P

pleura The membrane that covers the exterior
surface of the LUNGS and lines the inside of the
thoracic cavity. The pleura has the consistency
of wet tissue paper and appears to cling to the
lungs. The pleural space (thin area between the
two layers of pleura) protects the lungs from con-
tact with other structures within the thoracic cav-
ity, and contains a very small amount of fluid that
reduces friction with BREATHING. The pleural space
can become irritated, inflamed, and infected, caus-
ing conditions such as PLEURISY and PLEURAL
EFFUSION.
For further discussion of the pleura within the
context of pulmonary structure and function
please see the overview section “The Pulmonary
System.”
See also ALVEOLI; BRONCHUS; INFECTION; INFLAMMA-
TION; THORACIC DUCT; TRACHEA.

pleural effusion An increase in the amount of
fluid between the PLEURA. In health there is a very
small amount of fluid, only 10 to 20 milliliters,
within the pleural cavity. A pleural effusion can
contain upward of 2 liters of fluid, though much
smaller quantities (less than 400 milliliters) are
more common. Pleural effusion compresses the
LUNGS, preventing them from fully expanding.
Many conditions can cause pleural effusion.
Pleural effusion is exudative when it results from
INFLAMMATIONof the pleura (PLEURISY). Pleural effu-
sion is transudative when pressure changes in the
body’s fluid balance (osmotic) mechanisms allow
more fluid to cross the pleural membrane such as
with HEART FAILURE. A hemothorax exists when the
excess fluid is BLOOD, and a chylothorax occurs
when the excess fluid is LY M P H.
Many people who have pleural effusion have
no symptoms. When present, symptoms include

• DYSPNEA (shortness of breath or difficulty
  BREATHING)


• CHEST PAIN, primarily with inhalation


• fatigue or weakness

The diagnostic path typically includes chest X-

RAY, COMPUTED TOMOGRAPHY (CT) SCAN or

ULTRASOUND, and THORACENTESIS (withdrawing a

sample of the fluid using a syringe with a large

needle). Treatment aims to reduce the volume of

fluid as well as identify the underlying cause (such

as infection). Thoracentesis may also be therapeu-

tic, allowing the pulmonologist to drain away the

excess fluid. Doctors generally drain no more than

1.5 liters of fluid at a time because more substan-

tial withdrawal can result in rapid fluid shifts,

causing cardiovascular instability and the develop-

ment of pulmonary edema (fluid accumulation in

the lung tissue). Recovery depends on the condi-

tion causing the pleural effusion.

See also LUNG CANCER; PULMONARY EDEMA.

pleurisy INFLAMMATIONof the PLEURA, also called

pleuritis. Pleurisy can develop as a consequence of

direct irritation or INFECTIONin the pleural space,

or as a consequence of infection or INFLAMMATION

involving the LUNGSsuch as TUBERCULOSISor PNEU-

MONIA. AUTOIMMUNE DISORDERScan cause inflam-

mation, such asSYSTEMIC LUPUS ERYTHEMATOSUS(SLE)

andSARCOIDOSIS. The characteristic symptom of

pleurisy is sudden, sharp, and often severe PAIN

during inhalation and exhalation that subsides

with holding the breath. The pain may occur on

only one side of the chest or both sides, and may

feel as though it comes from the back or under the

shoulder blades, depending on the site of the

inflammation. Some people also have a persistent,

dry COUGH.

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