Lecture 11: Acid-Base Balance (Bolsor) Flashcards
Which buffering system (intracellular or extracellular) is sensitive to regulation and can respond more rapidly to changes in acid/base balance?
extracellular
What are the major buffering systems?
intracellular (primarily protein and phosphates) and extracellular (primarily CO2/HCO3 system)
Can the intracellular buffer system remove excess acid or base from the body?
NO
3 stage response to acid/base change:
1) chemical buffering (rapid)
2) respiratory system (rapid; +/- alveolar ventilation of CO2 in response to changes in extracellular pH)
3) kidneys (delayed response; responds to chronic acidemia/alkalemia by controlling excretion/production of HCO3)
Why is the amount of free H2CO3 in the body not equal to the amount of H+ ions?
H2CO3 doesn’t completely dissociate in the blood
Equation for acid/base balance. What drives this equation left or right?
CO2 + H2O H2CO3 HCO3 + H
- Equation shifts to the right when HCO3 is lost (tries to replace HCO3 that was lost)
- Equation shifts to the left when HCO3 gained (tries to get rid of excess HCO3)
Respiratory acidemia and possible causes
failure of the lungs to excrete adequate amounts of CO2.
Causes: alveolar hypoventilation due to pulmonary disease or central respiratory depression
How is respiratory acidemia compensated for?
Metabolic compensation: increased renal reabsorption of bicarbonate and excretion of acidic urine
How can you gauge how much H+ is released into circulation?
How much HCO3 lvls decreased in order to buffer the H+. Greater decrease in HCO3 = more H+ present
What does an “isobar” represent?
constant CO2 level while pH and HCO3 concentrations change
If pCO2 is increased and pH remains the same, what must happen to HCO3?
Must increase
If pCO2 is constant and pH decreases, what happens to HCO3?
decreases
metabolic acidemia and possible causes
abnormal retention of fixed (i.e. non-CO2) acids.
Causes: diabetes, trauma, shock, diarrhea
Most common type of acidosis
metabolic acidemia. It is also the most common acid/base imbalance seen in animals with kidney failure
How is metabolic acidemia compensated for?
- hyperventilation to eliminate CO2
- increased reabsorption and synthesis of HCO3 by kidneys
- excretion of an acidic urine
What is anion gap, what is it used for, and how is it calculated?
Measures the excess of unmeasured anions over unmeasured cations and is an index of whether or not the metabolic acidemia is due to loss of HCO3 or addition of H (determines what TYPE of metabolic acidosis is occuring). Calculated by subtracting concentrations of total chloride + bicarbonate from concentration of anions (primarily Na and K)
Inadequate O2 delivery to tissues triggers switch to what kind of metabolism?
anaerobic metabolism. Produces a by-product of lactic acid which stimulates hyperventilation
Which two cations constitute the vast majority of extracellular cations?
Na and K (95% of extracellular cations)
Which 2 anions constitute 85% of extracellular anions?
Cl and HCO3
Metabolic acidemia with normal anion gap is usually assoc. with:
- loss of bicarbonate with no increase in unmeasured anions
- hyperchloremia (kidney absorbs more Cl in response to low HCO3)
- often caused by chronic diarrhea
Metabolic acidemia with elevated anion gap is assoc. with:
- increase in non-HCO3 and non-Cl anions (i.e. lactic acid)
- increased H+
Common cause of metabolic acidosis with elevated anion gap
shock due to increased lactic acid. Lactic acid effectively replaces the HCO3 in circulation
respiratory alkalemia and possible causes
hyperventilation with excessive loss of CO2
Causes: CNS lesions, anemia
How is respiratory alkalemia compensated for?
increased renal excretion of bicarbonate and thus an alkaline urine
metabolic alkalosis and possible causes
Excessive loss of H ions or excessive intake or retention of base
Causes: vomiting (loss of HCl), hypokalemia (H moves into cells to exchange for K ions that move into the extracellular fluid to replace lost K)
How is metabolic alkalosis compensated for?
hypoventilation (increased CO2 retention), increased renal excretion of HCO3 and alkaline urine
How does acid/base status alter plasma K concentration?
During acidemia, H+ moves into cells to be buffered. However, K+ moves out of the cell to counterbalance this change and result = hyperkalemia
During alkalemia, H+ moves out of cell to compensate, and K+ moves into cell to counteract it. Result = hypokalemia
BOTTOM LINE: acidemia or alkalemia can also elicit disturbances in K balance
If VOMITING (loss of HCl) is the cause of metabolic alkalosis, how is kidney’s ability to compensate for the alkalosis compromised?
Loss of HCl results in hypochloremia (low Cl), which inhibits kidney’s ability to properly excrete HCO3. In severe cases, aldosterone secretion responding to low extracellular volume results in even more K excretion and exacerbates the situation.
Define 4 regions on HCO3 vs. pH Davenport diagrams
Upper left (low pH, high HCO3): respiratory acidosis Bottom left (low pH, low HCO3): metabolic acidosis Upper right (High pH, high HCO3): metabolic alkalosis Bottom right (high pH, low HCO3): respiratory alkalosis
what do you look at first when reading a blood gas measurement? Second? Third? What do these values tell you?
1st: pH (tells you if disturbance is acidosis or alkalosis)
2nd: PCO2 (tells you if disturbance is respiratory or metabolic in origin)
Low pH indicates
acidosis
High pH indicates
alkalosis
High PCO2 with low pH indicates
pH disturbance (acidosis) has a respiratory component
Low pH, high PCO2, and low HCO3 indicates:
Acidosis with ONLY a respiratory component (no metabolic component)
Low pH, low PCO2, and low HCO3 indicates:
no respiratory component to pH disturbance
