===== Interstitial Fluid Volume (ISF Volume) =====

**Definition:**  
'''Interstitial Fluid Volume''' refers to the total volume of fluid occupying the '''extracellular space''' between cells within a tissue. In the central nervous system (CNS), it is the fluid that lies between neurons, glia, and capillaries—distinct from [[cerebrospinal fluid]] (CSF) and intracellular fluid.

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**Physiological Role:**
  * Serves as a medium for:
    - **Nutrient transport**
    - **Waste removal**
    - **Ion exchange**
    - **Intercellular signaling**
  * Provides mechanical support and maintains the extracellular matrix (ECM) environment.
  * Interfaces dynamically with [[CSF]] through the [[glymphatic system]] and perivascular pathways.

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**Brain-Specific Characteristics:**
  * Estimated to occupy ~15–20% of total brain tissue volume under physiological conditions.
  * Closely regulated by:
    - Capillary filtration through the [[blood–brain barrier]] (BBB)
    - Aquaporin-4 channels on astrocytic endfeet
    - Arterial pulsatility and sleep-wake cycles (via glymphatic function)

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**Alterations in ISF Volume:**
↑ ISF Volume:
  * [[Idiopathic normal pressure hydrocephalus]] (iNPH)
  * [[Vasogenic edema]]
  * Aging-related extracellular matrix degradation

↓ ISF Volume:
  * [[Cytotoxic edema]] (e.g., ischemic stroke)
  * Cellular swelling or inflammation

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**Measurement Techniques:**
  * [[Spectral diffusion analysis]] (→ F<sub>int</sub>)
  * [[Diffusion MRI]] (indirect estimation via ADC)
  * Tracer-based imaging in experimental models
  * Optical or microdialysis methods in animal studies

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**Distinction from Related Terms:**
  * **Interstitial fluid volume fraction (F<sub>int</sub>):**  
    Dimensionless ratio of ISF volume to total tissue volume.
  * **CSF volume:** Located in ventricles and subarachnoid space; not part of ISF.
  * **Extracellular volume:** Includes ISF + CSF + intravascular space (context-dependent).
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