2.3.3 Acid-Base Balance Flashcards
What are the normal blood gas values for PaO2, PaCO2, pH, and HCO3- ?
PaO2 = 95 mm Hg
PaCO2 = 40 mm Hg
pH = 7.4
HCO3- = 24mM
If someone has a low PaO2, does this mean that alveolar ventilation is lower than normal?
Alveolar ventilation is defined by PaCO2
Decreased alveolar ventilation would lower PaO2. However, PaO2 may be low even though alveolar ventilation is increased. In this situation PAO2 - PaO2 would be greater than normal.
What are the important stimuli to increase alveolar ventilation?
Decreased PaO2
Increased PaCO2
Decreased arterial pH (increased H+ concentration)
What is the difference between respiratory and metabolic acid base changes?
Respiratory - Due to change in alveolar ventilation causing a change in PaCO2.
Metabolic - Change in production or loss of acid
How will increasing PaCO2 affect H+ and HCO3- concentrations?
Increased PaCO2 will be accompanied by increased H+ and HCO3, while decreased PaCO2 will have the opposite effect and decrease both
Each change in 10mm Hg of PaCO2 will change the pH by how much?
It will change by 0.08
If a person has a change in PaCO2 by 20mm Hg, how will this affect the pH in an acute setting vs a chronic setting?
In a person with an acute drop, such as an asthma attack, it will drop by .16 to 7.24.
For a patient with a chronic condition, such as COPD, there will be time for compensatory mechanisms to come in to effect. This will cause the pH to be lowered, but the pH will be above 7.24.
The same applied to alkalosis
What is the normal ratio of CO2 to H2CO3?
CO2 is higher by a ratio of 500 to 1
How is pH related to HCO3- and PCO2? What does this mean?
pH is directly related to HCO3- and inversely related to PCO2.
This means that a high PCO2 will tend to lower the pH and a high bicarb will tend to increase the pH
What are the main buffers of the body?
The bicarbonate system and the non-bicarb system.
How is hypoventilation going to create acid-base disturbances?
This is going to lead to an increase in CO2 and respiratory acidosis
Describe respiratory acidosis without metabolic compensation.
This is going to be caused by an increase in PCO2, increase in HCO3- and a decrease in pH.
The decrease is pH is going to be caused by the shift of the reaction to the right and generation of H+ ions.
The response
Describe respiratory acidosis with metabolic compensation.
If PCO2 remains elevated for some time, HCO3- increases along the high PCO2 isobar (red arrow, points a-b); this is largely due to the compensatory mechanisms of the kidney which result in the production of a more acid urine.
The increase in HCO3- at constant PCO2 results in an increase in pH towards normal.
What type of acid base disturbance can hyperventilation in a normal person cause?
It will cause the removal of CO2, thus leading to respiratory alkalosis
Describe non compensated respiratory alkalosis?
Hyperventilation will produce increased elimination of CO2 and a decrease in PCO2 . The direction of the changes will be opposite to that of respiratory acidosis.
The decrease in PCO2 will cause the pH to rise.
Describe respiratory alkalosis with metabolic compensation.
If hyperventilation continues, plasma HCO3- decreases further at constant PCO2 (c-d) . The decrease in HCO3- from c to d is due to the renal compensatory mechanisms that result in the excretion of a more alkaline urine. The composition of blood moves from point C to point D: PCO2 is unchanged (hyperventilation continues) but bicarbonate decreases; this leads to a decrease in pH towards normal.
Notice that in both respiratory alkalosis and acidosis, the compensation for the changes in PCO2 were changes in HCO3- in the same direction as the change in PCO2;
Describe metabolic acidosis.
Metabolic acidosis occurs when non-volatile acids are added to the body fluids. This can occur in strenuous exercise or hypovolemic shock, in which large amounts of lactic acid are produced, or in cases such as uncontrolled diabetes, where organic acids are produced in excess.
The excess H+ will be buffered by both HCO3- and the non-bicarbonate buffers as shown in the diagram. The portion of H+ buffered by HCO3- will be transformed into CO2 , which will be eliminated in the lungs, keeping the PCO2 constant. Blood composition will change from o to e. HCO3- will decrease, PCO2 will remain constant and pH will decrease. This is a non-compensated metabolic acidosis
Describe metabolic acidosis with respiratory compensation.
Elimination of CO2 by hyperventilation will cause a shift in the blood pH back to normal.
Describe metabolic alkalosis without compensation.
Metabolic alkalosis can occur when base is added to or acid removed from the body fluids. Metabolic alkalosis is seen less frequently than metabolic acidosis. Persistent vomiting in patients with pyloric obstruction, leading to loss of acid gastric juice, or salicylate intoxication, may produce metabolic alkalosis.
The decrease in H+ increases the dissociation of HA and of H2CO3. pH and plasma HCO3- increase at constant PCO2: non-compensated metabolic alkalosis. Blood acid-base composition moves from o to g
Describe metabolic alkalosis with respiratory compensation.
The high pH decreases the input to the pH chemoreceptors and ventilation decreases. This increases PCO2, and the blood moves from point g to h. The slope of the line connecting points g and h is -10mM/pH . This is a partially compensated metabolic alkalosis
Summarize this one image.
Respiratory disorders are those in which the initial disturbance is a change in PCO2
- Respiratory acidosis (Hypoventilation):
a. not-compensated: low pH, high PCO2, slightly elevated [HCO3-], such that the ratio
(24 -actual HCO3-) / ( 7.4 -actual pH) = ~10 mmol /(L x pH
b. compensated: [HCO3-] increased due to renal compensation, pH becomes closer to 7.4 ,
ratio ∆HCO3-/ ∆pH >> -10 mmol / (L x pH)
- Respiratory alkalosis (Hyperventilation):
a. not compensated: high pH, low PCO2, slightly decreased HCO3-; ratio ∆HCO3- / ∆pH ~ 10 mmol / (L x pH)
b. compensated: HCO3- decreased due to renal compensation, ratio >>> -10 mmol / (L x pH)
Metabolic disorders are produced by causes other than respiratory changes.
- Metabolic acidosis:
a. not compensated: low pH, normal PCO2, low HCO3-
b. compensated: PCO2 decreases due to hyperventilation, pH increases towards 7.4 - Metabolic alkalosis:
a. not compensated: high pH, high HCO3-, normal PCO2
b. compensated: PCO2 increases due to hypoventilation, pH decreases towards 7.4
C
2
5
B
7
