I’m not the best at the calculations of ICF/ECF, but basically you are infusing a hypertonic solution into the animal. Initially, this is all going to go into the extracellular space, as any IV infusion will do. Since it is higher than isotonic solution, water is going to go from the intracellular space to the extracellular space to try to balance it out, so the intracellular space will have decreased volume and increased osmolality (since only water is leaving, making it more concentrated).

So you know for sure ICF volume is decreased and osmolality increased, and the extracellular volume will be increased. I think the osmolality of the extracellular space is the tricky part and the part where maybe someone else can help with the calculations but basically it’s hypertonic enough that the osmolality will still be increased.

To answer this question, it may be easier to think of it as adding two different things: first, an isotonic saline solution up to the total volume added then later adding salt (thereby making a hypertonic solution). This will effectively demonstrate that the only real important addition is adding the salt.

Adding the isotonic solution will by definition not change the osmolality of ECF. Therefore, the only change is increased ECF volume.

Next, adding the salt obviously increases ECF osmolality, which increases ICF osmolality and decreases ICF volume as water moves into the ECF from the ICF. Since this change only further increases ECF volume, no further investigation is necessary.