Finally, when you ingest a food that is high in salt con-
tent, such as theater popcorn or several strips of bacon,
the renin-angiotensin-aldosterone system is suppressed,
and the release of ANP is increased to remove excess so-
dium chloride and water.
up into smaller pieces by using sound waves, ultrasound, or
lasers; or letting them pass naturally through the tubing.
Stone formation can often be prevented by consuming ad-
equate amounts of fluids and reducing the intake of certain
foods that promote release of excess amounts of mineral
substances into the urine.
How Do Kidney Stones Form?
Although regulation of blood water levels is critical for
survival, when very concentrated urine is produced, min-
erals such as calcium, phosphorus, and oxalates may begin
to crystallize out of solution. This can lead to formation
of urinary calculi (or
stones;
singular is calculus). Alkaline
urine pH and urinary tract infections also increase the
chance of calculus formation. These stones can form in ei-
ther the kidneys or the urinary bladder.
Treatments for urinary stones can involve dissolving
them by using urinary acidifier medications; breaking them
CONCEPT CHECK
1.
Which
electrolytes are the most important for
driving the movements of water?
2.
By
what
routes does your body lose water?
3.
What
does aldosterone do to regulate blood
pressure in the body?
4.
How
does the kidney regulate Ca2+ levels in the
blood?
The Kidneys Help Maintain the
Acid-Base Balance of Body Fluids
LEARNING OBJECTIVES
1.
List
the various pH buffer systems in the body.
2.
Describe
how the lungs maintain normal blood pH.
3.
Explain
how the kidneys maintain normal blood
pH.
4.
Identify
the physiologic changes and compen-
sations in acidosis and alkalosis.
our body continuously produces acid in the
form of hydrogen ions (H+) as a result of me-
tabolism. Your diet also contributes to H+
production; diets rich in meat lead to more
acid production than those rich in fruits and vegetables.
Yet the H+ concentration (pH) of the blood must be main-
tained within a narrow range, between 7.35 and 7.45, for
the cells of the body to function properly. The H+ produced
when you eat a bucket of chicken is released into the blood,
and the body needs a way to neutralize it or remove it from
the body to maintain a normal blood pH. Your body has
three major mechanisms that help to keep the blood pH
within the normal range. We will discuss each one in turn.
Buffer systems in the blood and intracellular compart-
ments.
Exhalation of carbon dioxide.
Kidney excretion.
As you will see, kidney excretion is the slowest mechanism,
but the only one that eliminates acids.
Buffers Help to Maintain pH Levels
Within a Narrow Range
Recall from Chapter 2 that a buffer can rapidly absorb or
release H+ in response to changes in the H+ levels in order
to maintain normal or nearly normal pH. Only the H+ that
is “free” in solution can contribute to pH, so binding H+
raises the pH, and releasing H+ helps lower the pH. The
body’s major pH buffer systems consist of protein, carbonic
acid-bicarbonate, and phosphate buffers (Figure 15.14).
All three buffer systems work together, as a change in H+
concentration in one fluid compartment causes a change
in other compartments.
The protein buffer system is the most abundant buf-
fer in intracellular fluid and blood plasma. Proteins such
458 CHAPTER 15
The Urinary System and Fluid, Electrolyte, and Acid-Base Balance
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