The Excretory System
The Kidneys
The kidneys make up a filter system for the blood, reabsorbing almost 99%% of the fluid into the blood, and sending only two to four pints of waste (urine) into the bladder for storage until it can be disposed of. The kidneys allow the blood to keep glucose, salts and minerals after cleansing it of poisonous materials which will be passed out in the urinary tract.
The Urinary Tract
Urine is produced in the kidneys and trickles down twenty-four hours a day through two ten to twelve inch long tubes called ureters, which connect the kidneys to the bladder. The ureters are about one-fourth inch in diameter and their muscular walls contract to make waves of movement to force the urine into the bladder. The bladder is expandable and stores the urine until it can be conveniently disposed of. It also closes openings into the ureters so that urine cannot flow back into the kidneys. The tube through which the urine flows out of the body is called the urethra. Did you know that less than half of one single kidney can do all the work that two kidneys usually do? The structure of the urinary tract includes: the kidneys, two ureters, (tubes leading from the kidneys to the bladder,) and the urethra, a tube leading from the bladder to the exterior of the body. The urinary tract is a little like a plumbing system, with special pipes that allow water and salts to flow through them.
The Bowel
Substances which have not been absorbed in the small intestine enter the large intestine in the form of liquid and fiber. The large intestine or "bowel" is sometimes called the "garbage dump" of the body, because the materials that reach it are of very small use to the body and are sent on to be disposed of. The first half of the colon absorbs fluids and recycles them into the blood stream. The second half compacts the wastes into feces, secretes mucus which binds the substances, and lubricates it to protect the colon and ease its passage. Of the two to two and one-half gallons of food and liquids taken in by the average adult, only about twelve ounces of waste enters the large intestine. Feces are comprised of about three quarters water. The remainder is protein, fat, undigested food roughage, dried digestive juices, cells shed by the intestine, and dead bacteria.