What is the primary function of the kidneys?

The primary function of the kidneys is to filter blood and form urine, which is essential for maintaining the body's homeostasis. The kidneys play a critical role in regulating fluid balance, electrolytes, and the overall composition of blood. They remove waste products and excess substances, such as urea and creatinine, from the bloodstream, which are then excreted as urine.

Additionally, the kidneys help control blood pressure by regulating the volume of blood (through fluid balance) and by producing the enzyme renin, which is involved in blood pressure regulation. The process of filtering blood occurs in microscopic structures called nephrons, where blood is filtered, and necessary substances are reabsorbed while wastes are excreted. This vital function ensures that harmful substances do not accumulate in the body, thereby contributing to overall health.

While the kidneys also contribute to hormone production (such as erythropoietin for red blood cell production and calcitriol for calcium metabolism), the overarching function that encompasses their role in waste removal and fluid balance is filtration and urine formation. Thus, that aspect is considered their primary function.