The hormone that helps regulate blood sodium concentration, by stimulating \( \mathrm{Na}^{+} \)conservation in the kidneys is: Aldosterone Antidiuretic hormone Cortisol

The hormone that regulates blood sodium concentration by stimulating \( \mathrm{Na}^{+} \) conservation in the kidneys is **Aldosterone**. **Explanation:** - **Aldosterone** is a mineralocorticoid hormone produced by the adrenal cortex. It plays a crucial role in regulating sodium and potassium levels in the blood. Aldosterone promotes the reabsorption of sodium (\( \mathrm{Na}^{+} \)) and the excretion of potassium (\( \mathrm{K}^{+} \)) in the distal tubules and collecting ducts of the nephron in the kidneys. This action helps maintain blood sodium balance and, consequently, blood pressure. - **Antidiuretic Hormone (ADH)**, also known as vasopressin, primarily regulates water balance in the body by increasing water reabsorption in the kidneys' collecting ducts. While it indirectly affects sodium concentration by influencing water retention, its primary role is not the direct regulation of sodium ions. - **Cortisol** is a glucocorticoid hormone that has multiple functions, including glucose metabolism, immune response modulation, and stress response. While high levels of cortisol can influence electrolyte balance, it is not the primary hormone responsible for sodium conservation in the kidneys. **Correct Answer:** **Aldosterone**