The Urinary System Plays a Vital
Role in Maintaining Homeostasis
the components of the urinary system
and their functions.
the macroscopic and microscopic struc-
tures of the kidneys.
aintaining homeostasis—keeping a stable in-
ternal environment—is critical for survival,
and the urinary system plays a vital role in
this process. As body cells carry out their met-
abolic functions, they consume oxygen and nutrients and
produce substances, such as carbon dioxide, that have no
useful functions and need to be eliminated from the body.
As we saw in the chapter opener, these substances or
metabolites can change the composition and pH of your
blood, not only during exercise but in the course of your
normal day-to-day life. While the respiratory system rids
the body of carbon dioxide, the urinary system disposes of
most other unneeded substances, including the nitrogen-
containing wastes, toxins, and drugs. As you will see later
in this chapter, the urinary system also helps maintain
fluid and electrolyte balance.
The urinary system consists of the kidneys, ureters,
urinary bladder, and urethra (Figure 15.1). The ure-
ters connect the kidneys to the urinary bladder, and the
urethra connects the urinary bladder to the outside. To-
gether these organs make up an elaborate system that
carry out the following functions:
Filtration of blood plasma.
Reabsorption of essential substances.
Secretion of nonessential substances.
The kidneys do the major work of the urinary system,
filtering gallons of fluid from the bloodstream every day.
The other parts of the system are primarily passageways
and temporary storage areas. After the kidneys filter
blood, they return most of the water and many of the sol-
utes to the bloodstream. The remaining water and solutes
make up urine, which passes from the kidneys through
the ureters to the urinary bladder. Urine is stored in the
the processes involved in urine
the role of each portion of the nephron
in urine formation.
urinary bladder until it is expelled from the body through
the urethra.
Let’s begin our journey through the urinary system
by examining the kidneys. Along the way, we will see how
your kidneys help you maintain homeostasis, even during
strenuous exercise.
^ I L E Y O
The Kidneys Do the .PLUS
Major Work of the Urinary System
The kidneys are a pair of reddish organs shaped like kid-
ney beans. They lie on either side of the vertebral column
between the peritoneum and the back wall of the abdomi-
nal cavity, at the level of the 12th thoracic and first three
lumbar vertebrae. The 11th and 12th pairs of ribs provide
some protection for the superior parts of the kidneys. The
right kidney is slightly lower than the left because the liver
occupies a large area above the kidney on the right side.
An adult kidney is about the size of a bar of bath soap.
Surrounding each kidney is the smooth, transparent re-
nal capsule, a connective tissue sheath that
helps main-
tain the shape of the kidney and acts as a barrier against
trauma. Adipose (fatty) tissue surrounds the renal capsule
and cushions the kidney. Along with a thin layer of dense
irregular connective tissue, the adipose tissue anchors the
kidney to the posterior abdominal wall.
The major processes of filtration, reabsorption, and
secretion occur in the kidneys. Specifically, the functions
of the kidneys include the following.
Regulation of blood volume and blood pressure.
The kidneys adjust the volume of water in the blood
by returning water to the blood or eliminating it in the
urine. They help regulate blood pressure by secreting
the enzyme renin.
438 CHAPTER 15
The Urinary System and Fluid, Electrolyte, and Acid-Base Balance
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