Acid –Base balance
• An acid is any substance capable of liberating
a hydrogen ion. An acid can be strong or
weak, depending on the degree to which it
breaks down to liberate hydrogen ions.
For example, hydrogen chloride (HCl) rapidly
and totally breaks down into hydrogen and
chloride ion; therefore, it is considered a
strong acid.
In contrast, few lactic acid molecules break
down to hydrogen ion and lactate; therefore,
lactic acid is considered a weak acid.
• A base is any substance that can accept a
hydrogen ion, thereby taking it out of solution.
A base can be strong or weak, depending on the
degree to which it accepts a hydrogen ion . Most
acids and bases found in the body are weak.
Buffers
A buffer is a substance that can either take free
hydrogen ions from a solution or release
hydrogen ions to a solution, thereby preventing
large fluctuations in pH. Weak acids and weak
bases make good buffers.
There are three important buffer systems in the
body:
1-Bicarbonate-Carbonic Acid Buffer System
This is the main buffer system in the body.
When free hydrogen ions are added to blood
containing bicarbonate, the bicarbonate ions
bind with the hydrogen ions, becoming
carbonic acid (H2CO3) thereby preventing a
significant decrease in blood pH. Carbonic
acid is considered a weak acid; bicarbonate
ion is considered its weak, conjugate
(complementary) base.
Carbonic acid can also dissociate to form carbon •
dioxide and water; therefore, the bicarbonate buffer
system is primarily used to eliminate hydrogen ion
from the body through the elimination of the
volatile gas carbon dioxide. The breakdown of
carbonic acid to carbon dioxide and water requires
the enzyme carbonic anhydrase, which is present in
red blood cells. The reaction of carbonic acid to
carbon dioxide and water is reversible, and carbon
dioxide and water can rejoin to form carbonic acid.
2-Phosphate Buffer System
Phosphoric acid (H2PO4-) is a weak acid; it dissociates in
plasma to phosphate (HPO42-) and hydrogen ion (H+).
Phosphate is a weak base. The kidney uses this system
to buffer the urine as it excretes hydrogen ion..
3-Hemoglobin Buffer System
The third buffer system is provided by proteins in the
blood, especially hemoglobin present in red blood cells.
Hemoglobin binds to free hydrogen ions so blood pH
decreases .As the blood flows through the lungs,
hydrogen ions dissociate from the hemoglobin and join
with bicarbonate to become carbonic acid which breaks
down to carbon dioxide and water.
Respiratory Control of Acid-Base Balance
The lungs rid the body of carbon dioxide.
Although carbon dioxide itself is not an acid, it
joins with water to form carbonic acid . The
minute-by-minute regulation of plasma pH is
controlled by an increase or decrease in the
rate of respiration. This system is possible
because of the sensitivity of the respiratory
center in the brain to free hydrogen ions
which usually vary in accordance with carbon
dioxide.
Renal Control of Acid-Base Balance(Urine Buffers)
Non-volatile acids(as lactic acid, acetic acid, acetoacetic
acid) produced during metabolism are excreted in the
urine. Their excretion occurs as a result of active
secretion of hydrogen ions by cells of the kidney into
the urine filtrate. In the filtrate, hydrogen ions join
with phosphate, sulfate, or ammonia (NH3) buffers and
are then excreted in the urine as salts of phosphoric
acid, sulfuric acid, or ammonium ion (NH4+).
Normal values:
pH
: 7.35-7.45
PaO2 : 80-100 mmHg
PaCO2: 35-45mmHg
HCO3 : 22-26mEq/
Acidemia
The decrease in arterial pH to less than 7.35 is called
acidemia. Acidemia may result from respiratory,
renal, or metabolic causes.
Acidosis
*Is a systemic increase in hydrogen ion concentration.
*Causes :
-failure of the lungs to eliminate Co2
-excess production of volatile or non-volatile acids.
-loss of bicarbonate base caused by persistent
diarrhea
-failure of the kidney either to reabsorb bicarbonate
or to secrete hydrogen ions.
Alkalemia
Alkalemia is the increase in arterial blood pH above
7.45.
Alkalemia may result from respiratory, renal, or
metabolic causes.
Alkalosis
Alkalosis is a systemic decrease in hydrogen ion
concentration
Causes :
-excess loss of carbon dioxide during
hyperventilation.
-excess loss of non-volatile acids during vomiting.
- excess ingestion of a base.
Compensation
• If acidosis or alkalosis results from a metabolic or a
renal disorder, the respiratory system responds by
increasing or decreasing respiratory rate, this is called
respiratory compensation. It occurs immediately upon
changes in hydrogen ion concentration, because H+
controls respiratory center .
• If acidosis or alkalosis results from a respiratory
disorder, the kidneys respond by altering their handling
of hydrogen ion and bicarbonate base to return the pH
back toward normal, this is called renal compensation.
Renal compensation begins to have an effect
approximately 24 hours after a respiratory alteration in
pH. Despite this delay, renal compensation is powerful.
Respiratory Acidosis
Is the decrease in arterial pH resulting from a primary
respiratory disorder. If respiration is impaired
,carbon dioxide levels increase, leading to increase
in H+ concentration which if not buffered causes pH
to decrease.
Causes of Respiratory Acidosis
- all obstructive pulmonary disorders (chronic
obstructive lung disease, asthma).
- hypoventilation of any origin, including drug
overdose or airway obstruction.
- severe pulmonary congestion .
- reduced pulmonary blood flow .
Compensation for Respiratory Acidosis
When acidosis is caused by a respiratory problem,
renal compensation occurs. Renal compensation
results in :
- increasing its secretion and excretion of acid
- increasing its reabsorption of base.
*Renal compensation takes at least 24 hours to
begin.
- Clinical Manifestations
-Neurologic symptoms such as headache,
behavioral changes, and tremors.
-Respiratory depression from increased carbon
dioxide may occur.
Diagnostic Tools
-The partial pressure of carbon dioxide is greater than 45
mmHg .
- Plasma bicarbonate levels are increased (greater than 28
mEq/L), reflecting the fact that the kidney is excreting more
hydrogen ion and reabsorbing more base.
-Urine pH is acidic as the kidneys excrete more hydrogen ion.
N.B. If renal compensation is successful, plasma pH will be
low, but in normal range
Complications
Paralysis and coma may result from cerebral vasodilatation in
response to increased carbon dioxide concentration if levels
become toxic.
Treatment
Improvement of ventilation is essential. Mechanical
ventilation may be required.
Respiratory Alkalosis
The increase in arterial pH resulting from any primary
respiratory disorder is called respiratory alkalosis.
Respiratory alkalosis results when carbon dioxide
partial pressure levels decrease to less than 35 mmHg.
With decreased carbon dioxide, equation Co2+H2O ---H2CO3- -- H+--- +HCO3- is driven to the left, resulting in
a decrease in free hydrogen ion concentration and an
increase in pH.
Causes of Respiratory Alkalosis
-Hyperventilation (fever and anxiety).
-Hypoxemia if the partial pressure of oxygen in arterial
blood decreases to less than 50 mmHg (normal is
approximately 100 mmHg).
- Salicylate toxicity & brain infections can directly
stimulate the respiratory center .
Compensation for Respiratory Alkalosis
Renal compensation involves decreasing its secretion
and excretion of hydrogen ion and actively secreting
bicarbonate ion into the urine. Again, this requires
24 hours to become effective.
Clinical Manifestations
- Rapid respirations.
- Neurologic disturbances as dizziness, muscle
contractions, and changes in consciousness.
Diagnostic Tools
-Decreased partial pressure of carbon dioxide of less than
35 mmHg .
-For respiratory alkalosis lasting longer than 24 hours,
bicarbonate blood levels are decreased (less than 22
mEq/L), reflecting the fact that the kidney is
reabsorbing less base or secreting base into the urine.
-Urine pH is basic as the kidneys attempt to excrete more
bicarbonate base and return pH toward normal.
Complications :
-Convulsions and coma if the condition persists or
becomes very severe.
Treatment
-Determining and treating the cause of
hyperventilation is the most successful therapy.
-Increasing partial pressure of carbon dioxide by
breathing into a bag and rebreathing the expired air
may reverse the alkalosis in an acute situation.
Metabolic Acidosis
The decrease in arterial pH resulting from a nonrespiratory problem is called metabolic acidosis.
Metabolic acidosis is characterized by the
accumulation of non-volatile acids.
Causes of Metabolic Acidosis
1-Increase in non-volatile Acids caused by:
-prolonged hypoxia(lactic acid)
- fat metabolism in diabetics(ketones)
-overdose of drugs such as salicylates .
-excess protein metabolism ( starvation or
malnutrition)
2-Decrease in the Renal Clearance of Hydrogen Ion
caused by:
- renal failure
- interruption in renal blood flow.
As a result of these conditions, the kidney, which
normally reabsorbs all filtered bicarbonate and actively
secretes hydrogen ion, cannot function properly,
causing hydrogen ion to accumulate.
3-Loss of Bicarbonate caused by:
- decreased renal function( loss of bicarbonate
occurs)leads to acidosis.
- chronic diarrhea (bicarbonate is concentrated in
intestinal secretions).
- high levels of extracellular chloride (hyperchloremia)
cause metabolic acidosis because bicarbonate ions shift
intracellularly( hyperchloremic acidosis).
Compensation for Metabolic Acidosis
- Respiratory compensation which involves increases in
the rate and depth of respirations.It begins almost
immediately with the onset of acidosis.
* success depends on the severity of the acidosis.
- Renal compensation(if healthy) by excreting more acid.
Clinical Manifestations
-weakness and fatigue occurring from poor muscle function.
-Anorexia, nausea, and vomiting.
-Warm flushed skin resulting from decrease in vascular response
to sympathetic stimuli.
*If metabolic acidosis is caused by diabetic ketoacidosis,
additional manifestations will include:
- Ketone smell (fruity) on breath
- Anorexia, nausea and vomiting, abdominal pain
- Increased rate and depth of respirations
- Decreasing level of consciousness, leading to coma
*If metabolic acidosis is caused by diarrhea, additional
manifestations will include:
- Signs of dehydration, including decreased blood pressure and
loss of skin turgor
- Abdominal pain and cramping
- Frequent, loose stools
Diagnostic Tools
- decreased bicarbonate levels to less than 22 mEq/L .
- Partial pressure of carbon dioxide is less than 35 mmHg(rapid deep
respiration ).
* If respiratory compensation is successful, pH is low but in the normal range.
* If compensation is unsuccessful, plasma pH will reflect high plasma acidity
and be less than 7.35, even in the face of reduced carbon dioxide.
- Urine pH will be acidic if renal function is normal, because the kidneys will
attempt to excrete more acid to return pH toward normal.
* If metabolic acidosis is caused by diabetic ketoacidosis, additional diagnostic
tools will include:
-Increase in blood and urine glucose
-Ketonuria and decreased urine pH
* If metabolic acidosis is caused by chronic renal failure, additional diagnostic
tools will include:
-Urine pH only slightly acidic or non-acidic
- Increased blood urea nitrogen (BUN), reflecting excess protein catabolism
(breakdown) and decreased GFR
Complications
*If metabolic acidosis is caused by chronic renal failure,
complications may include renal osteodystrophy (bone
dissolution due to renal disease) and renal encephalopathy.
*If pH falls below 7.0, cardiac dysrhythmia can occur. This
happens as a result of changes in cardiac conduction, which
occur in direct response to a decrease in pH and because of
the effects of increased hydrogen ion concentration on
plasma and intracellular potassium.
Treatment
-treating the cause.
-for patients with renal disease, excess base in the diet is given.
-Administration of sodium bicarbonate infusion may be used to
raise pH rapidly if the person is at risk of dying. This
procedure must be undertaken carefully because it may cause
brain swelling.
The increase in arterial pH resulting from a nonrespiratory problem is called metabolic alkalosis.
Causes of Metabolic Alkalosis
- excessive loss of acid
- increased base ingestion
- dehydration ( can lead to less bicarbonate to be filtered
across the glomerulus).
- excessive vomiting (chloride or potassium deficiency)
- ingestion of bicarbonate to treat indigestion or
heartburn
Compensation for Metabolic Alkalosis
Respiratory compensation ( decrease in the rate and
depth). It can occur almost immediately upon onset of
alkalosis. The kidneys will also participate in
compensation when possible.
Clinical Manifestations
-Neurologic manifestations are slow to develop, but may
include confusion, hyperactive reflexes, spasms, and
tetany (sustained muscle contraction).
Diagnostic Tools
-Increased bicarbonate levels greater than 28 mEq/L
(because increased bicarbonate either is the direct
cause of the alkalosis or reflects a decrease in hydrogen
ion concentration).
-Because of respiratory compensation, carbon dioxide
levels is greater than 45 mmHg. The rate and depth of
respirations are reduced, possibly causing hypoxemia.
-
-If respiratory compensation is successful, plasma pH
will be high but in the normal range. If
compensation is unsuccessful, plasma pH will
reflect the high plasma base concentration and will
be greater than 7.45 in the face of elevated carbon
dioxide.
-Urine pH will be basic if renal function is normal,
because the kidneys will attempt to excrete less
acid and more base to return pH toward normal.
Complications
With pH greater than 7.55, dysrhythmia and
coma may result from alterations in neuronal
and cardiac muscle cell depolarization.
Treatment
-If the cause is due to chloride or potassium
deficiency, these ions must be replaced.
-If decreased extracellular volume is the cause,
saline solution replacement is required
Summary
*Note, if the pH is low (acidosis),if high
(alkalosis)
*if PaCO2 is outside the normal range, the
problem is respiratory
*if HCO3 is outside the normal range, the
problem is metabolic
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Respiratory acidosis:
Ph
= 7.32
PaCO2 = 67
PaO2 = 47
HCO3 = 37
*Respiratory alkalosis:
Ph
= 7.6
PaCO2 = 30
PaO2 = 60
HCO3 = 22
*Metabolic acidosis:
Ph
= 7.18
PaCO2 = 38
PaO2 = 70
HCO3 = 15
*Metabolic alkalosis:
Ph = 7.58
PaCO2 = 35
PaO2 = 75
HCO3 = 50