HUMAN BIOLOGY

the UrInary SyStem 225

What is the main function of the kidneys?

• The kidneys adjust the volume and chemical composition of
  the blood. In this way they help maintain homeostasis in the
  extracellular fluid.


• The body gains water consumed in liquids and solid foods
  and from metabolism. It loses water through urinary excretion,
  evaporation, sweating, and elimination in feces.


• The body gains solutes from digested food, respiration,
  secretion by cells, and metabolism. Excess or harmful solutes
  are removed by urinary excretion, respiration, and sweating.

taKe-Home message

electrolytes Ions that carry

an electric current when dis-

solved in fluid.

urea The main waste prod-

uct from protein breakdown.

urinary excretion Removal

of excess or unwanted

water and solutes from the

body, in urine.

urine Fluid produced and

eliminated from the body

during the process of uri-

nary excretion.

solution in which they are dissolved

will carry an electric current. Chap-

ter 13 describes the crucial roles elec-

trolytes have in the nervous system.

Normally only a little of the

water and solutes that enter the

kidneys leaves as urine. In fact,

except when you drink lots of fluid

(without exercise), all but about

1 percent of the water is returned

to the blood. However, the chemi-

cal composition of the fluid that is

returned has been adjusted in vital

ways. Just how this happens will be

our focus in the next few sections.

or harmful solutes, in the form of urine. Some water also
evaporates from our skin and from the respiratory surfaces
of the lungs. These are sometimes called “insensible” water
losses, because a person is not always aware they are tak-
ing place. As noted in Chapter 11, normally very little water
that enters the GI tract is lost; most is absorbed and only a
little is eliminated in feces.

solutes enter extracellular fluid from food,
respiration, and metabolism

Three main sources add solutes to the body’s extracellu-
lar fluid. Food supplies nutrients (including glucose) and
mineral ions (such as potassium and sodium ions) that
are absorbed from the GI tract. Many of us also consume
many drugs and food additives. The respiratory system
brings oxygen into the blood. Last but not least, living cells
continually secrete substances, including carbon dioxide,
into tissue fluid and circulating blood. Figure 12.2 gives a
snapshot of major interactions between the urinary system
and other organ systems.

solutes leave the eCF by urinary excretion,
in sweat, and during breathing

Metabolic wastes, mineral ions, and other solutes leave
extracellular fluid in several ways. Metabolism produces
more than 200 waste substances. Carbon dioxide is the
most abundant one, and we get rid of it by exhaling it from
our lungs. All other major wastes leave in urine.
Important metabolic wastes include by-products of
processes that break down nucleic acids and proteins.
Dismantling nucleic acids produces one of these wastes,
uric acid. Another one, ammonia, forms
in “deamination” reactions, which remove
the nitrogen- containing amino groups from
amino acids. Ammonia is highly toxic if it
accumulates in the body. Reactions in the
liver combine ammonia with carbon diox-
ide, producing the much less toxic urea.
Accordingly, urea is the main waste product
when cells break down proteins. About half of the urea
filtered from blood in the kidneys is reabsorbed. The rest
is excreted. Protein breakdown also produces creatine,
phosphoric acid, sulfu ric acid, and small amounts of other
nitrogen-containing compounds, some of which are toxic.
These also are excreted.
Sweat carries away a small percentage of urea, but most
nitrogen-containing wastes are removed by the kidneys
while they filter other wastes and excess water from the
blood. The kidneys also help maintain the balance of
important ions such as sodium, potassium, and calcium.
These ions are sometimes called electrolytes because a

Figure 12.2 The activities of the urinary system coordinate

with those of other organ systems. (© Cengage Learning)

food, water intake

elimination

of carbon

dioxide

elimination of

excess water,

salts, wastes

water,

solutes

carbon

dioxide

oxygen

nutrients,

water,

salts

elimination

of food

residues

rapid transport

to and from all

living cells

oxygen intake

Urinary

System

Circulatory

System

Respiratory

System

Digestive

System

Lawrence Lawry/

Science Source

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