Chapter 13 - Serous Fluid

# Physiology

* The closed cavities of the body - the pleural, pericardial and peritoneal cavities - are each lined by two membranes referred to as the serous membranes.
* One membrane lines the cavity wall (parietal membrane).
* The other covers the organs within the cavity (visceral membrane).
* The fluid between the membranes is called serous fluid, which provides lubrication between the parietal and visceral membranes.
* Normally, only a small amount of serous fluid is present, because production and reabsorption take place at a constant rate.
* Production and reabsorption are subject to hydrostatic and colloidal (oncotic) pressures from the capillaries that serve the cavities and the capillary permeability.
* Under normal conditions, colloidal pressure from serum proteins is the same in the capillaries on both sides of the membrane.
* The hydrostatic pressure in the parietal and visceral capillaries causes fluid to enter between the membranes.
* A slightly unequal pressure in the parietal and visceral capillaries creates a small excess of fluid that is reabsorbed by the lymphatic capillaries.
* Serous fluids are formed as ultrafiltrates of plasma, with no additional material contributed by the mesothelial cells that line the membranes.
* The filtration of the plasma ultrafiltrate results in increased oncotic pressure in the capillaries that favors reabsorption of fluid back into the capillaries.
* This produces a continuous exchange of serous fluid and maintains the normal volume of fluid beween the membranes.

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# Specimen Collection and Handling

* Collection is done by needle aspiration, namely thoracentesis (pleural), pericardiocentesis (pericardial), and paracentesis (peritoneal).
* Abundant fluid (greater than 100 mL) is usually collected.
* An ethylenediaminetetraacetic acid (EDTA) tube is used for cell counts and differential counts.
* Sterile heparinized evacuated tubes are used for microbiology and cytology.
* For better recovery of microorganisms and abnormal cells, concentration of large amounts of fluid is performed by centrifugation.
* Chemistry tests can be run on clotted specimens in plain tubes or on heparinized tubes.
* Blood specimens should be obtained at the time of collection for comparison with chemical tests.
* Specimens for pH must be maintained anaerobically in ice.

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# Transudates and Exudates

* Effusions that form because of a systemic disorder that disrupts the balance in the regulation of fluid filtration and reabsorption are called transudates.
* Testing transudate fluids is usually not necessary.
* Effusions that form because of conditions that directly involve the membranes of the particular cavity are called exudates.
* Differentiation is done by determining the fluid-to-blood ratios for protein and lactic dehydrogenase.

| | Transudate | Exudate |
|----|----|----|
| **Appearance** | clear | cloudy |
| **Fluid-Serum Protein Ratio** |