It helps maintain normal blood pressure, supports the work of your nerves and muscles, and regulates your body’s fluid balance. A normal blood sodium level is between 135 and 145 milliequivalents per liter (mEq/L). Hyponatremia occurs when the sodium in your blood falls below 135 mEq/L
https://www.mayoclinic.org/diseases-conditions/hyponatremia/symptoms-causes/syc-20373711
https://www.osmosis.org/notes/Hypernatremia_and_hyponatremia#page-1