Water can be found inside or outside cells – The intracellular or extracellular space.
The extracellular space is divided into the interstitial space and intravascular (IV) space. The interstitial space is the fluid surrounding cells and outside the vasculature. The IV space aka plasma volume is the fluid inside blood vessels. Plasma volume is not the same as the blood volume because the blood volume includes cells, which are considered part of the intracellular space.
The cell membrane separates the intracellular space. Capillary walls separate the intravascular space from the interstitial space. Both cell membranes and capillary walls are freely permeable to water.
Take home message: These fluid compartments help us understand where our patients are losing fluid from as well as where our intravenous fluids go.
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