Here's why:

* Filtering and Reabsorption: The kidneys filter blood and remove waste products, including excess sodium. They also have the crucial ability to reabsorb sodium back into the bloodstream as needed to maintain proper electrolyte balance.

* Hormonal Control: The kidneys are influenced by hormones like aldosterone (which promotes sodium reabsorption) and atrial natriuretic peptide (ANP, which promotes sodium excretion). These hormones help fine-tune sodium levels.

* Blood Pressure Regulation: Sodium plays a significant role in blood pressure regulation. The kidneys help maintain proper sodium levels to ensure blood pressure stays within a healthy range.

While other organs and systems contribute to sodium regulation, the kidneys are the primary and most important regulators.