Normal saline is isosmotic and isotonic to your blood cells and is about 0,9% NaCl.
Explanation:
Hello!
To answer this question you have to take into account the properties of body fluids, in particular of plasma and intracellular fluids.
The most important property to consider when rehydrating a patient is osmolarity.
Remember, osmolarity measures the solute(st) concentration (number of particles) dissolved in a solution(sn). (expressed 
)
Considering two compartments separated by a selectively semipermeable membrane you can categorize solutions by osmolarity in:
>Hiposmotic: The number of particles dissolved in the solution is less than in plasma (and intracellular fluid).
>Isosmotic: The number of particles dissolved in the solution is equal to plasma (and intracellular fluid).
>Hiperosmotic: The number of particles dissolved in the solution is higher than in plasma (and intracellular fluid).
Tonicity is related to osmolarity, but instead of taking all particles dissolved in a solution into account, it only measures the non-freely penetrating solutes (meaning, particles that cannot penetrate through the biological membrane)
Why is this important? Quick reminder, biological membranes are selectively permeable membranes and liquids from two compartments separated by a semipermeable selective membrane always try to establish an osmotic equilibrium with which water is always attracted to the compartment with greater osmolarity (def. Osmosis).
Taking two compartments separated by a selectively semipermeable membrane, there are three types of tonic solutions:
>Isotonic: Same osmolarity in both compartments (for example, plasma and blood cells)
>Hipotonic: Less osmolarity in one of the compartments.
>Hipertonic: Higher osmolarity in one of the compartments.
So if you put a blood cell, for example, erythrocytes, in an isotonic solution, since the osmolarity inside the cell and outside the cell are equal, nothing happens.
If you put a blood cell in a hypotonic solution, water from the solution will enter the cell (attracted to the higher concentration of solutes), causing its volume to rise and eventually the cell will suffer cytolysis. (not compatible with life)
If the blood cell is put in a hypertonic solution, the water inside the cell will exit it reducing the volume of intracellular fluids. This phenomenon is called "crenation" and is not compatible with life.
Since blood cells will die if you use a solution with different osmolarity as plasma NSS must have the same osmolarity as plasma.
Human plasma has an osmolarity in range of 275-299mOsm/Kg, NSS (0,9%NaCl) has an osmolarity of 300 mOsm/Kg this allows rehydrating a patient without compromising the integrity of blood cells.
I hope you have a SUPER day!
