Which hormone is primarily responsible for the regulation of blood sodium levels?

Aldosterone is primarily responsible for the regulation of blood sodium levels. It is a steroid hormone produced by the adrenal cortex and plays a crucial role in maintaining electrolyte balance. Aldosterone acts on the kidneys, specifically targeting the distal convoluted tubules and collecting ducts, where it promotes the reabsorption of sodium ions back into the bloodstream while facilitating the excretion of potassium ions. This reabsorption of sodium is vital for regulating blood volume and pressure, as sodium retention leads to increased water retention through osmotic processes.

While cortisol, another hormone produced by the adrenal cortex, has some influence on sodium balance, its primary role is related to metabolic stress responses and the regulation of glucose metabolism. Adrenaline, also known as epinephrine, is more associated with the 'fight or flight' response and does not directly regulate blood sodium levels. Insulin primarily regulates glucose levels in the blood and has little to no direct effect on sodium. Thus, aldosterone stands out as the key hormone that directly controls sodium homeostasis in the body.