If the concentration of blood is 0.5 M, and red blood cells that are bathed in blood neither shrink nor swell and burst, the concentration of cytoplasm of RBCs is 0.5m. An isotonic solution relates to two solutions that have the same water concentration on both sides of a membrane. So, cells in isotonic solution have the same water concentration as the isotonic solution and thus they will neither shrink nor swell and burst. If the concentration of blood is 0.5 M, the concentration of RBC cytoplasm must be the same which is 0.5 M.