The primary function of the kidneys in humans is to filter and remove waste products from the blood, as well as maintain the body's fluid and electrolyte balance. Each kidney contains millions of tiny filtering units called nephrons, which play a critical role in this process. They filter blood, reabsorbing essential substances like glucose and electrolytes while excreting waste products and excess substances as urine. In addition to waste removal, the kidneys help regulate blood pressure, produce hormones such as erythropoietin (which stimulates red blood cell production), and maintain acid-base balance, contributing significantly to overall homeostasis in the body.