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Chapter 30
Acid-Base Balance
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 1
Introduction

Acid-base balance is one of the most important
of the body’s homeostatic mechanisms
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Acid-base balance refers to regulation of
hydrogen ion concentration in body fluids

Precise regulation of pH at the cellular level is
necessary for survival
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Slight pH changes have dramatic effects on
cellular metabolism
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 2
Mechanisms That
Control pH of Body Fluids
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Meaning of pH—negative logarithm of hydrogen ion
concentration of solution (Figure 30-1)

Sources of pH-influencing elements
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Carbonic acid—formed by aerobic glucose metabolism
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Lactic acid—formed by anaerobic glucose metabolism
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Sulfuric acid—formed by oxidation of sulfur-containing amino acids
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Phosphoric acid—formed in breakdown of phosphoproteins and
ribonucleotides
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Acidic ketone bodies—formed in breakdown of fats
• Acetone
• Acetoacetic acid
• Beta-hydroxybutyric acid
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 3
Mechanisms That
Control pH of Body Fluids


Acid-forming potential of foods—determined by chloride, sulfur,
and phosphorus content
Types of pH control mechanisms
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Chemical—rapid action buffers
• Bicarbonate buffer system
• Phosphate buffer system
• Protein buffer system
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Physiological—delayed action buffers
• Respiratory response
• Renal response
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Summary of pH control mechanisms
• Buffers
• Respiration
• Kidney excretion of acids and bases

Effectiveness of pH control mechanisms; range of pH—
extremely effective, normally maintain pH within very narrow
range of 7.36 to 7.40
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 4
Mechanisms That
Control pH of Body Fluids


Buffers defined
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Substances that prevent marked change in pH of
solution when an acid or base is added to it

Consist of weak acid (or its acid salt) and basic salt
of that acid
Buffer pairs present in body fluids—mainly
carbonic acid, proteins, hemoglobin, acid
phosphate, and sodium and potassium salts of
these weak acids
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 5
Buffer Mechanisms for
Controlling pH of Body Fluids

Action of buffers to prevent marked changes in pH of body fluids
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Nonvolatile acids, such as hydrochloric acid, lactic acid, and ketone bodies,
buffered mainly by sodium bicarbonate
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Volatile acids, chiefly carbonic acid, buffered mainly by potassium salts
of hemoglobin and oxyhemoglobin (Figure 30-5)

Chloride shift makes it possible for carbonic acid to be buffered in red
blood cell and then carried as bicarbonate in plasma (Figure 30-6)

Bases buffered mainly by carbonic acid (when homeostasis of pH
at 7.4 exists)
Ratio B • HCO3/H2CO3=20/1

Evaluation of role of buffers in pH control—cannot maintain
normal pH without adequate functioning of respiratory and
urinary pH control mechanisms
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 6
Respiratory Mechanism of pH Control

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Explanation of mechanism
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Amount of blood carbon dioxide directly relates to amount
of carbonic acid and therefore to concentration of H+
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With increased respirations, less carbon dioxide remains in
blood, hence less carbonic acid and fewer H+; with
decreased respirations, more carbon dioxide remains in
blood, hence more carbonic acid and more H+
Adjustment of respirations to pH of arterial blood
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 7
Respiratory Mechanism of pH Control

Some principles relating respirations and pH of body fluids
Acidosis → hyperventilation
↓
increases elimination of CO2
↓
decreases blood CO2
↓
decreases blood H2CO3
↓
decreases blood H+; that is, increases blood pH
↓
tends to correct acidosis; that is, to restore normal pH
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 8
Respiratory Mechanism of pH Control

Some principles relating respirations and pH
of body fluids (cont.)
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Prolonged hyperventilation, by decreasing blood H+
excessively, may produce alkalosis
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Alkalosis causes hypoventilation, which tends to
correct alkalosis by increasing blood CO2 and therefore
blood H2CO3 and H+
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Prolonged hypoventilation, by eliminating too little CO2,
causes increase in blood H2CO3 and consequently in
blood H+, thereby may produce acidosis
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 9
Urinary Mechanisms of pH Control

General principles about mechanism—plays vital
role in acid-base balance because kidneys can
eliminate more H+ from body while reabsorbing
more base when pH tends toward acid side, and
eliminates fewer H+ while reabsorbing less base
when pH tends toward alkaline side
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 10
Urinary Mechanisms of pH Control
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Mechanisms that control urine pH
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Secretion of H+ into urine—when blood CO2, H2CO3, and
H+ increase above normal, distal tubules secrete more
H+ into urine to displace basic ion (mainly sodium) from
a urine salt and then reabsorb sodium into blood in
exchange for the H+ excreted
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Secretion of NH3—when blood hydrogen ion
concentration increases, distal tubules secrete more
NH3, which combines with H+ of urine to form
ammonium ion, which displaces basic ion (mainly
sodium) from a salt; basic ion then reabsorbed back
into blood in exchange for ammonium ion excreted
Mosby items and derived items © 2007, 2003 by Mosby, Inc.
Slide 11