The primary function of the kidneys in the human body is to filter blood, removing waste products and excess substances, and excreting them as urine. Kidneys maintain the body's fluid balance, regulating electrolytes, blood pressure, and acid-base balance. They also produce hormones such as erythropoietin, which stimulates red blood cell production, and renin, which helps regulate blood pressure. As crucial organs for homeostasis, kidneys ensure that essential substances like glucose and certain ions are reabsorbed into the bloodstream while disposing of toxic byproducts generated from metabolism, thus safeguarding overall health.