ACID BASE BALANCE

ACID BASE
BALANCE

ACID BASE BALANCE
• General Principles
– Dynamic relationship which reflects the
concentration of hydrogen (H+) ions
– Hydrogen ions are acids which must be
maintained within strict limits
» Any deviation from norm affects biochemical
events
– Measured in a negative log (pH) which is
inversely proportional to the hydrogen ion
concentration
» High H+ ion concentration = Low pH
» Low H+ ion concentration = High pH

THE pH SCALE
• Most frequently used measure of acid base balance
• Ranges from 1-14
– 1 means only hydrogen ions present
– 14 means no hydrogen ions present
– pH of water is 7.0 (neutral)
• pH of human body is 7.35-7.45
• pH compatible with life = 6.9-7.8
– pH of < 7.35 indicates a state of acidosis
– pH of > 7.45 indicates a state of alkalosis
– Variation of 0.4 in either direction can be fatal

Acid-Base Balance
• A pH below 7.35 is
referred to as
acidosis
• A pH above 7.45 is
referred to as
alkalosis

• Body constantly produces acids through
metabolism
– Carbon dioxide
– Metabolic acids (lactic and pyruvic acid)
• Acids must be constantly eliminated from the body
– As acids increase, bodily functions decrease
– Example
» Diminishes myocardial contractility
» Reduces vascular response to catecholamines
» Interferes with the actions of
pharmacological agents

BUFFER SYSTEM
• Fastest acting defense mechanism (seconds)
• Chief buffer is bicarbonate
– Bicarbonate combines with excess hydrogen ions to form
carbonic acid in a dynamic relationship:
HCO3 + H+ H2CO3
– For every molecule of carbonic acid, there are 20 molecules
of bicarbonate
– Any change in 20:1 ratio is immediately corrected to
maintain pH
» An increase in H+ causes an increase in H2CO3
» A decrease in H+ causes a decrease in H2CO3

Bicarbonate Buffer System
• Fastest mechanism
• Hydrogen will bind with bicarbonate
• Results in formation of carbonic acid
Hydrogen
H HCO H CO
3 2 3
ion bicarbonate ion carbonic
acid

Carbonate Buffer System
• An increase in hydrogen ions leads to an increase
in carbonic acid.
• A deficit in hydrogen ions (alkalosis); carbonic
acid will dissociate into bicarbonate ion and
hydrogen ion
Increased
Decreased
Acid:
H
Acid: H
HCO
3
HCO
3
H
2
H
CO
2
3
CO
3

BUFFER SYSTEM (CONT.)
• Carbonic acid is a weak, volatile acid
which must be eliminated
• Enzyme carbonic anhydrase
catalyzes the carbonic acid to
convert to carbon dioxide and water:
H2CO3--------->CO2 + H2O
• CO2 and H2O are easily eliminated
by the lungs and kidneys

• Functions within minutes (1-3)
• Lungs eliminate excess CO2 by
increasing respirations, causing a
decrease in H+ ion and an increase in
pH
• Lungs can retain more CO2 by
decreasing respirations, causing an
increase in H+ ions and a decrease in
pH

Respiratory Mechanisms
• Increased respirations cause increased elimination of CO 2
and H 2 O.
– An increase in pH
• Decreased respirations cause retention of CO 2 and H 2 O.
– Decreasing pH

• Slowest onset and generally important in
long term maintenance of acid-base
balance (hours to days)
• Function
– Kidneys retain bicarbonate, causing a
decrease in H+ ions and an increase in pH
– Kidneys excrete bicarbonate, causing an
increase in H+ ions and a decrease in pH

Respiratory Acidosis
• Caused by abnormal retention of CO 2 from
impaired ventilation due to problems
occurring in the lungs or respiratory center
of the brain
Respiration = CO 2 + H 2 O H 2 CO 3 H + + HCO 3
-

Respiratory Alkalosis
Caused by increased respiration and
excessive elimination of CO 2
The CO 2 level is decreased and the pH is
increased.
Respiration = CO 2 + H 2 O H 2 CO 3 H + + HCO 3
-

ACID-BASE
DERANGEMENTS (CONT.)
• Metabolic acidosis
– Increased production of acids (lactic and pyruvic)
– Can also result from diarrhea, vomiting, diabetes and
medications (ASA)
– pH is decreased and CO2 level is normal
– Treatment is aimed at improving ventilations to
eliminate CO2
– Sodium bicarbonate with documented metabolic
acidosis
» Rarely administered during initial resuscitative
strategies

ACID-BASE
DERANGEMENTS (CONT.)
• Metabolic alkalosis
– Caused by increased vomiting, overdosing on
antiacids, diuretics
– pH is increased and CO2 remains normal
– Treatment is aimed at underlying cause
• Combinations
– Any combination of acid-base derangements
– Only ABG can accurately diagnose problem

Thank you!