[Haematology] Atlas of Serous Body Fluids

Serous Body Fluids, Atlas of Serous Body Fluids, Graff's Textbook of Urinalysis and Body Fluids

Serous organs (heart, lungs, abdominal) and are lined with serous membranes. The serous membrane that covers body cavities are those which surround various the organ is the visceral portion of the membrane, whereas the serous membrane that lines the body wall is the parietal portion of the membrane. Serous fluid fills the space between the visceral portion and the parietal portion and functions as a lubricant between the membranes of the body wall and organs. Figure 10-1 shows the mesothelial lining of serous body cavities. This chapter contains information regarding the analysis of these fluids (pleural, pericardial, and peritoneal) obtained by paracentesis. 


The name serous is given to this fluid because of its serum- like composition. Serous fluid is an ultrafiltrate of plasma and is maintained by the pressure forces (tissue colloidal osmotic pressure, capillary hydrostatic pressure, capillary colloidal osmotic pressure, and tissue hydrostatic pressure) and by the absorption of fluid into the lymphatic system. The accumulation of serous fluid is called an effusion. Effu- sions may result from a disruption in the balance of these pressures or in response to infection and inflammatory processes. Depending on which pressure forces predominate effusions are further classified into transudates or exu- dates. Correctly classifying effusions assists physicians in determine a diagnosis. These classifications are based on results from various laboratory tests. 


Nearly every section of the laboratory can be involved in the evaluation of serous body fluids. Laboratory tests that may be performed on serous body fluids include, but are not limited
Mesothelial lining of serous body cavities
Figure 10-1. Mesothelial lining of serous body cavities

to, macroscopic evaluation of fluid appearance, microscopic evaluation of cell count and type, chemical analysis, microbiology cultures, and immunologic and molecular analyses. Some of these tests are beyond the scope of this text. However, the more common laboratory procedures are included.


Serous body fluids normally resemble serum, clear and pale yellow. Chapter 7 includes an overview of the terminology used to describe abnormal body fluid color and clarity. San- guineous specimens may indicate a hemorrhage but also appear similar in traumatic taps. Differentiation between the two is made by collecting several specimens. Those from traumatic taps will become clear as additional fluid is removed. Purulent specimens indicate the presence of white blood cells (WBCs), which correlates with bacterial infec- tions. Milky fluids may contain chyle or be pseudochylous (explained later in the chapter). Cholesterol crystals, if present in a fluid, will contribute a golden-green iridescence often termed shimmery or shimmering. Clotted specimens can be reported as clotted or fibrinous.


Standard chemical tests that are performed on serous fluids include glucose, lactate dehydrogenase (LD), and protein. These are the most common tests used to categorize effu- sions as transudates or exudates (explained later in this chapter). Tests less commonly performed on body fluids include alkaline phosphatase, ammonia, amylase, bilirubin, chloride, lipids, and pH. 

Peritoneal fluid alkaline phosphatase will be increased when the small intestine is perforated. Peritoneal fluid ammonia levels are higher than serum levels in cases of bowel strangulation, perforated peptic ulcer, ruptured appendix, and ruptured bladder.

Ammonia and amylase levels are increased in bowel necrosis. Amylase is also increased in esophageal perforation, metastatic adenocarcinoma, pancreatitis, and bowel necro- sis. Esophageal ruptures will cause fluids to become more acidic than their normal pH of 7.3 or higher. The chloride levels of body fluids will be less than that of serum when a bacterial infection is present, due to the presence of both bac- teria and WBCs.3 Testing lipid levels in serous fluids assists in the differentiation between chylous and pseudochylous effusions. Triglycerides are higher in chylous effusions, whereas cholesterol is higher in pseudochylous effusions.


Chapter 8 includes an explanation of microscopic examina- tion of body fluids in general. Normally, cells counts and differential are performed. In addition, the presence of crystals is noted.

Red blood cells (RBCs) are not normally seen in body fluids. When present RBCs may indicate hemorrhage or traumatic specimen collection procedure. WBCs are normally present in low numbers with mononuclear cells predominating. The presence of increased numbers of WBCs correlates with various pathologies and is reflected by their dis- tribution. Types of blood cells that can be seen in serous body fluids include neutrophils, eosinophils, basophils, lymphocytes, plasmacytes, monocytes, histiocytes, and macro-phages. 

Mesothelial cells that line the serous cavities may also be present in body fluids due to normal sloughing of cells. Mesothelial cells may exhibit reactive morphology that can be confused with plasmacytes, histiocytes, or tumor cells.

Mesothelial cells are large with dark blue cytoplasm. Histiocytes (tissue monocytes) may be of similar size to mesothelial cells but have a lighter colored cytoplasm. 

Effusions from patients with neoplasms may contain malignant cells. Malignant cells commonly occur in clumps. A pathologist’s consult in the identification of malignant cells is a must when suspect cells are seen.1 

The WBC count that is performed usually includes mesothelial and malignant cells, because all nucleated cells are counted for the WBC count. Therefore, the differential count often includes mesothelial cells and tumor cells. Cytologic examination should be performed when malignancies are suspected, or to assist in the differentiation between tumor cells and reactive mesothelial cells. 

Sometimes, microorganisms may be seen on Wright stained smears while performing differential counts. Though these cells can easily be detected, their identifica- tion must be made using microbiology procedures. 

Figures 10-2–10-7 (page 248) illustrate the cells that may be seen in various body fluid effusions.

Cytospin preparation of pleural fluid containing RBCs  and lymphocytes in acute inflammation
Figure 10-2. Cytospin preparation of pleural fluid containing RBCs 
and lymphocytes in acute inflammation. Wright stain 400x    .

Cytospin preparation of pleural fluid containing RBCs,  neutrophils, and a mesothelial cell in bacterial infection
Figure 10-3. Cytospin preparation of pleural fluid containing RBCs,
neutrophils, and a mesothelial cell in bacterial infection. Wright stain 200x

Cytospin preparation of peritoneal fluid containing RBCs,   lymphocytes, monocytes, and mesothelial cells.
Figure 10-4. Cytospin preparation of peritoneal fluid containing RBCs, 
lymphocytes, monocytes, and mesothelial cells. Wright stain 200x

ytospin preparation of pericardial fluid containing RBCs,   WBCs, and cells resembling adenocarcinoma
Figure 10-5. Cytospin preparation of pericardial fluid containing RBCs, 
WBCs, and cells resembling adenocarcinoma. Wright stain 200x  

Cytospin preparation of peritoneal fluid containing RBCs,  WBCs, and many bacteria
Figure 10-6. Cytospin preparation of peritoneal fluid containing RBCs, 
WBCs, and many bacteria. Identified by culture as Escherichia coli. Wright stain 1000x.

Cytospin preparation of peritoneal fluid containing  WBCs and few bacteria
Figure 10-7. Cytospin preparation of peritoneal fluid containing 
WBCs and few bacteria. Identified by culture as Staphylococcus. Wright stain 1000x

Gram stain and both aerobic and anaerobic cultures should be set up on body fluid specimens to increase the rate of microbial recovery. Pleural fluids should routinely have an acid-fast stain performed for the identification of tuberculosis. In addition, fungal stains and cultures may be set up if a yeast infection is suspected. Table 10-1 summarizes common bacteria observed in effusions.

Effusions are the accumulation of fluids in the tissue spaces and result from an imbalance in pressures between the tis- sues and the capillaries. The laboratory tests described earlier assist in the classification of effusions as transudates or exudates.

Transudate effusions occur during various systemic disorders that disrupt fluid filtration, fluid reabsorption, or both. Examples of systemic disorders that may result in the formation of transudates include congestive heart failure, hepatic cirrhosis, or nephrotic syndrome.

Exudate effusions occur during inflammatory processes that result in damage to blood vessel walls, body cavity membrane damage, or decreased reabsorption by the lymphatic system. Examples of these pathologic processes include infections, inflammations, hemorrhages and malignancies. Each of these processes can damage tissues, body cavity membranes, and alter lymphatic function.

Various laboratory tests are used to differentiate between transudates and exudates including fluid appearance, specific gravity, amylase, glucose, LD, and proteins. Table 10-2 outlines this differentiation. Additional tests such as ammonia, lipids, and pH may be useful in confirming the cause of an effusion for specific body sites.

A chylous effusion is an effusion that contains an emulsion of lymph and chylomicrons. Obstruction or damage of lymphatic vessels contributes to the development of a chylous effusion. Chylous effusions appear milky and may appear shimmery (resembling milk mixed with honey) if cholesterol crystals are present.

Sheets of cholesterol crystals may be present in serous flu- ids when a lymphatic vessel located near a cavity is damaged. Figures 10-8 and 10-9 show a pleural fluid with cholesterol crystals from a lymphatic vessel rupture.

Chronic effusions present in disorders, such as rheumatoid arthritis and tuberculosis, may resemble chylous effusions because of the high amount of cellular debris and

cholesterol present. These effusions are termed pseudochy- lous and can be differentiated from chylous effusions using various laboratory tests such as pH and lipid analysis. Table 10-3 lists laboratory test differences between chylous and pseudochylous effusions.


Serous body fluids are found in the cavities surrounding the vital organs. This fluid is normally clear and slightly yellow in appearance, resembling serum. Serous cavities include the pericardium, pleura, and peritoneum.
Cholesterol crystals in pleural fluid
Figure 10-8. Cholesterol crystals in pleural fluid. Bright light (400x)

Pericardial effusions are an accumulation of fluid around the heart. Figure 10-10 illustrates the pericardium surrounding the heart. Normally, the pericardium contains less than 50 mL of fluid. The procedure for removing excess pericardial fluid, pericardiocentesis, is dangerous and therefore rarely performed. See Figure 10-11 for an illustration of how this procedure is performed. However, this procedure is necessary to obtain a sample if cultures are needed to investigate an infection or if cytology is needed for suspected malignancy.

Normal pericardial fluid is pale yellow and clear. Sanguineous (bloody) effusions may be present in pericardial fluid due to a number of causes. Pericardial effusions are all caused by damage to the mesothelium and not by mechan- ical factors. Therefore, pericardial effusions are usually always exudates. Table 10-4 outlines causes for pericardial exudates as well as causes for other effusions.

Pleural effusions occur when fluid accumulates around the lungs. Figure 10-12 illustrates the pleural cavity and its lining. The pleural cavity normally contains less than 30 mL of fluid. Abnormal accumulation of pleural fluid usually begins at the base of the lungs. Factors that contribute to

Cholesterol crystals in pleural fluid.
Figure 10-9. Cholesterol crystals in pleural fluid. A. Polarized light. 
B. Polar- ized, compensated light (400 )
Figure 10-10. The heart and pericardium

Aspirating pericardial fluid. In pericardiocentesis, a needle and syringe are inserted through the chest wall into the pericardial sac
Figure 10-11. Aspirating pericardial fluid. In pericardiocentesis, a needle and syringe are inserted through the chest wall into the pericardial sac (as shown below). Electrocardiographic monitoring, with a lead wire attached to the needle and electrodes placed on the limbs (right arm, left arm, and left leg), helps ensure proper needle placement and avoids damage to the heart

the formation and the removal of pleural fluid include the draining function of the lymphatic system and the exchange of fluids in the capillaries. As explained in Chapter 7, fluids enter the tissue space, in this case the pleural sac, when there is an increase in capillary hydrostatic pres- sure and/or a decrease in plasma osmotic pressure.

A thoracentesis is performed to remove this excess fluid (over 30 mL). Removal of pleural fluid not only provides a specimen for laboratory examination but also helps improve patient symptoms and allows for better visualization of the lungs and pleural cavity upon radiological procedures.3 An illustration of a thoracentesis is seen in Figure 10-13.

Pleural effusions may be primary or can be secondary to accumulation of peritoneal fluid (ascites). Secondary accumulation occurs because the lymphatic system drains the abdomen toward the right side passing through the diaphragm.

Normal pleural fluid is pale yellow and clear. Abnormal colors and turbidity of pleural fluid indicate various patho- logic processes. Among the various colors abnormal pleural fluid may exhibit are sanguineous (if not a traumatic tap), milky, and shimmery. Table 10-4 outlines causes for pleural transudates and exudates as well as causes for other effusions.

The pleural cavity with effusion
Figure 10-12. The pleural cavity with effusion

Figure 10-13. Thoracentesis. A needle is inserted 
into the pleural space to withdraw fluid.

A peritoneal effusion is the accumulation of peritoneal fluid, also called ascites, in the abdominal cavity. Figure 10-14 illustrates the organs that are contained within the peritoneal cavity. Fluid may accumulate in the abdomen as a result of a specific clinical disorder or because of generalized edema (accumulation of fluid in tissues). Ascites is removed by abdominal paracentesis, as illustrated in Figure 10-15. The fluid that accumulates during chronic liver disease is a result of a decrease in the plasma colloidal pressure because of the liver’s impaired ability to synthesize proteins.

Removal of more than 1000 mL of ascites can cause hypovolemia and shock. Another procedure used to collect peritoneal fluid is peritoneal lavage. Peritoneal lavage is used when the patient has had a blunt or penetrating abdominal trauma.

Normal peritoneal f luid is pale yellow. Abnormal appearances of peritoneal f luid indicate various pathologic processes. Abnormal colors that peritoneal fluid can show include sanguineous (if not a traumatic tap), brown, green, and milky. Table 10-4 outlines causes for peritoneal transudates and exudates as well as causes for other effusions.

The organs of the abdomen
Figure 10-14. The organs of the abdomen. IVC, inferior vena cava. 

Paracentesis of the abdominal cavity in midline.
Figure 10-15. Paracentesis of the abdominal cavity in midline.


Lillian A. Mundt and Kristy Shanahan, Graff's Textbook of Urinalysis and Body Fluids, Second Edition 2011



Free Medical Atlas: [Haematology] Atlas of Serous Body Fluids
[Haematology] Atlas of Serous Body Fluids
Serous Body Fluids, Atlas of Serous Body Fluids, Graff's Textbook of Urinalysis and Body Fluids
Free Medical Atlas
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