278 CHAPTER 7• Urinary System
ANATOMY AND PHYSIOLOGY OVERVIEW
The urinary system is composed of the kidneys, ureters, bladder, and urethra. Its purpose is to regulate the volume
and composition of fluids in the body and remove waste substances and excess fluid from the blood. Waste substances
are filtered from the blood by the kidneys and excreted in the urine, which exits via the ureters into the urinary bladder.
Urine is stored in the bladder until the urge to urinate occurs, at which point the muscles at the bladder outlet relax,
allowing the urine to be expelled through the urethra. Review Figure 7–1 to identify the location of urinary structures
within the body.
The kidneys perform the major work of the urinary system. The other parts of the system are mainly passageways
and storage areas. Functions of the kidneys include regulating the amount of water in the body and keeping body
fluids at a constant concentration and acid-base level. They achieve these functions by filtering blood and excreting
waste substances and excess water as urine. Other essential substances are reabsorbed into the bloodstream by the
process called reabsorption.
The filtering–reabsorption process is necessary to maintain the balance of substances required for a relatively stable
internal body environment. This stable internal environment, known as homeostasis,is necessary for the cells of the
body to survive and carry out their functions effectively. If the kidneys fail, waste substances cannot be eliminated
from the body. Thus, the substances accumulate in the blood to toxic levels and the cells can no longer function. Death
ultimately results unless impurities are filtered out of the blood by means of an artificial kidney known as kidney dial-
ysisor the nonfunctioning kidneys are replaced with a healthy kidney through kidney transplantation.