Acute sequestration in a body compartment not in equilibrium with extracellular fluid is called what?

• A. Intravascular shift
• B. Third space
• C. Interstitial edema
• D. Intracellular sequestration

Answer: (B)

Fluid sequestered in a body compartment that is not in equilibrium with the extracellular fluid is called third spacing. This means the fluid has moved into spaces such as the peritoneal cavity, pleural or pericardial spaces, or various tissue compartments where it’s not readily exchanged back into the intravascular pool. Because this fluid is trapped away from the circulating blood, it lowers effective circulating volume even if overall body water hasn’t dramatically changed, which is a key issue during surgery or critical illness. Interstitial edema is still part of the extracellular fluid and usually more readily mobilized back into the circulation, whereas intracelluar sequestration is fluid inside cells and intravascular shifts stay within the blood compartment. Therefore, when fluid shifts into a nonexchangeable space, the best term is third spacing.