ANATOMY & PHYSIOLOGY II
Lecture #17: Acid-Base Balance
I. Acid-Base Balance
Background:
Acid – a proton (H+ ion) donor; releases H+ ions into solution.
Base – a proton (H+ ion) acceptor; receives H+ ions from acids. It is usually a negative ion that accepts the H+
ion.
The pH scale measures acidity or alkalinity:
pH: 1 2 3 4 5 6 7 8 9 10 11 12 13 14
└──────────────┘ └───────────────────┘
Acid
Neutral
Basic
This is a logarithmic scale:
A change of 1 pH unit = a 10 -fold difference
The pH of blood has a normal range of 7.35 - 7.45. This is a very narrow range (it can only vary by
0.0000000092 g/l of H+ ions).
A. Mechanisms of pH control
Three mechanisms are used for maintaining the pH in this range:
1) Buffers
2) Respiratory system
3) Renal system
1. Buffers
a. A buffer solution resists a change in pH.
b. Consists of a buffer pair - a weak acid and a salt of the same weak acid. The negative ion in the acid and
the salt are the same, but in the salt the H is replaced with something else like Na or K.
c. Carbonic acid/bicarbonate system (H2CO3 + NaHCO3) – most important buffer system in the blood.
This system resists changes in pH when acid is added as follows:
HCl + NaHCO3 ──── NaCl + H2CO3
acid
salt of a
salt weak acid
weak acid
H2CO3 is a weak acid (ionizes <1%) so it will not change the pH as much as a strong acid, like HCl
(which ionizes 100 %).
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This system resists changes in pH when base is added as follows:
NaOH + H2CO3 ──── NaHCO3 + H2O
base
weak acid
salt of a water
weak acid
d. Four main buffer systems in the body:
1) Carbonic acid/bicarbonate system – primarily acts in blood plasma
2) Protein buffer systems – involves plasma proteins as well as proteins in cells
3) Hemoglobin system - localized in RBCs
4) Phosphate buffers (KH2PO4/K2HPO4) – primarily acts within cells
e. Buffers are immediate in action, but of short duration.
To Do: Indicate whether each of the following statements refers to an acid (A) or a base (B).
Proton acceptor
Receives H+ ions
Solution has a pH above 7
Proton donor
Releases H+ ions
Solution has a pH below 7
Answer the following questions:
A solution that resists a change in pH is a: a) neutral solution; b) buffer solution; c) salt solution
A buffer pair is a: a) weak base and its salt; b) weak acid and a weak base; c) weak acid and its salt
Which of the following is the salt of a weak acid? a) NaHCO3; b) NaCl; c) H2CO3
Match the buffer system with the location where it mainly is important:
a. blood plasma
b. cells
c. red blood cells d. both plasma and inside cells
Phosphate buffers
Hemoglobin system
Protein buffers
Carbonic acid/bicarbonate system
2. Respiratory system
a. Recall the reaction:
────────
CO2 + H2O ──────── H2CO3
carbonic acid
────────
──────── HCO3- + H+
bicarbonate
b. When the H+ ion concentration increases, these reactions are driven to the left causing the amount of CO2
to increase (thus high Pco2 levels signal that the H+ level is also high). High Pco2 stimulates the
respiratory centers and increases ventilation so that more CO2 is blown off.
c. When the H+ ion concentration decreases, these reactions are driven to the right causing the amount of
CO2 to decrease. Low PCO2 inhibits the respiratory centers and ventilation decreases, allowing more
CO2 to build up in the blood.
d. The respiratory response takes a few minutes to occur, but it has a longer term effect than the buffer
systems.
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3. Renal system
a. This adjusts for changes in pH by adjusting tubular secretion of H+ and reabsorption of HCO3-.
b. When the H+ ion concentration is high (acidic), tubular secretion of H+ ions increases. This causes the
urine to become more acidic and the blood to become more basic.
c. When the H+ ion concentration is low (basic), less HCO3- is reabsorbed so it is excreted in the urine. This
causes the urine to become more basic and the blood to become more acidic.
d. The renal system has the slowest response time to acid/base imbalances, but provides the most effective
long term compensation.
All three systems (buffers, respiratory, and renal) must work together to maintain the proper pH in the body.
To Do: Indicate whether each of these would occur to try to correct a too acidic (A) or a too basic condition (B).
Increased ventilation rate
Increased HCO3- reabsorption.
Increased H+ secretion.
Decreased ventilation rate
Decreased HCO3- reabsorption.
Decreased H+ secretion.
Rank these from fastest (1) to slowest (3):
Respiratory system
Renal system
Buffer systems
Rank these from longest duration (1) to shortest duration (3):
Respiratory system
Renal system
Buffer systems
B. Acid-Base Imbalances
1. Normal conditions
a. The acid/base status of the body can be analyzed based on the pH, Pco2, and HCO3- levels in the blood.
b. Normal values:
pH
= 7.35 - 7.45 (7.4 average)
Pco2
= 40 mm Hg
HCO3- = 24 mEq/l
c. Pco2 is considered to be the respiratory indicator, since this responds to changes in the ventilation rate.
As shown earlier, an increase in CO2 levels corresponds to an increase in H+ ions (the more CO2 present,
the more acidic the blood).
d. HCO3- is the metabolic indicator since bicarbonate levels can be altered by the kidneys, and vary in
response to the levels of metabolic acids (like lactic acid) and gastric acid in the body. HCO3- is a base
because it can accept H+ ions to form H2CO3 (so the more HCO3- present, the more alkaline the blood).
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2. Analyzing the indicator values
a. Acidosis or Alkalosis – For purposes of simplicity, we will say that any variation from the normal blood
pH of 7.4 indicates either acidosis (pH less than 7.4) or alkalosis (pH greater than 7.4) (although
clinically we are only concerned with changes that take it outside the normal range).
b. The abnormal pH could be the result of a variation in either the Pco2 or the HCO3- level:
pH
7.4
(7.35-7.45)
7.4 = acid
7.4 = base
Normal:
Abnormal:
Respiratory indicator
Pco2
40 mm Hg
Metabolic indicator
HCO324 mEq/l
40 = acid
40 = base
24 = acid
24 = base
To Do: Tell whether each of these sets of values represents acidosis (A) or alkalosis (B).
pH
Pco2
7.50
32
7.36
70
HCO3-
HCO3-
pH
Pco2
24
7.30
32
15
38
7.77
35
50
c. Respiratory or Metabolic Imbalance – To figure out which system is causing the pH imbalance, we look
for the one whose indicator value is changing the same way as the pH (becoming more acidic in acidosis
or more basic in alkalosis).
Example:
pH = 7.3 (acidic), Pco2 = 50 (acidic), HCO3- = 24 (normal)
In this example, we can conclude that the problem is respiratory acidosis since CO2 is our respiratory
indicator and its level is becoming more acidic, like the pH.
*Sometimes both the respiratory and metabolic systems are contributing to the problem, in which case
we call it a combined acidosis or alkalosis.
To Do: Tell whether each of these is a respiratory (R), metabolic (M), or combined (C) imbalance.
pH
Pco2
7.50
32
7.36
70
HCO3-
HCO3-
pH
Pco2
24
7.30
32
15
38
7.77
35
50
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d. Compensation - Within a few minutes or hours after a pH imbalance occurs, the body's systems adjust to
try to compensate. By examining the Pco2 and HCO3- values we can determine whether the imbalance is
fully compensated, partially compensated, or uncompensated.
1) Uncompensated - If the Pco2 and HCO3- measurements are taken before any adjustment occurs, no
change will be seen in the indicator for the system which is not causing the problem. In the example
below, the HCO3- value is normal, so no compensation has occurred. Any time a combined acidosis or
alkalosis occurs, it is automatically considered to be uncompensated.
Example: pH = 7.3 (acidic), Pco2 = 50 (acidic), HCO3- = 24 (normal)
So uncompensated = one normal indicator OR a combined problem.
2) Partially compensated - If the system which is not responsible for the problem is no longer
at its normal value, but the pH is still outside the range of 7.35 - 7.45, then we refer to this as being
partially compensated. This would be the case if the HCO3- level in the above example were 30
(basic) (see below), since this would mean that the metabolic system was attempting to compensate
for the respiratory acidosis.
Example: pH = 7.3 (acidic), Pco2 = 50 (acidic), HCO3- = 30 (basic)
This would be partially compensated respiratory acidosis.
So partially compensated = no normal indicator value and pH OUTSIDE normal range.
3) Fully compensated - If the system which is not causing the problem has been able to compensate
enough to bring the pH into the range of 7.35 - 7.45, then we say the problem is fully compensated.
An example is shown below:
Example: pH = 7.37 (acidic), Pco2 = 50 (acidic), HCO3- = 34 (basic)
This would be fully compensated respiratory acidosis.
So fully compensated = no normal indicator value and pH INSIDE normal range.
To Do: Tell whether each of these is uncompensated (U), partially compensated (P), or fully compensated (F).
HCO3-
pH
Pco2
What we have figured out so far:
7.50
32
24
Respiratory alkalosis
7.36
70
38
Respiratory acidosis
7.30
32
15
Metabolic acidosis
7.77
35
50
Combined alkalosis
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3. Acidosis and Alkalosis
a. Causes of Respiratory Acidosis
1) Depression of the respiratory centers - could result from taking barbiturates or tranquilizers.
2) Impaired alveolar ventilation as a result of infections, asthma, emphysema, etc.
3) Impaired respiratory movements caused by poisons like curare or by paralysis of the diaphragm.
* All these lead to a higher than normal level of CO2 in the blood causing it to become acidic.
b. Causes of Respiratory Alkalosis
1) Hyperventilation - can be the result of hysteria, fever, pain, damage to the respiratory centers of the
brain, or the initial stages of aspirin overdose.
2) Hypoxia of high altitudes - at high altitudes the body breathes more deeply and rapidly to try to get
enough oxygen to the cells, which causes more CO2 to be blown off.
* All these lead to an excessive amount of CO2 being lost from the body through ventilation, and a
decreased amount in the blood, causing basic conditions.
c.
Causes of Metabolic Acidosis
1) Excess of metabolic acids due to diabetes – this produces high levels of ketone bodies in the blood.
2) Diarrhea resulting in excessive loss of HCO3-.
3) Deep vomiting from the small intestine results in loss of HCO3- from bile and pancreatic secretions.
4) Late stages of aspirin (salicylic acid) poisoning - absorption of salicylic acid raises the H+
concentration, since this is an acid.
* These cause either an increase in acidic compounds entering the blood or a loss of basic bicarbonate
from the body, leading to acidic conditions.
d. Causes of Metabolic Alkalosis
1) Stomach vomiting - results in the loss of hydrochloric acid from the body.
2) Intake of excessive base – like antacids, or baking soda (sodium bicarbonate).
* These result in either loss of acid or increase in base in the body, leading to basic conditions.
e. Causes of Combined Acidosis
This would always result from two separate problems, one that causes respiratory acidosis and one that
causes metabolic acidosis (see above), such as a person with a flu that caused both congestion of the
lungs and diarrhea.
Example of uncompensated combined acidosis:
pH = 7.3 (acidic), PCO2 = 50 (acidic), HCO3- = 21 (acidic)
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f.
Causes of Combined Alkalosis
Results from two separate problems that cause both respiratory alkalosis and metabolic alkalosis.
Example of uncompensated combined alkalosis:
pH = 7.48 (basic), PCO2 = 35 (basic), HCO3- = 28 (basic)
*Combined disorders are always uncompensated since neither system is working to correct the problem.
To Do: What kind of acid-base imbalance would most likely be caused by each of the following? Use these choices:
a. Respiratory acidosis
b. Respiratory alkalosis
c. Metabolic acidosis
d. Metabolic alkalosis
e. Combined acidosis
f. Combined alkalosis
Diarrhea
Emphysema
Hyperventilation
Stomach vomiting
Deep vomiting
Ingesting a bottle of aspirin 15 minutes ago
Ingesting a bottle of aspirin 1 hour ago
Pneumonia
A diabetic making too many ketone bodies who has bronchiole congestion
A person who has just moved from sea level to the mountains
Overdose of tranquilizers
Someone with fever and pain who ingested baking soda to settle the stomach
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Learning Objectives for Lecture #17: Acid-Base Balance
After studying this material you should be able to:
1.
Define the terms acid and base.
2.
Describe in general terms how the pH scale works and be able to tell for any given pH value whether it is
acidic, basic or neutral.
3.
List the three mechanisms of pH control, and tell how quickly each takes effect and about how long it lasts.
4.
Describe what a buffer solution does and what two things are needed for a buffer pair.
5.
List the four main buffer systems in the body and for each know where it primarily acts.
6.
Describe how the respiratory system and the renal system adjust to compensate for too much acid in the body
and too much base in the body.
7.
List the normal values for pH, Pco2, and HCO3-, and for each tell whether the value increases or decreases to
produce more acidic or more basic conditions.
8.
Analyze a set of blood gas values (pH, Pco2, and HCO3-) and be able to tell what type of acid-base imbalance
is indicated and what type of compensation is taking place.
9.
Identify the possible causes for each type of acid-base imbalance (respiratory acidosis, respiratory alkalosis,
metabolic acidosis, metabolic alkalosis, combined acidosis, and combined alkalosis).
Lecture #17: Acid-Base Balance – Answer Key
Page 2 - Indicate whether each of the following statements refers to an acid (A) or a base (B).
B
Proton acceptor
A
B
Proton donor
A
Receives H+ ions
+
Releases H ions
B
Solution has a pH above 7
A
Solution has a pH below 7
Answer the following questions:
B
A solution that resists a change in pH is a: a) neutral solution; b) buffer solution; c) salt solution
C
A buffer pair is a: a) weak base and its salt; b) weak acid and a weak base; c) weak acid and its salt
A
Which of the following is the salt of a weak acid? a) NaHCO3; b) NaCl; c) H2CO3
Match the buffer system with the location where it mainly is important:
a. blood plasma
b. cells
c. red blood cells
d. both plasma and inside cells
B
Phosphate buffers
C
Hemoglobin system
D
Protein buffers
A
Carbonic acid/bicarbonate system
Page 3 – Indicate whether each of these would occur to try to correct a too acidic (A) or a too basic condition (B).
A
Increased ventilation rate
A
Increased HCO3- reabsorption.
A
Increased H+ secretion.
B
Decreased ventilation rate
B
Decreased HCO3- reabsorption.
B
Decreased H+ secretion.
Rank these from fastest (1) to slowest (3):
2
Respiratory system
3
Renal system
1
Buffer systems
3
Buffer systems
Rank these from longest duration (1) to shortest duration (3):
2
Respiratory system
1
Renal system
Page 4 - Tell whether each of these sets of values represents acidosis (A) or alkalosis (B).
HCO3-
pH
Pco2
B
7.50
32
24
A
7.36
70
38
HCO3-
pH
Pco2
A
7.30
32
15
B
7.77
35
50
Page 4 - Tell whether each of these is a respiratory (R), metabolic (M), or combined (C) imbalance.
HCO3-
pH
Pco2
R
7.50
32
24
R
7.36
70
38
HCO3-
pH
Pco2
M
7.30
32
15
C
7.77
35
50
10
Page 5 – Tell whether each of these is uncompensated (U), partially compensated (P), or fully compensated (F).
HCO3-
pH
Pco2
What we have figured out so far:
U
7.50
32
24
Respiratory alkalosis
F
7.36
70
38
Respiratory acidosis
P
7.30
32
15
Metabolic acidosis
U
7.77
35
50
Combined alkalosis
Page 7 - What kind of acid-base imbalance would most likely be caused by each of the following? Use these choices:
a. Respiratory acidosis
b. Respiratory alkalosis
c. Metabolic acidosis
d. Metabolic alkalosis
e. Combined acidosis
f. Combined alkalosis
C
Diarrhea
A
Emphysema
B
Hyperventilation
D
Stomach vomiting
C
Deep vomiting
B
Ingesting a bottle of aspirin 15 minutes ago
C
Ingesting a bottle of aspirin 1 hour ago
A
Pneumonia
E
A diabetic making too many ketone bodies who has bronchiole congestion
B
A person who has just moved from sea level to the mountains
A
Overdose of tranquilizers
F
Someone with fever and pain who ingested baking soda to settle the stomach