What is the primary function of the kidneys in the human body?

The primary function of the kidneys in the human body is to excrete waste and balance fluids. The kidneys play a crucial role in the urinary system by filtering blood to remove waste products, such as urea and creatinine, which are byproducts of metabolism. This filtration process not only eliminates these wastes but also regulates the volume and composition of bodily fluids, maintaining homeostasis.

Additionally, the kidneys help regulate electrolyte levels, such as sodium, potassium, and calcium, which are vital for normal cell function. Through their ability to adjust the concentration of urine, they also contribute to the body’s fluid balance, effectively responding to changes in hydration status. By excreting excess water and solutes, the kidneys ensure that the body does not become overloaded with fluids, thereby preventing conditions like edema or dehydration.

While the kidneys are involved in hormone regulation and participate in the synthesis of erythropoietin to stimulate red blood cell production, their central function remains the filtration and excretion process that maintains overall fluid and waste balance. This makes option C the most accurate representation of the kidneys' primary role in the body.