What is the primary function of the kidneys?

The primary function of the kidneys is to filter blood and produce urine. This vital role involves removing waste products and excess substances from the bloodstream, which are then excreted as urine. The kidneys constantly regulate the composition of the blood by filtering it multiple times a day—each kidney containing approximately one million microscopic units called nephrons that perform this filtration and excretion. Through this process, the kidneys help maintain the body’s homeostasis, ensuring that waste products like urea and creatinine, as well as other substances, are efficiently eliminated.

While the kidneys also play significant roles in regulating blood pressure, producing hormones, and maintaining electrolyte balance, these functions are secondary to their primary role of filtering blood. The regulation of blood pressure, for instance, involves the release of renin, a hormone that affects blood volume and vascular resistance, while hormone production includes erythropoietin and calcitriol, which influence red blood cell production and calcium metabolism. Thus, while these functions are essential, they stem from the kidneys' primary role in filtering blood and producing urine.