Acid-Base Balance II Flashcards
When would a person be considered to be of normal acid-base balance?
Plasma pH close to 7.4 (range 7.35-7.45)
[HCO3-]p close to 25 mmol/l (range 23-27)
Arterial PCO2 close to 40mmHg (range 35-45)
What is compensation?
Restoration of pH irrespective of what happens to [HCO3-]p and PCO2
What is correction?
Restoration of pH, [HCO3-]p and PCO2 to normal
What is the first priority if normal acid-base balance is disturbed?
To restore pH
What acid-base disturbances cause plasma pH to fall?
Respiratory acidosis and metabolic acidosis
What acid-base disturbances cause plasma pH to rise?
Respiratory alkalosis and metabolic alkalosis
What occurs in immediate buffering of pH change?
Immediate dilution of the acid or the base in ECF
What carries out the immediate buffering of pH change?
Blood buffers or buffers in the ECF
What is the issue with immediate buffering of pH change?
Response is very quick but the buffer stores are quickly depleted = kidney must rectify stores
How is blood-gas analysis carried out?
A blood gas analyser can measure pH and PCO2
[HCO3-]p can then be calculated
What is used to plot the results of blood-gas analysis?
A Davenport diagram
What is respiratory acidosis?
Retention of CO2 by the body
What are some causes of respiratory acidosis?
Chronic bronchitis, chronic emphysema, airway restriction, chest injuries, respiratory depression
What does CO2 retention due to respiratory acidosis do to the equilibrium?
Drives it to the right = both [H+]p and [HCO3-] rise
Increased [H+] results in acidosis
What is uncompensated respiratory acidosis?
pH < 7.35 AND PCO2 > 45 mmHg
How are respiratory disorders compensated for?
Virtually no extracellular buffering in respiratory disorders so renal system must compensate
What drives H+ secretion by the kidney?
PCO2 = CO2 retention stimulates H+ secretion into the filtrate
How does the renal compensate in respiratory acidosis?
H+ secretion stimulated
All filtered HCO3- reabsorbed
H+ continues to be secreted and generates titratable acid and NH4+
Acid secreted and new HCO3- is added to blood
What causes [HCO3-] to rise in respiratory acidosis?
Both the disorder and the renal compensation
What does correction of respiratory acidosis and alkalosis require?
Restoration of normal ventilation
What is respiratory alkalosis?
Excessive removal of CO2 by the body
What are some causes of respiratory alkalosis?
Low inspired PO2 at altitude, hyperventilation, hysterical overbreathing
What does respiratory alkalosis do to the equilibrium?
Drives it to the left = [H+]p and [HCO3-]p fall
Decreased [H+]p results in alkalosis
What is uncompensated respiratory alkalosis?
pH > 7.45 AND PCO2 < 35 mmHg
What effect does respiratory alkalosis have on the kidney?
Excessive removal of CO2 reduces H+ secretion into the tubule
Why does respiratory alkalosis cause alkaline urine?
H+ secretion insufficient to reabsorb filtered HCO3- = HCO3- excreted in urine
How does the renal system compensate for respiratory alkalosis?
HCO3- excreted in urine
No titratable acid or NH4+ produced so there is no new HCO3- generated
What is metabolic acidosis?
Excess H+ from any source other than CO2
What are some cause of metabolic acidosis?
Ingestion of acid/acid producing foods
Excessive metabolic production of H+ (e.g DKA)
Excessive loss of base (e.g diarrhoea)
What cause [HCO3-] depletion in metabolic acidosis?
Buffering excess H+ or loss of HCO3- from the body
What is uncompensated metabolic acidosis?
pH < 7.35, [HCO3-]p is low
What stimulates the respiratory system in metabolic acidosis and alkalosis?
Peripheral chemoreceptors = decrease/increase in plasma pH respectively
How does the respiratory system compensate for metabolic acidosis?
Ventilation is quickly increased and more CO2 is blown off, [H+]p is lowered raising the pH towards normal, [HCO3-]p is also lowered
Why is respiratory compensation essential in correcting metabolic acidosis?
Acid load cannot be excreted immediately
When does ventilation return to normal in metabolic acidosis?
Once acid load is excreted (acidic urine) and [HCO3-]p is restored
How is metabolic acidosis corrected?
Filtered HCO3- is very low and very readily reabsorbed
H+ secretion continues and produces titratable acid and NH4+ to generate more new HCO3-
Acid excreted in urine
What is metabolic alkalosis?
Excessive loss of H+ from body = less common than metabolic acidosis
What are some causes of metabolic alkalosis?
Loss of HCl from stomach (e.g vomiting), ingestion of alkali/alkali-producing foods, hypersecretion of aldosterone
What causes the rise in [HCO3-]p in metabolic alkalosis?
H+ loss or addition of base
What is uncompensated metabolic alkalosis?
pH > 7.4, [HCO3-]p is high
How does the respiratory system compensate for metabolic alkalosis?
Increased pH slows ventilation = CO2 retained, PCO2 rises
[H+]p rises which lowers pH and [HCO3-] rises further
Why can’t all of the filtered HCO3- be reabsorbed in metabolic alkalosis?
Filtered HCO3- load is so large compared to normal that there is too much to completely reabsorb
How is metabolic alkalosis corrected?
No titratable acid or NH4+ generated so no new HCO3- produced, HCO3- is excreted in urine (alkaline urine), [HCO3-]p falls back towards normal
