Osmosis, tonicity, and cell volume are regulated by the permeability of cell membranes to water and solutes. Water moves across membranes through aquaporin channels and by osmosis down its concentration gradient. The osmolarity of extracellular fluid is normally 300 mOsm, and cells maintain volume by moving water in or out depending if the extracellular fluid is hypo- or hypertonic. The kidneys help maintain electrolyte and acid-base balance by reabsorbing or secreting ions like sodium, potassium, hydrogen, and bicarbonate to keep fluid volumes and pH within normal ranges.