The excretory system, or urinary system, is composed of multiple organs.  The main organ of the urinary system is the kidney.  The kidney performs dual tasks of filtering out wastes and purifying blood,   The main function of the kidneys is to filter out wastes.  Other organs in the urinary system include;  the urinary bladder, the ureters, and the urethra.  The ureters are two tubes that transport urine from the kidneys to the urinary bladder.  The urinary bladder is a large hollow muxcle sac that holds urine.  The urethra is a tube that transports urine from the bladder to an opening outside the body.