What happens to water movement in an isotonic solution?
Isotonic relationship exists when two compartments separated by semipermeable membrane possess identical effective osmolarity of impermeable solutes, typically around 300 milliosmoles per kilogram for mammalian cells corresponding to 0.9 percent sodium chloride or 5 percent dextrose clinically used for infusion. Osmotic pressure pi equals iCRT is identical on both sides, so water chemical potentials equal and thermodynamic driving force zero. Nevertheless molecular agitation does not cease; water molecules continuously cross lipid bilayer by solubility diffusion and through aquaporin-1 channels via single-file hydrogen bonded chain in both directions at rates exceeding ten to the ninth per second per pore due to thermal motion. Because probabilities of forward and reverse jumps are balanced by equal activity, influx equals efflux, net flux zero, and compartment volumes remain constant over time with no change in cell diameter. This dynamic equilibrium prevents shrinkage or swelling and preserves discoid shape and deformability for microcirculation. Hypertonic condition would cause net efflux and crenation, hypotonic influx and hemolysis. Physiological saline therefore formulated to be isotonic avoids damage during transfusion.
Ref: Alberts et al., Molecular Biology of the Cell, Chapter 11: Isotonic Solutions and Dynamic Equilibrium of Water Flux.