Final answer:
The kidneys monitor the Na+ concentration in the extracellular fluid (ECF) using osmoreceptors in the brain. ADH is released to regulate Na+ concentration by controlling water reabsorption in the kidneys.
Step-by-step explanation:
The kidneys monitor the Na+ concentration in the extracellular fluid (ECF). Osmoreceptors, specialized cells in the brain, detect changes in the concentration of solutes in the blood. If the levels of Na+ increase beyond a certain range, a hormone called antidiuretic hormone (ADH) is released to regulate Na+ concentration indirectly by controlling the reabsorption of water in the kidneys.