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Chapter 30Chapter 30Acid-Base BalanceAcid-Base Balance

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Introduction Introduction

Acid-base balance is one of the most important Acid-base balance is one of the most important of the body’s homeostatic mechanismsof the body’s homeostatic mechanisms

Acid-base balance refers to regulation of Acid-base balance refers to regulation of hydrogen ion concentration in body fluidshydrogen ion concentration in body fluids

Precise regulation of pH at the cellular level is Precise regulation of pH at the cellular level is necessary for survivalnecessary for survival

Slight pH changes have dramatic effects on Slight pH changes have dramatic effects on cellular metabolismcellular metabolism

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Mechanisms That Mechanisms That Control pH of Body FluidsControl pH of Body Fluids

Meaning of pH—negative logarithm of hydrogen ion Meaning of pH—negative logarithm of hydrogen ion concentration of solution (Figure 30-1)concentration of solution (Figure 30-1)

Sources of pH-influencing elementsSources of pH-influencing elements Carbonic acid—formed by aerobic glucose metabolismCarbonic acid—formed by aerobic glucose metabolism

Lactic acid—formed by anaerobic glucose metabolismLactic acid—formed by anaerobic glucose metabolism

Sulfuric acid—formed by oxidation of sulfur-containing amino acidsSulfuric acid—formed by oxidation of sulfur-containing amino acids

Phosphoric acid—formed in breakdown of phosphoproteins and Phosphoric acid—formed in breakdown of phosphoproteins and ribonucleotidesribonucleotides

Acidic ketone bodies—formed in breakdown of fatsAcidic ketone bodies—formed in breakdown of fats

• AcetoneAcetone

• Acetoacetic acidAcetoacetic acid

• Beta-hydroxybutyric acidBeta-hydroxybutyric acid

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Mechanisms That Mechanisms That Control pH of Body FluidsControl pH of Body Fluids

Acid-forming potential of foods—determined by chloride, sulfur, Acid-forming potential of foods—determined by chloride, sulfur, and phosphorus contentand phosphorus content

Types of pH control mechanismsTypes of pH control mechanisms Chemical—rapid action buffersChemical—rapid action buffers

• Bicarbonate buffer systemBicarbonate buffer system• Phosphate buffer systemPhosphate buffer system• Protein buffer systemProtein buffer system

Physiological—delayed action buffersPhysiological—delayed action buffers• Respiratory responseRespiratory response• Renal responseRenal response

Summary of pH control mechanismsSummary of pH control mechanisms• BuffersBuffers• RespirationRespiration• Kidney excretion of acids and basesKidney excretion of acids and bases

Effectiveness of pH control mechanisms; range of pH—Effectiveness of pH control mechanisms; range of pH—extremely effective, normally maintain pH within very narrow extremely effective, normally maintain pH within very narrow range of 7.36 to 7.40range of 7.36 to 7.40

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Mechanisms That Mechanisms That Control pH of Body FluidsControl pH of Body Fluids

Buffers definedBuffers defined

Substances that prevent marked change in pH of Substances that prevent marked change in pH of solution when an acid or base is added to itsolution when an acid or base is added to it

Consist of weak acid (or its acid salt) and basic salt Consist of weak acid (or its acid salt) and basic salt of that acidof that acid

Buffer pairs present in body fluids—mainly Buffer pairs present in body fluids—mainly carbonic acid, proteins, hemoglobin, acid carbonic acid, proteins, hemoglobin, acid phosphate, and sodium and potassium salts of phosphate, and sodium and potassium salts of these weak acidsthese weak acids

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Buffer Mechanisms for Buffer Mechanisms for Controlling pH of Body FluidsControlling pH of Body Fluids

Action of buffers to prevent marked changes in pH of body fluidsAction of buffers to prevent marked changes in pH of body fluids

Nonvolatile acids, such as hydrochloric acid, lactic acid, and ketone bodies, Nonvolatile acids, such as hydrochloric acid, lactic acid, and ketone bodies, buffered mainly by sodium bicarbonatebuffered mainly by sodium bicarbonate

Volatile acids, chiefly carbonic acid, buffered mainly by potassium salts Volatile acids, chiefly carbonic acid, buffered mainly by potassium salts of hemoglobin and oxyhemoglobin (Figure 30-5)of hemoglobin and oxyhemoglobin (Figure 30-5)

Chloride shift makes it possible for carbonic acid to be buffered in red blood cell 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)and then carried as bicarbonate in plasma (Figure 30-6)

Bases buffered mainly by carbonic acid (when homeostasis of pH Bases buffered mainly by carbonic acid (when homeostasis of pH at 7.4 exists)at 7.4 exists)

Ratio B • HCORatio B • HCO33/H/H22COCO33=20/1=20/1

Evaluation of role of buffers in pH control—cannot maintain normal Evaluation of role of buffers in pH control—cannot maintain normal pH without adequate functioning of respiratory and urinary pH control pH without adequate functioning of respiratory and urinary pH control mechanismsmechanisms

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Respiratory Mechanism of pH ControlRespiratory Mechanism of pH Control

Explanation of mechanismExplanation of mechanism

Amount of blood carbon dioxide directly relates to amount Amount of blood carbon dioxide directly relates to amount of carbonic acid and therefore to concentration of Hof carbonic acid and therefore to concentration of H++

With increased respirations, less carbon dioxide remains With increased respirations, less carbon dioxide remains in blood, hence less carbonic acid and fewer Hin blood, hence less carbonic acid and fewer H++; with ; with decreased respirations, more carbon dioxide remains in decreased respirations, more carbon dioxide remains in blood, hence more carbonic acid and more Hblood, hence more carbonic acid and more H++

Adjustment of respirations to pH of arterial bloodAdjustment of respirations to pH of arterial blood

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Respiratory Mechanism of pH ControlRespiratory Mechanism of pH Control

Some principles relating respirations and pH of body fluidsSome principles relating respirations and pH of body fluidsAcidosis → hyperventilationAcidosis → hyperventilation

↓↓

increases elimination of COincreases elimination of CO22

↓↓

decreases blood COdecreases blood CO22

↓↓

decreases blood Hdecreases blood H22COCO33

↓↓

decreases blood H+; that is, increases blood pHdecreases blood H+; that is, increases blood pH

↓↓

tends to correct acidosis; that is, to restore normal pHtends to correct acidosis; that is, to restore normal pH

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Respiratory Mechanism of pH ControlRespiratory Mechanism of pH Control

Some principles relating respirations and pH Some principles relating respirations and pH of body fluids (cont.)of body fluids (cont.)

Prolonged hyperventilation, by decreasing blood HProlonged hyperventilation, by decreasing blood H++ excessively, may produce alkalosisexcessively, may produce alkalosis

Alkalosis causes hypoventilation, which tends to correct Alkalosis causes hypoventilation, which tends to correct alkalosis by increasing blood COalkalosis by increasing blood CO22 and therefore blood and therefore blood

HH22COCO33 and H and H++

Prolonged hypoventilation, by eliminating too little COProlonged hypoventilation, by eliminating too little CO22, ,

causes increase in blood Hcauses increase in blood H22COCO33 and consequently in and consequently in

blood H+, thereby may produce acidosisblood H+, thereby may produce acidosis

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Urinary Mechanisms of pH ControlUrinary Mechanisms of pH Control

General principles about mechanism—plays vital General principles about mechanism—plays vital role in acid-base balance because kidneys can role in acid-base balance because kidneys can eliminate more Heliminate more H++ from body while reabsorbing from body while reabsorbing more base when pH tends toward acid side, and more base when pH tends toward acid side, and eliminates fewer Heliminates fewer H++ while reabsorbing less base while reabsorbing less base when pH tends toward alkaline sidewhen pH tends toward alkaline side

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Urinary Mechanisms of pH ControlUrinary Mechanisms of pH Control Mechanisms that control urine pHMechanisms that control urine pH

Secretion of HSecretion of H++ into urine—when blood CO into urine—when blood CO22, H, H22COCO33, and H, and H++

increase above normal, distal tubules secrete more Hincrease above normal, distal tubules secrete more H++ into into urine to displace basic ion (mainly sodium) from a urine salt urine to displace basic ion (mainly sodium) from a urine salt and then reabsorb sodium into blood in exchange for the Hand then reabsorb sodium into blood in exchange for the H++ excretedexcreted

Secretion of NHSecretion of NH33—when blood hydrogen ion concentration —when blood hydrogen ion concentration

increases, distal tubules secrete more NHincreases, distal tubules secrete more NH33, which combines , which combines

with Hwith H++ of urine to form ammonium ion, which displaces basic of urine to form ammonium ion, which displaces basic ion (mainly sodium) from a salt; basic ion then reabsorbed ion (mainly sodium) from a salt; basic ion then reabsorbed back back into blood in exchange for ammonium ion excretedinto blood in exchange for ammonium ion excreted