Isotonic solutions have the same osmolality as body fluids. It is used to replace extracellular volume. Replaces fluid loss when there is not an electrolyte imabalance. Stay in ECF/ICF. Does not cause cell shrinking or hydration. It is 0.9% NaCl, Lactated Ringers. 0.9% NS contains 9g/L of sodium chloride, which is closest to the bodies concentration. That is why it does not move from the ICF/ECF. Now 0.45% NaCL contains 4.5g/L sodium chloride, half of the bodies normal concentration. Think about it: if you have a solution that is 0.45% solutes compared to the 0.9%, then the new solution is more dilute. It is hypotonic because there is less concentration of solutes compared to the normal body. When you infuse that into the body, the body wants to even out the electrolytes, so the 0.45% solution will be pulled into the intracellular fluid to even out the electrolytes because there is too little concentration of them with hypotonic solution.
D5W is isotonic out of the body, but then turns hypotonic when infused in the body. D5W is a carbohydrate solution that uses glucose as the solute dissolved in sterile water. Five percent dextrose in water is packed as an isotonic solution but becomes hypotonic
in the body because the glucose dissolved in sterile water is metabolized rapidly by the body’s cells. This leaves primarily water and causes IV fluid to become hypotonic in relation to the plasma surrounding the cells. Accordingly, the now hypotonic solution causes an osmotic shift of water to and from the bloodstream and into the cells, so fluid goes from the ECF to the ICF.
I hope that I am explaining this correctly , and that it makes sense.