Two smaller tubes about the size of a crow quill, the waste pipes of the kidneys (the _ureters_), carry the water containing urea and other waste substances strained out by the kidneys and called urine, down into a large pouch, the _bladder,_ to be stored there until it can be got rid of.
[Illustration: THE URINARY SYSTEM _K_, kidneys; _U_, ureters; _B_, bladder; _A_, artery; _V_, vein.] The kidneys then are big filter-glands.

They, like the lungs, are made up of a mesh, or network, of thousands of tiny tubes of two kinds, one set of tubes being blood vessels, and the other set the tiny branches of the kidney tubes which finally run together to form the ureters.

The urine filters through from the spongy mesh of blood tubes (capillaries) into the kidney tubes and is poured out through the ureters.

It is very important that the urine should be discharged as fast as it fills the bladder, that is, about once every three hours during the day.

Nothing should be allowed to interfere with this; and whenever nature tells you that the bladder is full, it should be emptied promptly, or the poisons which nature is trying to get rid of in the urine may get back into the blood and cause serious trouble.
Diseases of the Kidneys.