Pleural, Peritoneal, and Pericardial Effusions
Fig. 19.23 Cell block of pleural effusion containing a large fragment of
hyperplastic mesothelium with a collagenous stroma. The patient had
foreign body granulomatous pleuritis (H&E x MP).
tubes), and each contains a small amount of fluid. The outer layer
of each serous cavity is the parietal layer; the layer directly in con-
tact with the enveloped organ is the visceral layer. Apart from a
thin film of fluid, these layers are in contact with each other; thus,
under normal conditions, each cavity is only a potential cavity.
When a cavity contains excess fluid, it becomes an actual cavity.
Each cavity is lined by a monolayer of mesothelial cells beneath
which is a layer of connective tissue, supplied with blood vessels,
lymphatics, and nerves (Fig. 19.24). The close proximity of blood
and lymphatic vessels to mesothelium may partly account for the
ready spread of neoplastic cells into a serous cavity.
Types of Effusions
Accumulation of fluid in a serous cavity in excess of the normal
small amount is referred to as an effusion. Effusions are desig-
nated according to their location: pleural, peritoneal, or pericar-
dial. The condition of peritoneal effusion is frequently referred
to as ascites and the fluid as ascitic fluid. Air may sometimes
be introduced into a serous cavity by trauma (including surgi-
cal trauma), for radiologic diagnostic purposes, for therapy, or
secondary to a pathologic process, producing the condition of
pneumothorax, pneumoperitoneum, or pneumopericardium.
When these conditions are accompanied by effusion, the terms
are expanded to indicate its presence: pneumohydrothorax,
pneumohydroperitoneum, and pneumohydropericardium.
Transudates and Exudates
The number and type of non-neoplastic cells commonly found
in serous effusions depend to a large extent on the pathogenetic
mechanisms of fluid formation, which determine whether an
effusion is classified as a transudate or an exudate. Transudates
are effusions characterized by a low protein content, usually less
than 3.0 g/dL, and low specific gravity, usually less than 1.015.
Transudates accumulate by the filtration of serum across physi-
cally intact capillary walls under conditions in which the out-
flow of fluid through a serous membrane exceeds the normal
reabsorptive process. This may take place as a result of increased
venous pressure, as in congestive heart failure or cirrhosis of
Fig. 19.24 Peritoneum. A monolayer of mesothelial cells beneath which is
a layer of fibrous connective tissue (H&E x MP).
the liver, or in hypoproteinemia in renal failure. Transudates
generally have a lower cellular content than exudates, and their
fibrin content is also lower. The cellular content usually consists
of mesothelial cells and macrophages, with an occasional
lymphocyte or neutrophilic leukocyte.
Exudates result from damage to the capillary walls that ram-
ify in the serosal connective tissue. This damage allows escape of
protein and various cellular constituents of the blood into the
serous cavity, resulting in an effusion that has a higher protein
content (3.0 g/dL or more) and specific gravity (>1.015) than
that of the typical transudate. Furthermore, the cellular con-
tent is higher and is likely to contain many inflammatory cells
in exudates caused by inflammation or, in exudates caused by
neoplasm on the serosal surface, many neoplastic cells.
Pleural inflammatory exudates are likely to be caused by
pneumonia, pulmonary infarct, pulmonary abscess, pleuritis, or
secondary bacterial infection of a transudate. Peritoneal inflam-
matory exudates are likely to be caused by peritonitis, either
spontaneous bacterial or secondary to infarct or inflammation
of the bowel, spontaneous or traumatic rupture of a viscus, or
pelvic inflammatory disease in women. Pericardial effusions
that are exudates are likely to be caused by viral or bacterial
inflammation of the pericardium or uremic pericarditis. In the
less prosperous nations of the world, tuberculous inflammation
of a serous membrane is always foremost in the minds of clini-
cians, whereas in affluent societies it is far less frequent.
Serous effusions caused by neoplasm may be transudates,
resulting from failure of resorption of serous fluid due to
mechanical interference by neoplasm, or they may be exudates
caused by a neoplasm that damages the capillaries of the serous
Every pleural, peritoneal, and pericardial effusion contains
cells, often numerous, occasionally scanty. The non-neoplas-
tic cells commonly found in serous effusions are those derived
from blood (erythrocytes, leukocytes, and histiocytes) and from
the serosal lining, the mesothelial cells. The proportion of the
different types of these cells varies considerably, depending on