Final answer:
The sodium ion content of the extracellular fluid (ECF) is largely regulated by an adrenal cortex hormone called Aldosterone.
Step-by-step explanation:
Aldosterone is the adrenal cortex hormone responsible for regulating the sodium ion content of the extracellular fluid (ECF). It enhances sodium reabsorption and potassium secretion and is stimulated by changes in blood sodium, volume, pressure, or potassium levels. The adrenal cortex hormone responsible for regulating the sodium ion content of the extracellular fluid (ECF) is called aldosterone. It is produced in the adrenal cortex and is referred to as a mineralocorticoid. Aldosterone enhances the reabsorption of sodium ions and secretion of potassium ions and is stimulated by a decrease in blood sodium levels, blood volume, and blood pressure, or an increase in blood potassium levels.