What is the primary function of the kidneys?

The primary function of the kidneys is to filter waste from the blood and produce urine. This critical process helps maintain homeostasis in the body by removing excess substances, including toxins and metabolic byproducts, while also regulating the balance of water and electrolytes. As blood passes through the kidneys, they filter out waste materials and excess ions, which are then excreted as urine. This function is essential for keeping the body's internal environment stable, preventing the accumulation of harmful substances that can affect bodily functions.

Other options, though they describe functions relevant to overall health, do not accurately represent the kidneys' primary role. For instance, while the kidneys do produce certain hormones, such as erythropoietin and renin, hormone production is secondary to their main function of filtration. Absorbing nutrients is primarily the role of the intestines, not the kidneys, which focus on waste elimination. Similarly, regulating heart rate is a function of the cardiovascular system and specific neural mechanisms, again illustrating a different biological focus than that of the kidneys.