major excretory organ, maintain the body's internal environment

regulating total water volume and total solute concentration in water, regulating ion concentrations in ECF, ensuring long-term acid base balance, excreting metabolic wastes, toxins, drugs, producing erythropoietin and renin (regulate RBC production and BP respectively), activating vitamin D, and carrying out gluconeogenesis if needed

kidneys, ureters, urinary bladder, urethra