The urinary system is also called the excretory system because one of its main functions is excretion, removal and elimination of metabolic waste products from the blood. It has many other functions as well, including regulation of the volume, acid-base balance (pH), and electrolyte composition of body fluids. Although the focus of this page is the urinary system, certain aspects of other systems are also discussed, because body systems work interdependently to maintain homeostasis (internal balance). The systems active in excretion and some of the substances they eliminate are the following:
* The urinary system excretes water, nitrogen-containing waste products, and salts. These are all constituents of the urine.
* The digestive system eliminates water, some salts, and bile in addition to digestive residue, all of which are contained in the feces. The liver is important in elimieliminating the products of red blood cell destruction and in breaking down certain drugs and toxins.
Figure 18-1 Male urinary system, showing blood vessels.
* The respiratory system eliminates carbon dioxide and water. The latter appears as vapor, as can be demonstrated by breathing on a windowpane.
* The skin, or integumentary system, excretes water, salts, and very small quantities of nitrogenous wastes. These all appear in perspiration, although water also evaporates continuously from the skin without our being conscious of it.

Organs of the Urinary System

The main parts of the urinary system, shown in Figure 18-1, are as follows:
Male urinary system, showing blood vessels
* Two kidneys. These organs extract wastes from the blood, balance body fluids, and form urine.
* Two ureters. These tubes conduct urine from the kidneys to the urinary bladder.
* A single urinary bladder. This reservoir receives and stores the urine brought to it by the two ureters.
* A single urethra. This tube conducts urine from the bladder to the outside of the body for elimination.
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