At the cellular level, what happens when the blood sodium concentration falls below normal?

1. Sodium is the major cation of the extracellular fluid and sets much of its osmotic pull. 2. In hyponatremia the extracellular fluid is diluted, so its solute concentration drops. 3. Solutes inside the cell now exceed those in the diluted fluid outside it. 4. Water follows that gradient and moves into the cell by osmosis. 5. The cells therefore swell rather than shrink. 6. Swollen red cells carry oxygen less efficiently and may be too large to pass through capillaries. 7. Swelling of neurons in the brain can lead to brain damage or death. 8. Water leaving cells would need the outside fluid to be more concentrated, which is the opposite state. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 26 "Fluid, Electrolyte, and Acid-Base Balance", section Sodium_