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Acid-Base Balance
Chapter 20
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2. Learning Objectives Lesson 20.1: Acid-Base Balance
Define the term acid-base balance and discuss the concept of pH.
Define the terms buffer and buffer pair and contrast strong and weak acids and bases.
Contrast the respiratory and urinary mechanisms of pH control.
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3. Learning Objectives Lesson 20.1: Acid-Base Balance (Cont.)
Define acidosis and alkalosis, and compare and contrast metabolic and respiratory types of pH imbalances.
Discuss compensatory mechanisms that may help return blood pH to near-normal levels in cases of pH imbalances.
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pH of Body Fluids
Definition of pH
A number that indicates the hydrogen ion (H+) concentration of a fluid; pH 7.0 indicates neutrality, pH higher than 7.0 indicates alkalinity, and pH less than 7.0 indicates acidity (Figure 20-1)
Normal arterial blood pH
About 7.45
Normal venous blood pH
About 7.35
The pH unit is based on exponents of 10 from one unit to the next
Acid-base balance is one of the most important of the body’s homeostatic mechanisms.
The pH of pure water is 7.0.
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The pH Range
Figure 20-1 shows the pH range and various examples.
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6. Mechanisms that Control pH of Body Fluids
Buffers
Definition
Substances that prevent a sharp change in the pH of a fluid when an acid or base is added to it (Figures 20-3 and 20-4)
“Fixed” acids are buffered mainly by sodium bicarbonate (NaHCO3)
The body has three mechanisms for regulating the pH of its fluids. They are (1) buffer mechanism in blood, (2) respiratory mechanism, and (3) urinary mechanism.
The lungs remove the equivalent of more than 30 L of carbonic acid each day from the venous blood by elimination of carbon dioxide.
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7. Mechanisms that Control pH of Body Fluids (Cont.)
Respiratory mechanism of pH control
Lungs are the primary regulator for respiratory imbalances.
CO2 (Carbon Dioxide) is the primary gas we exhale. Remember that H+ ions ALWAYS go in the same direction as CO2. That means if CO2 increases then
H+ increases too! Keep in mind H+ is acidic. This will
create more acid in the body (pH will decrease).
When there is too much CO2 in the body, the body tries to get rid of it by combining with H2O to become Carbonic Acid (H2CO3). Some of the H2CO3 separates to become HCO3 (Bicarbonate) a base and H+, which neutralize each other.
Any factor that causes an appreciable decrease in respiration may in time produce acidosis.
Any factor that causes an excessive increase in respirations may in time produce alkalosis.
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8. Mechanisms that Control pH of Body Fluids (Cont.)
Urinary mechanism of pH control
Being the body’s most effective regulator of blood pH, kidneys usually acidify urine by the distal tubules secreting hydrogen ions and ammonia (NH3) into the urine from blood in exchange for NaHCO3 being reabsorbed into the blood
More acids than bases usually enter blood, so more acids than bases are usually excreted by the kidneys.
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pH Imbalances
Acidosis and alkalosis – pH or acid-base imbalances
Disturbances in acid-base balance depend on relative quantities of NaHCO3 and H2CO3 in the blood
Body can regulate both of the components of the NaHCO3–H2CO3 buffer system
Blood levels of NaHCO3 regulated by kidneys
H2CO3 levels regulated by lungs
Disturbances in acid-base balance can be considered dependent on the relative quantities of H2CO3 to NaHCO3 in the blood.
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pH Imbalances (Cont.)
Metabolic and respiratory disturbances (pH disturbances) can alter the normal 20:1 ratio of NaHCO3 to H2CO3 in blood
Metabolic disturbances affect the NaHCO3 levels in blood
Respiratory disturbances affect the H2CO3 levels in blood
Hyperventilation leads to a H2CO3 deficit caused by excessive loss of CO2 in expired air.
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pH Imbalances (Cont.)
Metabolic disturbances
Metabolic acidosis
Bicarbonate (NaHCO3) deficit
Metabolic alkalosis
NaHCO3 excess; complication of severe vomiting
Respiratory disturbances
Respiratory acidosis
Carbonic acid (H2CO3) excess
Respiratory alkalosis
H2CO3 deficit
When acidosis or alkalosis occurs in the body, our various pH-balancing mechanisms try to restore balance as soon as possible.
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Questions?
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