Acid Base Balance
An acid is a substance that releases a H+ ion when dissolved in water.
A base is a substance that will bind to a H+ ion when dissolved in water.
pH scale is 1 to 14, with 7 being neutral. 1 to 6.9 is considered acid while 7.1 to 14 is alkaline.
Normal plasma pH is 7.35 to 7.45. Acidosis is below 7.35 and alkalosis is above 7.45.
Death usually occurs below 6.8 or above 7.8.
The number of free hydrogen ions present determines pH.
Increase in free hydrogen ions is acidosis while a decrease in free hydrogen ions is alkalosis. Or a
decrease in HCO3- is acidosis and an increase in HCO3- is alkalosis, since it is the balance between the
two that keeps the pH within normal limits.
The ratio of hydrogen ions to bicarbonate ions is 1 to 20 in normal plasma pH.
pH 7.35 to 7.45
One H+

20 HCO3-
Hydrogen ions circulate in two ways: (Note: H20 is water which in this case is the water in plasma, HCO 3is bicarbonate, CO2 is carbon dioxide, H2CO3 is carbonic acid, and NaCO3 is sodium bicarbonate.)
A.
Volatile hydrogen ions: The ionization of H2CO3 into H+ and HCO31.
2.
B.
Occurs in erythrocytes: CO2 + H2O yields H+ and HCO3
Tissue to lungs: H+ and HCO3 yields H2O + CO2
Non Volatile hydrogen ions:
1.
2.
3.
Produced as a result of metabolism.
Include organic acid, lactic acid and ketone bodies.
Normal diet produces 50 to 100 mEqs of hydrogen ions.
Control of Acid Base Balance:
A.
Buffers: Tend to prevent changes in hydrogen ion concentration when an acid or alkali is
added to the body.
Occur in pairs:
1.
Bicarbonate pair: In the ECF and is the most important buffer.
CO2 + H2O = H2CO3
H2CO3 = H+ + HCO3
The HCO3 combines with Na+ to form NaHCO3. NaHCO3 is sodium
bicarbonate a strong alkali.
2.
Hemoglobin pair: blood
3.
Plasma protein pair: plasma
4.
Phosphate pair: intracellular
B. Lungs: Affect carbonic acid, a volatile form of H+.
1.
When H+ concentration is increased, respirations increase to blow off CO 2 and
decrease the amount H+ HCO3
2.
If the H+ concentration decreases, respirations also decrease to conserve CO 2
3.
The lungs are very effective in controlling acid base balance. Lack of ventilation
for 1 minute will cause the body to become acidotic. Hyperventilation for 1
minute will cause the body to become alkalotic.
When an acid base dysfunction is caused by the carbonic acid the condition is
called respiratory acidosis or alkalosis.
4.
C. Kidney: Non volatile hydrogen ions.
1.
Can conserve or excrete H+:
a.
b.
c.
Small amounts of H+ free in the urine determines urine pH.
60% of H+ excreted is in the form of
NH4Cl or ammonium.
40% is excreted in weak acids such as urea.
2.
Can conserve or excrete bicarbonate and sodium.
3.
When the acid base disturbance is not caused by carbonic acid, but is caused by
the accumulation of other acids or when there is an excess or deficit of
bicarbonate the condition is called metabolic acidosis or alkalosis.
Measurement of Acid Base:
1.
pH tells acidosis or alkalosis, but not if the problem is metabolic or respiratory.
2.
PCO2 measures H2CO3 or carbonic acid.
a.
b.
c.
d.
Normal is 38 to 42 mmHg if arterial.
Normal is 40 to 41 mmHg if venous.
If the PCO2 goes to 50, respirations tipple.
If the PCO2 drops to 30, respirations are reduced to 1/4.
3.
CO2 capacity is 24 to 33 mEqs.
4.
Bicarbonate is 22 to 26 mEqs.
Compensation:
1.
Occurs when the body uses regulatory mechanisms tp return the pH to normal
levels by transforming acids and bases within the body. The pH is kept within
normal levels but there are abnormal amounts of CO 2 or HCO3.
2.
A metabolic problem will cause respiratory compensation and a respiratory
problem will cause metabolic compensation. This is because if the problem is
respiratory the lungs are affected and cannot compensate. If the problem is
metabolic the kidneys will not be able to excrete the acids or alkalis fast enough
so the lungs will compensate. The lungs can compensate rapidly within a few
minutes, while the kidneys may take days to compensate.
Metabolic Acidosis: (Base bicarbonate deficit)
1.
2.
Causes:
a.
Gain of acids:
1)
Diabetic ketoacidosis
2)
Lactoc acidosis
3)
Starvation
4)
Shock
b.
Inability to excrete acids: Renal failure
c.
Loss of base
1)
Gastrointestional fistulas
2)
Chronic diarrhea
Manifestations: The acid pH causes calcium to increase ionization, even though
there are no changes in the serum calcium level. Consequently, many of the
manifestations associated with acidosis are similar to calcium excess.
Related to calcium ionization, decreased fluid volume (because the acids are
causing an osmotic diuresis), and an accumulation of acids in the body.
a.
b.
c.
d.
Drowsiness, lethargy, stupor, coma
Nausea, vomiting, diarrhea, abdominal pain
Hypotension, disrhythmias, peripheral vasodilation
Weakness, hypotonic skeletal muscles
Compensatory: Deep rapid respirations (Kussmaul’s respiration) to blow off CO 2
and lower the pH.
3.
Laboratory Values
a.
b.
c.
d.
e.
4.
Decreased pH (This is acidosis so the pH is decreased.)
Decreased HCO3- (The bicarbonate is decreased because the body is
using it to buffer the acids.)
Decreased PCO2 (The PCO2 is decreased because the body is blowing
off CO2.)
Decreased urine pH (Reflects the blood plasma pH.)
Increased potassium level (When acidosis occurs, potassium ions leave
the cells and is replaced with hydrogen ions. In severe acidosis
potassium can be low because the osmotic diuresis has washed out the
majority of the body’s potassium.)
Treatment:
a.
The cause of the metabolic acidosis must be treated to correct the
problem.
b.
Sodium bicarbonate may be given to treat the acidosis. This is done
only in severe acidosis, because there can be a rebound causing the
body to make more acids.
Metabolic Alkalosis: (Base Bicarbonate Excess)
1.
Causes:
a.
Loss of strong acids
1)
Severe vomiting
2)
Long-term gastric suctioning
b.
Gain of base
1)
Excessive ingestion of sodium bicarbonate in antacids.
2)
Poisoning with household cleaners (Most are strong alkalis.)
c.
Loss of potassium (When potassium is lost, chloride is lost. The body
compensates by increasing the production of bicarbonate to keep the
anions and cations in equal.)
1)
2)
3)
2.
Diuretic therapy
Excessive mineral corticosteroids (Increase potassium excretion
and sodium retention)
Potassium deficit
Manifestations: The alkali pH causes calcium to decrease ionization, even
though there are no changes in the serum calcium level. Consequently, many of
the manifestations associated with alkalosis are similar to calcium deficit.
a.
Related to calcium deficit
1)
Tremors, hypertonic muscles, muscle cramps, tetany, tingling of
fingers and toes, seizures
2)
Dizziness, irritability, tachycardia, disrythmias
Compensatory: Hypoventilation to conserve CO2 and increase the pH.
3.
Laboratory Values
a.
b.
c.
d.
e.
4.
Increased pH (This is alkalosis so the pH is increased.)
Increased HCO3- (The bicarbonate is increased because the body has
too much bicarbonate from the cause.)
Increased PCO2 (The PCO2 is increased because the body is conserving
CO2.)
Increased urine pH (Reflects the blood plasma pH.)
Decreased potassium level (When alkalosis occurs, potassium ions
move inside the cells.)
Treatment: The cause of the metabolic alkalosis must be treated to correct the
problem.