Renal System, Human

renal system, in humans, organ system that includes the kidneys, where urine is produced, and the ureters, bladder, and urethra for the passage, storage, and voiding of urine.

In many respects the human excretory, or urinary, system resembles those of other mammalian species, but it has its own unique structural and functional characteristics. The terms excretory and urinary emphasize the elimination function of the system. The kidneys, however, both secrete and actively retain within the body certain substances that are as critical to survival as those that are eliminated.

The system contains two kidneys, which control the electrolyte composition of the blood and eliminate dissolved waste products and excess amounts of other substances from the blood; the latter substances are excreted in the urine, which passes from the kidneys to the bladder by way of two thin muscular tubes called the ureters. The bladder is a sac that holds the urine until it is eliminated through the urethra.