A solution is only isotonic compared to another solution. The statement that .9 NaCl is isotonic is incomplete. You are referring to the fact that it is isotonic with blood serum. This means they have the same osmotic pressure as each other which controls how they pass through the semi permeable membrane of blood vessels. .9 NaCl is isotonic with blood serum and that means that it will diffuse out of the blood vessel after IV injection in a similar fashion to the components of the blood serum itself. .9Nacl contains about 154 mOsm/L of Na and 154 mOsm/L of Cl giving the solution an osmolarity of 308 mOsm/L. The osmolarity of blood is about 300-310 mOsm/L so the two are very close and that's why .9NaCl is considered isotonic for IV solution.