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, excess substances, and toxins from the bloodstream, which helps maintain the body's internal balance, or homeostasis. The kidneys regulate the volume and composition of bodily fluids, manage electrolytes, control blood pressure through the regulation of fluid levels, and maintain acid-base balance. The urine produced then serves as a way for the body to excrete these waste materials.

While the kidneys do produce certain hormones, such as erythropoietin and renin, this is not their primary function but rather an additional role they play. Likewise, gas exchange occurs primarily in the lungs, and the storage of nutrients occurs mainly in organs like the liver and muscles. Therefore, the most accurate and encompassing answer regarding the primary role of the kidneys focuses on their essential function in filtration and urine production.