What is the main function of the kidneys?

The primary role of the kidneys encompasses several critical functions that are essential for maintaining homeostasis in the body. The kidneys filter blood to remove waste products, which is vital for preventing the accumulation of toxins and waste substances that could be detrimental to health. This waste removal process is accomplished through nephron function and leads to the formation of urine, which is expelled from the body.

Additionally, the kidneys play a crucial role in regulating fluid balance. They adjust the volume and concentration of urine based on the body's hydration levels, ensuring that the body maintains the right amount of water. This regulation is fundamental, as it affects blood pressure and overall fluid equilibrium.

The kidneys also maintain electrolyte levels—important minerals such as sodium, potassium, and calcium are carefully balanced to support various physiological functions, including nerve transmission and muscle contraction. Moreover, the kidneys produce hormones like erythropoietin, which stimulates red blood cell production, and renin, which controls blood pressure.

The other options provided either describe functions not related to the kidneys or focus on processes associated with distinct organs. The kidneys are not primarily responsible for nutrient or energy storage, nor do they produce bile for digestion—that function is specific to the liver. Consequently, the comprehensive functions of waste removal, fluid and electrolyte