What regulates the main extracellular ion, sodium (Na+)?

Sodium (Na+) is primarily regulated by the kidneys through a process known as renal sodium handling. The kidneys play a pivotal role in maintaining sodium balance, which is crucial for fluid balance, blood pressure regulation, and overall homeostasis. They filter sodium from the blood and can reabsorb it back into circulation or excrete it in urine depending on the body's needs.

In cases of low blood sodium levels, the kidneys will conserve sodium by reabsorbing it from the renal tubules, often influenced by hormones such as aldosterone. Conversely, when sodium levels are adequate or elevated, the kidneys facilitate the excretion of excess sodium, promoting balance within the extracellular fluid.

The liver, lungs, and gastrointestinal tract also play roles in various aspects of fluid and electrolyte balance, but they do not have the direct regulatory function over sodium levels in the manner that the kidneys do. The liver is involved in metabolic processes and detoxification, the lungs facilitate gas exchange and play a minor role in fluid regulation via carbon dioxide management, and the gastrointestinal tract is primarily responsible for nutrient absorption and hydration status rather than direct sodium regulation. Thus, the kidneys are the key organ responsible for maintaining sodium homeostasis in the body.