Most of the water is in the intracellular space. Approximately 2/3 or 67% of TBW is found inside cells.
That leaves one-third or 33% in the extracellular space. Most of the extracellular fluid, 3/4 or 75%, is in the interstitial space. The last 1/4 or 25% is in the IV space.
If fractions work better for you:
If percentages work better for you:
If a flow chart works better for you:
For a 25 kg dog, the TBW is 60% BW, so 0.6 x 25 kg = 15 liters
Intracellular volume is 2/3 X 15 L = 10 L
Extracellular volume is 5 L. 1/3 of 15 L or 15 L – 10 L = 5 L
Interstitial volume is 3/4 x 5 L = 3.75 L. This is 25% TBW.
IV volume is 1/4 x 5 L = 1.25 L. This is 8% TBW.
Take home message: The intracellular space is quite large, while the IV space is relatively small.
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