What is the primary function of the kidneys?

The primary function of the kidneys is to filter waste products from the blood and regulate fluid balance. This function is crucial for maintaining homeostasis in the body. The kidneys accomplish this by functioning as a key component of the urinary system; they filter out toxins, excess salts, and urea from the bloodstream while reabsorbing essential substances like water and electrolytes back into the body.

Additionally, the regulation of fluid balance by the kidneys helps control blood pressure and overall fluid levels in the body, which is vital for maintaining healthy organ function and overall physiological stability. This filtration process occurs in functional units called nephrons, where blood is filtered, and urine is produced, ultimately excreting waste while preserving necessary components.

While the other options reference important functions that the kidneys are involved in—such as hormone production, vitamin D synthesis, and management of blood pH levels—they do not capture the kidneys' primary role, which is centered around waste removal and fluid homeostasis.