The concentration of sodium chloride, NaCl, in isotonic saline solution is 0.9 percent, or 9 grams NaCl per 1,000 milliliters of water. Saline is a salt and water mixture used internally for hydration and externally for rinsing the eyes and nose.
The concentration of NaCl in isotonic saline is close enough to the concentration within a healthy cell that it prevents osmosis of water into or out of the cell. Saline with a higher concentration of NaCl than inside a cell is said to be hypertonic. Cells in hypertonic saline lose water to the solution and shrivel. Hypotonic saline contains a lower concentration of NaCl than a cell. In hypotonic saline, cells draw in water until bursting.