The primary purpose of the kidneys is to filter waste products and excess substances from the blood, maintaining the body's fluid and electrolyte balance. Each kidney contains millions of nephrons, which are the functional units responsible for filtering blood and producing urine. Through this filtration process, the kidneys remove toxins, urea, and surplus salts while conserving essential nutrients and water. Additionally, kidneys play a vital role in regulating blood pressure, producing hormones that influence red blood cell production, and helping maintain pH balance. Thus, they are crucial for overall metabolic health and homeostasis.