A system and method for controlling a mammalian cell culture process are provided. Such control of the cell culture process involves control of the level of dissolved carbon dioxide in the cell culture media and the resulting ability to prevent increases in the osmolality level is achieved by enhanced stripping of carbon dioxide with little or no damage to the mammalian cells. The disclosed methods and systems of dissolved carbon dioxide stripping include enhanced surface gas exchange mechanisms within the bioreactor vessel through the use of an upward flow impeller combined with vertical baffles to convert swirling motions of the liquid into a largely vertical flow.