What is the primary effect of aldosterone on the kidneys?

Aldosterone primarily functions to regulate electrolyte balance in the body, particularly through its action on the kidneys. It promotes the reabsorption of sodium from the renal tubules into the bloodstream, which is essential for maintaining blood volume and blood pressure. As sodium is reabsorbed, water follows osmotically, which can also lead to an increase in blood volume. This effect is particularly significant in the distal convoluted tubule and the collecting duct of the nephron, where aldosterone increases the expression of sodium channels and sodium-potassium pumps.

In contrast, while calcium reabsorption, potassium excretion, and water retention are influenced by various hormones and mechanisms, they do not reflect the primary effect of aldosterone. Calcium reabsorption is mainly regulated by parathyroid hormone and calcitonin, potassium excretion increases as sodium is reabsorbed, and aldosterone does not reduce water retention; instead, it often increases water retention indirectly by promoting sodium reabsorption. Therefore, the key role of aldosterone in promoting sodium reabsorption is what makes this answer correct.