A2 Chemistry_D
Use of this content is subject to the Terms and Conditions of the Evolve web site.
A2
Chemistry
ACID-BASE IMBALANCE
ACID-BASE IMBALANCE
Review of Acid-Base Balance
Acid-base balance is very important in the body because cell enzymes can function only within a very narrow pH
range. The normal serum pH range is 7.35 to 7.45. Death usually results if serum pH is below 6.8 or above 7.8 (Fig.
6-8). For example, a pH of below 7.35 depresses central nervous system function and decreases all cell enzyme
activity.
FIGURE 6-8 The hydrogen ion and pH scale.
When serum pH is below 7.4, more hydrogen ions (H+) are present, and acidosis results. A serum pH of above 7.4
is more basic, indicating alkalosis or the presence of fewer hydrogen ions. The body normally has a tendency
toward acidosis, or a lower pH, because cell metabolism is constantly producing carbon dioxide
(CO2) or carbonic acid (H2CO3) and nonvolatile metabolic acids such as lactic acid, ketoacids, sulfates, or
phosphates. Lactic acid results from the anaerobic (without oxygen) metabolism of glucose, ketoacids result from
incomplete oxidation of fatty acids, and protein metabolism may produce sulfates or phosphates.
6-19
THINK ABOUT
a. When hydrogen ions are decreased, is the pH higher or lower?
b. State the optimal range of serum pH for normal cell function.
Control of Serum pH
,As the blood circulates through the body, nutrients diffuse from the blood into the cells, various metabolic
processes take place in the cells using these nutrients, and metabolic wastes, including acids, diffuse from the
cells into the blood (Fig. 6-9).
FIGURE 6-9 Changes in acids, bicarbonate ion, and serum pH in circulating blood.
Three mechanisms control or compensate for pH.
1. The buffer pairs circulating in the blood respond to pH changes immediately.
2. The respiratory system can alter carbon dioxide levels (carbonic acid) in the body by changing the
respiratory rate (see Chapter 19).
3. The kidneys can modify the excretion rate of acids and the production and absorption of bicarbonate
ion (see Chapter 21).
Note that the lungs can change only the amount of carbon dioxide (equivalent to the amount of carbonic acid)
in the body. The kidneys are slow to compensate for a change in pH but are the most effective mechanism
because they can excrete all types of acids (volatile or gaseous and nonvolatile) and can also adjust serum
bicarbonate levels.
6-20
THINK ABOUT
How does the respiratory rate change when more hydrogen ions enter the blood, and how does this change
affect acid levels in the body?
Buffer Systems
To control serum pH, several buffer systems are present in the blood. A buffer is a combination of a weak acid
and its alkaline salt. The components react with any acids or alkali added to the blood, neutralizing them and
thereby maintaining a relatively constant pH.
The body has four major buffer pairs:
1. the sodium bicarbonate-carbonic acid system,
2. the phosphate system,
3. the hemoglobin system,
4. the protein system.
The bicarbonate system is the major extracellular fluid buffer and is used clinically to assess a client's acid-
base status. The principles of acid-base balance are discussed here using the bicarbonate pair. Specific figures
are not used because the emphasis is on the basic concepts.
6-2
Challenge
, Predict three ways by which control of serum pH could be lost.
The Bicarbonate-Carbonic Acid Buffer System and Maintenance of Serum pH
The bicarbonate buffer system is composed of carbonic acid, which arises from the combination of carbon
dioxide with water, and bicarbonate ion, which is present as sodium bicarbonate. The balance of bicarbonate
ion (HCO3−), a base, and carbonic acid (H2CO3) levels is controlled by the respiratory system and the
kidneys (see Fig. 6-9). Cell metabolism produces carbon dioxide, which diffuses into the interstitial fluid and
blood, where it reacts with water to form carbonic acid, which then dissociates immediately under the
influence of the enzyme carbonic anhydrase, to form hydrogen ions and bicarbonate ions. This enzyme is
present in many sites, including the lungs and the kidneys. In the lungs, this reaction can be reversed to form
carbon dioxide, which is then expired along with water, thus reducing the total amount of carbonic acid or
acid in the body. In the kidneys, the reaction needed to form more hydrogen ions is promoted by enzymes; the
resultant hydrogen ions are excreted in the urine, and the bicarbonate ions are returned to the blood to restore
the buffer levels.
To maintain serum pH within the normal range, 7.35 to 7.45, the ratio of bicarbonate ion to carbonic acid (or
carbon dioxide) must be 20:1.
As one component of the ratio changes, the other component must change proportionately to maintain this
ratio and thus serum pH. For instance, if respiration is impaired, causing an increase in carbon dioxide in the
blood, the kidneys must increase serum bicarbonate levels to compensate for the change. The actual
concentrations are not critical as long as the proportions are sustained. It may help to remember that the
bicarbonate part or alkali part of the buffer ratio is 20, the higher figure, because more bicarbonate base is
required to neutralize the acids constantly being produced by the body cells.
6-21
THINK ABOUT
If bicarbonate ion is lost from the body, how will carbonic acid levels change?
Respiratory System
When serum carbon dioxide or hydrogen ion levels increase, chemoreceptors stimulate the respiratory control
center to increase the respiratory rate, thus removing more carbon dioxide or acid from the body. When
alkalosis develops, the respiratory rate decreases, thus retaining more carbon dioxide and increasing acid
levels in the body.
Renal System
The kidneys can also reduce the acid content of the body by exchanging hydrogen for sodium ions under the
influence of aldosterone and can remove acids (H+) by combining them with ammonia and other chemicals.
The kidneys also provide the bicarbonate ion for the buffer pair as needed. Urine pH may range from 4.5 to
8.0 as the kidneys compensate for metabolic conditions and dietary intake.
Use of this content is subject to the Terms and Conditions of the Evolve web site.
A2
Chemistry
ACID-BASE IMBALANCE
ACID-BASE IMBALANCE
Review of Acid-Base Balance
Acid-base balance is very important in the body because cell enzymes can function only within a very narrow pH
range. The normal serum pH range is 7.35 to 7.45. Death usually results if serum pH is below 6.8 or above 7.8 (Fig.
6-8). For example, a pH of below 7.35 depresses central nervous system function and decreases all cell enzyme
activity.
FIGURE 6-8 The hydrogen ion and pH scale.
When serum pH is below 7.4, more hydrogen ions (H+) are present, and acidosis results. A serum pH of above 7.4
is more basic, indicating alkalosis or the presence of fewer hydrogen ions. The body normally has a tendency
toward acidosis, or a lower pH, because cell metabolism is constantly producing carbon dioxide
(CO2) or carbonic acid (H2CO3) and nonvolatile metabolic acids such as lactic acid, ketoacids, sulfates, or
phosphates. Lactic acid results from the anaerobic (without oxygen) metabolism of glucose, ketoacids result from
incomplete oxidation of fatty acids, and protein metabolism may produce sulfates or phosphates.
6-19
THINK ABOUT
a. When hydrogen ions are decreased, is the pH higher or lower?
b. State the optimal range of serum pH for normal cell function.
Control of Serum pH
,As the blood circulates through the body, nutrients diffuse from the blood into the cells, various metabolic
processes take place in the cells using these nutrients, and metabolic wastes, including acids, diffuse from the
cells into the blood (Fig. 6-9).
FIGURE 6-9 Changes in acids, bicarbonate ion, and serum pH in circulating blood.
Three mechanisms control or compensate for pH.
1. The buffer pairs circulating in the blood respond to pH changes immediately.
2. The respiratory system can alter carbon dioxide levels (carbonic acid) in the body by changing the
respiratory rate (see Chapter 19).
3. The kidneys can modify the excretion rate of acids and the production and absorption of bicarbonate
ion (see Chapter 21).
Note that the lungs can change only the amount of carbon dioxide (equivalent to the amount of carbonic acid)
in the body. The kidneys are slow to compensate for a change in pH but are the most effective mechanism
because they can excrete all types of acids (volatile or gaseous and nonvolatile) and can also adjust serum
bicarbonate levels.
6-20
THINK ABOUT
How does the respiratory rate change when more hydrogen ions enter the blood, and how does this change
affect acid levels in the body?
Buffer Systems
To control serum pH, several buffer systems are present in the blood. A buffer is a combination of a weak acid
and its alkaline salt. The components react with any acids or alkali added to the blood, neutralizing them and
thereby maintaining a relatively constant pH.
The body has four major buffer pairs:
1. the sodium bicarbonate-carbonic acid system,
2. the phosphate system,
3. the hemoglobin system,
4. the protein system.
The bicarbonate system is the major extracellular fluid buffer and is used clinically to assess a client's acid-
base status. The principles of acid-base balance are discussed here using the bicarbonate pair. Specific figures
are not used because the emphasis is on the basic concepts.
6-2
Challenge
, Predict three ways by which control of serum pH could be lost.
The Bicarbonate-Carbonic Acid Buffer System and Maintenance of Serum pH
The bicarbonate buffer system is composed of carbonic acid, which arises from the combination of carbon
dioxide with water, and bicarbonate ion, which is present as sodium bicarbonate. The balance of bicarbonate
ion (HCO3−), a base, and carbonic acid (H2CO3) levels is controlled by the respiratory system and the
kidneys (see Fig. 6-9). Cell metabolism produces carbon dioxide, which diffuses into the interstitial fluid and
blood, where it reacts with water to form carbonic acid, which then dissociates immediately under the
influence of the enzyme carbonic anhydrase, to form hydrogen ions and bicarbonate ions. This enzyme is
present in many sites, including the lungs and the kidneys. In the lungs, this reaction can be reversed to form
carbon dioxide, which is then expired along with water, thus reducing the total amount of carbonic acid or
acid in the body. In the kidneys, the reaction needed to form more hydrogen ions is promoted by enzymes; the
resultant hydrogen ions are excreted in the urine, and the bicarbonate ions are returned to the blood to restore
the buffer levels.
To maintain serum pH within the normal range, 7.35 to 7.45, the ratio of bicarbonate ion to carbonic acid (or
carbon dioxide) must be 20:1.
As one component of the ratio changes, the other component must change proportionately to maintain this
ratio and thus serum pH. For instance, if respiration is impaired, causing an increase in carbon dioxide in the
blood, the kidneys must increase serum bicarbonate levels to compensate for the change. The actual
concentrations are not critical as long as the proportions are sustained. It may help to remember that the
bicarbonate part or alkali part of the buffer ratio is 20, the higher figure, because more bicarbonate base is
required to neutralize the acids constantly being produced by the body cells.
6-21
THINK ABOUT
If bicarbonate ion is lost from the body, how will carbonic acid levels change?
Respiratory System
When serum carbon dioxide or hydrogen ion levels increase, chemoreceptors stimulate the respiratory control
center to increase the respiratory rate, thus removing more carbon dioxide or acid from the body. When
alkalosis develops, the respiratory rate decreases, thus retaining more carbon dioxide and increasing acid
levels in the body.
Renal System
The kidneys can also reduce the acid content of the body by exchanging hydrogen for sodium ions under the
influence of aldosterone and can remove acids (H+) by combining them with ammonia and other chemicals.
The kidneys also provide the bicarbonate ion for the buffer pair as needed. Urine pH may range from 4.5 to
8.0 as the kidneys compensate for metabolic conditions and dietary intake.
