What is the primary function of the kidneys?

The primary function of the kidneys is to filter waste products from the blood and maintain fluid and electrolyte balance. The kidneys play a crucial role in homeostasis by removing metabolic waste, such as urea, creatinine, and excess salts, which are produced by the body's normal metabolic processes. This filtering process occurs in the nephrons, the functional units of the kidneys, where blood is filtered, and waste products are excreted as urine.

In addition to waste filtration, the kidneys help regulate the body's fluid balance by adjusting the amount of water excreted in urine based on the body's hydration status. They also maintain electrolyte balance by controlling the levels of sodium, potassium, calcium, and phosphate, which are vital for many physiological processes including nerve function and muscle contraction.

While hormone production is a function of the kidneys—such as the production of erythropoietin for red blood cell formation—this is a secondary function compared to their role in filtration and balance. The kidneys do not play a role in digestion, nor do they regulate body temperature directly, making the primary function of waste filtration and fluid balance critical to overall health.