the risk for cancer in people who smoke, making
any type of lung cancer more likely as well as pre-
senting the specific risk for malignant mesothe-
lioma. The most effective measures for preventing
lung cancer are not smoking and avoiding circum-
stances in which other people are smoking.
Exposure to radon, a naturally occurring gas
that comes from the soil and can become concen-
trated within indoor areas such as homes and
office buildings, is the second-leading cause of
lung cancer. Radon is odorless and invisible,
though radon detectors can measure its presence.
The U.S. Environmental Protection Agency (EPA)
has established a level of 4 picocuries per liter of
air (4 pCi/L) as the maximum acceptable level.
Simple ventilation measures can reduce or elimi-
nate radon from indoor air.
See also ADENOMA-TO-CARCINOMA TRANSITION;
CANCER PREVENTION; CANCER TREATMENT OPTIONS AND
DECISIONS; ENVIRONMENTAL CIGARETTE SMOKE; PAIN
MANAGEMENT IN CANCER; RADON EXPOSURE; SMOKING
AND CARDIOVASCULAR DISEASE; SMOKING AND PUL-
MONARY DISEASE.
lungs The paired organs in the chest that bring
oxygen-bearing air into the body and expel wastes
in the form of exhaled gases, primarily carbon
dioxide. The right lung has three lobes and the left
lung has two lobes. An indentation between the
left lung’s two lobes, called the cardiac notch, cra-
dles the HEART. The lungs and heart, along with
their supporting structures, fill the thoracic cavity
(chest). The heart pumps deoxygenated BLOODto
the lungs via the PULMONARY ARTERIESand receives
oxygenated blood back from the lungs via the PUL-
MONARY VEINS, circulating the body’s entire blood
volume through the lungs once every minute.
The TRACHEA (windpipe) carries air from the
THROATinto the lungs, branching into the right and
left BRONCHUSto deliver air to the right and left
lung, respectively. Each bronchus further subdi-
vides into mainstem bronchi going to each lobe of
the lung and into progressively smaller bronchial
branches within the lungs. The smallest branches
are the bronchioles which terminate in the alveoli,
grapelike clusters of tiny sacs where the
OXYGEN–CARBON DIOXIDE EXCHANGE takes place. A
weblike mesh of capillaries (tiny blood vessels)
covers each ALVEOLUS. Each lung contains about
300 million alveoli, which gives lung tissue a
spongelike appearance.
Each lobe of the lung consists of multiple seg-
ments, anatomically and physiologically distinct. A
bronchial structure—bronchi, bronchioles and
alveoli along with supporting nerves, arteries, and
veins—supplies each segment. The three lobes of
the right lung contain 10 segments; the two lobes
of the left lung contain 8 segments. This structural
and functional compartmentalization aids the effi-
ciency of the lung as well as helps protect it in the
event of injury (either traumatic or due to dis-
ease), enabling portions of the lung to function
when others are damaged or diseased.
Lung tissue contains elastin, a substance that,
as the name implies, gives the lung tissue elastic-
ity. The lungs have no ability to move on their
own but rather function as a pair of synchronized
bellows that stretch and rebound with contraction
and relaxation of the DIAPHRAGMand the inter-
costal muscles (the muscles between the ribs).
Contraction of these muscles expands the chest,
and the lungs stretch to fill the space which pulls
air into the lungs. When these muscles relax, the
chest returns to its normal size and the lungs
rebound, pushing air back out of the lungs. Each
combination of inhalation and exhalation consti-
tutes a RESPIRATORY CYCLE. The lungs complete 15
to 20 respiratory cycles each minute in a healthy
adult.HEALTH CONDITIONS THAT AFFECT THE LUNGS
ASBESTOSIS ASPERGILLOSIS
ASTHMA ATELECTASIS
BERYLLIOSIS BRONCHIECTASIS
BRONCHITIS BYSSINOSIS
CHRONIC OBSTRUCTIVE CYSTIC FIBROSIS
PULMONARY DISEASE(COPD) LEGIONNAIRES’ DISEASE
LUNG ABSCESS LUNG CANCER
PNEUMOCONIOSIS Pneumocystis carinii
PNEUMONITIS PULMONARY EDEMA
PULMONARY EMBOLISM PULMONARY FIBROSIS
PULMONARY HYPERTENSION SILICOSIS
TUBERCULOSISOxygen–carbon dioxide exchange, the process
of getting oxygen into and removing carbon diox-
ide from the blood, is the primary purpose of the
lungs and is a function of physics in which mole-212 The Pulmonary System