What is the main function of the kidneys?

The main function of the kidneys encompasses several critical roles in maintaining homeostasis within the body, with the elimination of water and excess waste products being a fundamental aspect of their operation. The kidneys filter waste and excess substances from the blood, which are then excreted as urine. This process not only helps to eliminate toxins but also plays a vital role in regulating the body's fluid balance. By adjusting the amount of water and electrolytes excreted, the kidneys help to maintain the appropriate volume and composition of body fluids.

While the kidneys do contribute to blood pressure regulation and hormone production, these functions are secondary to their primary role in waste elimination. The kidneys produce hormones such as erythropoietin, which stimulates red blood cell production, and renin, which plays a crucial part in blood pressure regulation, but these activities are ancillary to their main job of filtering blood and managing waste. Thus, the statement regarding the elimination of water and excess waste products accurately reflects a core function of the kidneys, making it the most comprehensive answer.