Final answer:
The hormone Aldosterone, secreted by the adrenal cortex, primarily promotes sodium reabsorption in the kidneys, which helps to regulate electrolyte balance and maintain proper blood pressure and fluid volume.
Step-by-step explanation:
The correct option is B:
Aldosterone is a hormone crucial in regulating the balance of electrolytes, specifically sodium and potassium, within the body. It is secreted by the adrenal cortex in response to various factors including low blood sodium levels, high blood potassium levels, low blood volume, and low blood pressure. The correct answer to the student's question is B. promotes sodium reabsorption in the kidneys.
Aldosterone facilitates the reabsorption of sodium ions from the kidney filtrate back into the bloodstream. This process aids in the retention of water in the body due to osmotic forces, which helps to increase blood volume and consequently elevate blood pressure. The hormone does this by acting on the distal tubules and collecting ducts of the nephron, where it prompts the cells to insert sodium channels and pumps, allowing more sodium to be reabsorbed. Concomitantly, aldosterone causes an increase in the excretion of potassium by promoting its secretion from these same renal cells. This dual role helps maintain crucial balances of these electrolytes and, by extension, contributes to the regulation of blood pressure and overall fluid balance.