ACID-BASE BALANCE
Healthy survival depends on the body’s maintaining a state of acid-base balance; more specifically, healthy
survival depends on the maintenance of a relative constant, slightly alkaline pH of blood and other fluids.
When the body is in a state of acid-base balance, it maintains a stable hydrogen ion concentration in body
fluids; specifically, blood pH remains relatively constant between 7.35 and 7.45
The body has three devices or mechanisms for maintaining acid-base balance; named in order of the speed with
which they act:
- the buffer mechanism,
- the respiratory mechanism
- the renal or urinary mechanism
A state of uncompensated acidosis exists if blood pH decreases below 7.35
A state of uncompensated alkalosis exists if blood pH increases above 7.45
The pH of body fluids shifts from the ideal of 7.35 to 7.45 for several reasons.
Glucose, used by almost all body cells, is oxidized; as a result, energy, water, and carbon dioxide are produced;
then the CO2 combines with the water to produce carbonic acid (H2CO3)
Metabolism of sulfur amino acids results in formation of sulfuric acid.
Metabolism of phospholipids and phosphoproteins results in formation of phosphoric acid.
Muscle metabolism under anaerobic conditions produces lactic acid.
Rapid weight loss results in extra fat metabolism, producing ketone bodies that include alpha keto acids.
The acid produced by the normal mechanisms just mentioned requires neutralization to avoid acidosis, coma
and death.
BUFFER MECHANISMS FOR MAINTAINING ACID-BASE BALANCE
The buffer mechanism consists of chemicals called buffers, which are present in the blood and other body fluid
and which combine with relatively strong acids or bases to convert them to weaker acids or bases; hence,
buffers function to prevent marked changes in blood pH levels either acids or bases enter the blood.
A buffer is often referred to as a buffer pair because it consists of not one but two substances: the chief buffer
pair in the blood consists of the weak acid, carbonic acid (H2CO3), and its base salts, collectively called base
bicarbonate (BHCO3); Na bicarbonate (NaHCO3), is by far the most abundant base bicarbonate present in blood
plasma.
When the body is in a state of acid-base balance, blood contains 27 mEq base bicarbonate per liter and 1.35
mEq carbonic acid per liter; usually this is written as a ratio, referred to as the base bicarbonate/carbonic acid
ratio:
27mEq B HCO3 = 20 / 1.35mEQ H2CO3 = 1
Whenever the base bicarbonate/carbonate acid ratio of blood equals 20/1, blood pH equals 7.4
Base bicarbonate buffers are nonvolatile acids that are stronger than carbonic acid and a basic salt. Buffering
does not prevent blood pH from decreasing, but it does prevent it from decreasing extreme amounts.
Buffering removes some NaBicarb from the blood and adds some carbonic acid to it; this necessarily decreases
the base bicarbonate/carbonic acid ratio, which in turn necessarily decreases the pH of blood as it flows through
capillaries (from its arterial level of about 7.4 to its venous level of about 7.38)
Anything that decreases the blood’s base bicarbonate/carbonic acid ratio necessarily decreases blood pH and
thus tends to produce acidosis; the opposite is also true; anything that increases the base bicarbonate/carbonic
acid ratio necessarily increases blood pH and thus tends to produce alkalosis.
Other buffer systems
- Protein buffer
Most plentiful; three fourths of all chemical buffering power lies in the proteins of the body fluids. It provides
support to other buffering systems such as the bicarbonate buffer and phosphate buffer.
- Phosphate buffer system
One sixth the neutralizing ability of bicarbonate buffer in extracellular fluid.
More important in intracellular fluids, where its concentration is considerably higher. Helps to buffer pH of
urine in kidney tubules.
- Bicarbonate buffer is the most important buffer in human body fluids because its components, base
bicarbonate and carbonic acid, are actively and constantly regulated by the action of the respiratory and urinary
systems
RESPIRATORY MECHANISM FOR MAINTAINING ACID-BASE
BALANCE
Respiratory controls acid-base balance by controlling rate of CO2 exhalation
from the lungs; during normal body metabolism CO2 is produced, which
reacts with warm hydrogen to form carbonic acid, resulting in a decrease in
pH (as acidity increases, pH decreases; when the respiratory system blows
CO2 out of the body, carbonic acid breaks down into CO2 and water,
resulting in an increase in pH (as acidity decreases, pH increases)
Conditions impairing the ability of the respiratory system to blow off CO2 will result in a buildup of CO2 in the
body; Excess CO2 combines with water (H2O) to form carbonic acid and hydrogen ions, resulting in a decrease
in pH.
Signs of Respiratory acidosis include:
dyspnea, irritability, tachycardia, and cyanosis
Common causes include:
emphysema, pneumonia, asthmatic attacks, atelectasis, pneumothorax, respiratory depression from drug
overdose
RESPIRATORY ACIDOSIS AS A RESULT OF FAILURE OF
MECHANISM
Hyperventilation blows off too much O2 from the body, causing an excessive breakdown of carbonic acid,
resulting in an increase of pH.
Signs of Respiratory alkalosis include deep and/or rapid breathing, lightheadedness, tetany, convulsions, and
unconsciousness.
Common causes include hysteria, prolonged crying, and mechanical ventilation
RESPIRATORY COMPENSATION OF METABOLIC IMBALANCE
In metabolic acidosis the respiratory system compensates by hyperventilation in attempt to blow off CO2 and
raise the pH. The respiratory system compensates by decreasing the rate and depth of breathing in an attempt to
retain CO2 and decrease pH.
RENAL MECHANISM FOR MAINTAINING ACID-BASE BALANCE
The renal mechanism is for maintaining acid-base balance; unless it operates
adequately, acid-base balance cannot be maintained.
The renal mechanism for maintaining acid-base balance makes the urine
more acidic and the blood more alkaline. This neutralizes the constant
production of acidic products from cells; the mechanism consists of two
functions performed by the distal renal tubule cells, both of which remove
hydrogen ions from blood to the urine and in exchange reabsorb sodium ions
from tubular urine to the blood.
Metabolic Acidosis as a result of failure of mechanism
Excess acid, other than carbonic acid, which is a respiratory acid, accumulates in the body beyond the body’s
ability to neutralize it.
Signs of metabolic acidosis include weakness, malaise, headache, disorientation, deep rapid breathing, fruity
odor to breath, coma, death
Common causes include: Diabetes mellitus (DKA), salicylate poisoning, severe diarrhea, vomiting of intestinal
contents, infection and renal failure
Metabolic Alkalosis as a result of failure mechanism
Excess base bicarbonate in the body
Signs of Metabolic Alkalosis include: muscle hypertonicity, tetany, confusion, shallow slow respiration,
convulsions, and coma
Common causes include: vomiting of stomach contents or prolonged gastric suctioning, excessive ingestion of
alkaline drugs, and potent diuretics
Metabolic compensation for respiratory imbalance
In respiratory acidosis the urinary system excreted hydrogen ions to compensate for the respiratory system’s
inability to blow off CO2
In respiratory alkalosis the urinary system may decrease excretion of hydrogen ions to compensate and maintain
the body’s pH in the normal range.
Interpretation of ABGs
Lemone -- Page 153
STEP 1
pH < 7.35 acidosis
pH > 7.45 alkalosis
STEP 2
Evaluate Respiratory Function (Ventilation)
PaO2 > 45 mm Hg = ventilatory failure and resp acidosis
PaO2< 35 mmHg = hyperventilation and resp alkalosis
STEP 3
Evaluate Metabolic Processes
Serum bicarbonate < 22 mEq/L and /or base excess <-3 mEq/L =metabolic acidosis
Serum bicarbonate > 26 mEq/L and/or base excess >-3 mEq/L = metabolic alkalosis
STEP 4
Determine the Primary and Compensating D/O
When both PaCO2 and Bicarbonate deviate from normal
Follow the deviation in pH and the one that deviates the most form normal
The value that follows the deviation in pH and has the greatest deviation from normal identifies the primary
disturbance
STEP 5
Evaluate Oxygenation
Checking the PaO2 and SaO2 to determine whether they are within normal limit, decreased, or increased.
Normally, the Pao2 is between 80-100 mmHg with a normal Sao2 of 95% or greater
Hypoxia PaO2 <60 == decreased SaO2
Normal ABG’s
pH 7.35 –7.45
PaCO2 35-45 mmHg
Hco3 22- 26 mEq L
PaO2 80-100mmHg