span. From moments after birth to near the end of
life the lungs function continuously and consis-
tently, bringing air into the body to deliver oxygen
to the BLOODand carrying air out of the body to
eliminate carbon dioxide and other gaseous meta-
bolic wastes from the body. The entire respiratory
process—BREATHINGand oxygenation—takes place
under the regulation of the brainstem, without
conscious awareness or control.
Before birth, though the lungs go through the
movements of breathing they do not oxygenate
the fetus’s blood. Rather, the fetus draws its oxy-
gen from the mother’s blood through the PLACENTA
where oxygen molecules migrate across the capil-
lary membranes from the mother’s blood supply
to the fetus’s blood supply. Differences in the car-
diovascular system of the fetus further support
this mechanism of oxygenation. In the unborn
child the HEARTshunts blood from the right atrium
to the left atrium through an opening in the atrial
septum (wall of muscle that separates the right
atrium and the left atrium) called the foramen
ovale. Blood also passes from the PULMONARY
ARTERIESto the AORTAthrough an opening called
the ductus arteriosus, bypassing the lungs.
The newborn’s first breath fills the lungs with
air, setting in motion a sequence of events that
results in the closure of these openings in the
heart and the rerouting of blood flow from the
right side of the heart to the lungs and from
the lungs to the left side of the heart. As the lungs
fully expand after a few breaths, they take over
complete responsibility for oxygenating the body.
The lung tissue produces a chemical called surfac-
tant, a fluid that coats the inner layer of the lung
surfaces to maintain appropriate surface tension to
keep the alveoli from collapsing with each exhaled
breath, much in the same fashion moisture inside
a balloon keeps the walls of the balloon from
sticking together when the balloon deflates. These
changes may lag in an infant born prematurely,
giving rise to breathing and oxygenation difficul-
ties until the lungs can more fully develop.
The lungs continue to function in the same
responsibility for the remainder of life, with few
changes beyond growing as the body grows. Lung
capacity (the ability of the lungs to hold air) and
diffusing capacity (the ability of the lungs to trans-
fer oxygen to the blood) peak in the early 20s,
after which both slowly but steadily decline until
by about age 75 they are roughly half what they
were at age 25. Beginning at age 35, lung capacity
diminishes about 5 percent every 10 years. In the
50s the muscles of breathing begin to stiffen and
lose resilience, though in counterbalance changes
within the lungs and airways occur to reduce the
resistance air encounters during inhalation. Regu-
lar aerobic CONDITIONINGthroughout life can offset
many of the functional implications of these
changes, allowing strong pulmonary performance
well into the 70s or beyond.
The health conditions affecting the lungs before
age 30 tend to be acute (of sudden onset and con-
tained duration), often INFECTIONsuch as viral or
bacterial BRONCHITIS, pleuritis, and PNEUMONIA.
Chronic (ongoing) health conditions affecting the
lungs become increasingly common with advanc-
ing age, in part because the natural changes in the
lungs may precipitate them and in part because
other health situations or environmental factors
begin to have cumulative consequences.
CARDIOVASCULAR DISEASE(CVD), which becomes
more common in middle age and beyond among
both men and women, can have as much effect on
the structures and functioning of the pulmonary
system as do conditions of the lungs. HEART
FAILURE, in which the heart cannot pump enough
blood to meet the body’s oxygen needs, allows
fluid to back up into the lungs. The resulting cir-
cumstances, pulmonary congestion andPULMONARY
EDEMA, flood the alveoli and prevent them from
conducting the OXYGEN–CARBON DIOXIDE EXCHANGE.
Chronic HYPERTENSION (high BLOOD PRESSURE)
diminishes the elasticity of all arteries in the body,
including the pulmonary arteries. The resulting
stiffness and inflexibility of the arteries can con-
tribute to or exacerbate cardiovascular conditions
such as heart failure. Other forms of cardiovascu-
lar disease may result in PULMONARY HYPERTENSION,
increased pressure within the pulmonary arteries
that damages the smaller arteries within the lungs.
External factors also influence pulmonary
health throughout life. Cigarette smoking is the
single-most destructive exposure the lungs typi-
cally face, causing an extremely high risk for pro-
gressive disorders such as CHRONIC OBSTRUCTIVE
PULMONARY DISEASE(COPD) and especially for LUNG
CANCER. Cigarette smoking accounts for 85 percent182 The Pulmonary System