEROBIC FITNESS
through regular physical activity keeps the pul-
monary system as healthy as possible for as long
as possible, helping offset the changes of advanc-
ing age. Aerobic fitness enables the lungs to effi-
ciently extract oxygen from the air and conduct it
into the bloodstream. As well, the lungs fill more
fully with air on each inhalation, keeping the
alveoli open and functioning. This helps keep fluid
from accumulating within the alveoli.
See also AGING, CARDIOVASCULAR CHANGES THAT
OCCUR WITH;AGING, CHANGES IN THE BLOOD AND LYMPH
THAT OCCUR WITH; ASPIRATION; CONGENITAL HEART DIS-
EASE; PLEURISY; SMOKING AND HEALTH.
alveolus A tiny, thin-walled sac, grouped in
clusters at the ends of the smallest airways (bron-
chioles) deep inside the LUNGS, that is the terminus
for each breath of air. A dense mesh of capillaries
entwines each alveolus. Oxygen from the air
within the alveolus passes across the alveolar
membrane to enter the bloodstream, while carbon
dioxide and other waste gases pass across the
membrane from the BLOODto the air within alveo-
lus. The clustered formations of the alveoli greatly
increase the surface area for OXYGEN–CARBON DIOX-
IDE EXCHANGE. Each lung contains about 300 mil-
lion alveoli, which, if spread out, would coat the
surface of a tennis court—about 290 square feet.
The large surface area is important but so is the
very thin interface between the airway and the
blood vessel. Many disease states cause this barrier
to thicken or distort, which impedes the exchange
of oxygen and carbon dioxide (called decreased
diffusing capacity).
For further discussion of the alveolus within
the context of pulmonary structure and function
please see the overview section “The Pulmonary
System.”
See also BRONCHUS; HEMOGLOBIN; TRACHEA.anthracosis A lung condition resulting from
long-term exposure to coal dust, also called coal
worker’s PNEUMONOCONIOSIS(CWP) and black lung
disease. There are two types of anthracosis: simple
and complicated. In simple anthracosis the coal
dust coats the LUNGSin wide distribution, and the
IMMUNE RESPONSEencapsulates the dust particles
without causing scarring (fibrosis). This is the
most common type of anthracosis and may gener-
ate no symptoms or mild symptoms such as DYSP-
NEA (shortness of breath) with exertion and
chronic COUGH. The doctor may detect simple
anthracosis during routine medical examination
or screening for lung disease. Diagnosis generally
considers X-RAYfindings in combination with his-
tory of exposure to coal dust. The doctor may also
conduct BRONCHOSCOPY to examine the inner
bronchial structures, which have a characteristic
black appearance.
Complicated anthracosis becomes aggressively
fibrotic, though doctors do not know what causes
it to do so. It continues to progress even after
exposure ends, and may result in disabling
obstructive lung disease. Symptoms include wors-
ening cough and dyspnea. About 15 percent of
coal workers who have simple anthracosis develop
complicated anthracosis.
Improvements in mining techniques and condi-
tions, including environmental filtration systems,
have greatly reduced the amount of dust coal min-
ing operations produce. Those who work as cutters,
loaders, and continuous mining operators face the
highest risk for exposure. In the United States, the
Occupational Safety and Health Administration
(OSHA) regulates permissible levels of dust and
worker exposure. Diagnosis of new cases of anthra-
cosis is steadily declining as a result. Federal health
programs provide medical care and other benefits
for coal workers who have anthracosis.
See also ASBESTOSIS; BERYLLIOSIS; BYSSINOSIS;
CHRONIC OBSTRUCTIVE PULMONARY DISEASE(COPD); SILI-
COSIS.apnea The temporary and involuntary cessation
of BREATHING. Apnea may result from neurologicapnea 183