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Chapter: Medical Physiology: The Body Fluid Compartments: Extracellular and Intracellular Fluids; Interstitial Fluid and Edema

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Fluids in the “Potential Spaces” of the Body

Perhaps the best way to describe a “potential space” is to list some examples: pleural cavity, pericardial cavity, peritoneal cavity, and synovial cavities, including both the joint cavities and the bursae.

Fluids in the “Potential Spaces” of the Body

Perhaps the best way to describe a “potential space” is to list some examples: pleural cavity, pericardial cavity, peritoneal cavity, and synovial cavities, including both the joint cavities and the bursae. Virtually all these potential spaces have surfaces that almost touch each other, with only a thin layer of fluid in between, and the surfaces slide over each other. To facilitate the sliding, a viscous proteinaceous fluid lubricates the surfaces.

Fluid Is Exchanged Between the Capillaries and the Potential Spaces. The surface membrane of a potential spaceusually does not offer significant resistance to the passage of fluids, electrolytes, or even proteins, which all move back and forth between the space and the interstitial fluid in the surrounding tissue with relative ease. Therefore, each potential space is in reality a large tissue space. Consequently, fluid in the capillar-ies adjacent to the potential space diffuses not only into the interstitial fluid but also into the potential space.

Lymphatic Vessels Drain Protein from the Potential Spaces.

Proteins collect in the potential spaces because of leakage out of the capillaries, similar to the collection of protein in the interstitial spaces throughout the body. The protein must be removed through lymphat-ics or other channels and returned to the circulation. Each potential space is either directly or indirectly connected with lymph vessels. In some cases, such as the pleural cavity and peritoneal cavity, large lymph vessels arise directly from the cavity itself.

Edema Fluid in the Potential Spaces Is Called “Effusion.”

When edema occurs in the subcutaneous tissues adja-cent to the potential space, edema fluid usually collects in the potential space as well, and this fluid is called effusion. Thus, lymph blockage or any of the multipleabnormalities that can cause excessive capillary filtra-tion can cause effusion in the same way that intersti-tial edema is caused. The abdominal cavity is especially prone to collect effusion fluid, and in this instance, the effusion is called ascites. In serious cases, 20 liters or more of ascitic fluid can accumulate.

The other potential spaces, such as the pleural cavity, pericardial cavity, and joint spaces, can become seriously swollen when there is generalized edema. Also, injury or local infection in any one of the cavi-ties often blocks the lymph drainage, causing isolated swelling in the cavity.

The dynamics of fluid exchange in the pleural cavity are discussed. These dynamics are mainly representative of all the other potential spaces as well. It is especially interesting that the normal fluid pressure in most or all of the potential spaces in the nonedematous state is negative in the same way that this pressure is negative (subatmos-pheric) in loose subcutaneous tissue. For instance, the interstitial fluid hydrostatic pressure is normally about 7 to –8 mm Hg in the pleural cavity, –3 to –5 mm Hg in the joint spaces, and –5 to –6 mm Hg in the peri-cardial cavity.


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