K8 - Acid Base Balance II- Whole body acid-base balance Flashcards
When will a person have a normal acid- base balance?
- Plasma pH close to 7.4
- [HCO3-]p close to 25 mol/l
- Arterial PCO2 close to 40mmHg
what is compensation?
if normal acid-base balance is disrupted then priority is to restore pH to 7.4 irrespective of bicarbonate levels and partial pressure of CO2
what is correction?
restoration of pH and bicarbionate and PCO2 to normal
Name two disturbances of respiratory origin
- respiratory acidosis (plasma pH falls)
- respiratory alkalosis (plasma pH rises)
Name two Disturbances of non-respiratory origin
- metabolic acidosis (plasma pH falls)
- metabolic alkalosis (plasma pH rises)
how the body initally limits pH changes?
- immediate dilution of acid or base in ECF
- blood buffers
- buffers in the ECF - bicarbonate
stores deplete very quickly so kidney has to rectify the problem
What can measure pH and PCO2?
A blood-gas analyser
What is the name for the diagram which shows [HCO3-]p and plasma pH?
Davenport Diagram
What is respiratory acidosis?
retention of CO2 by the body
Give 5 examples of respiratory acidosis
- chronic bronchitis
- chronic emphysema
- airway restriction (bronchial asthma, tumour)
- chest injuries
- respiratory depression
what causes respiratory acidosis?
CO2 retention drives equilibrium to the right
CO2 + H2O >< H2CO3 >< H+ + HCO3-
so both [H+]p and [HCO3-]p rise and the increases [H+] results in acidosis
what range of values does respiratory acidosis occur in?
pH < 7.35 and PCO2 > 45 mmHg
Is there extracellular buffering in respiratory disorders?
virtually none
What stimulates H+ secretion into the filtrate to compensate for respiratory acidosis?
CO2 retention stimulates H+ secretion into the filtrate
Describe the events that happen after H+ secretion is stimulated for respiratory acidosis compensation?
- all filtered HCO3- is reabsorbed
- H+ continues to be secreted and generates titratable acid (TA) and NH4+
- acid is excreted and “new” HCO3- is added to the blood
Why does [HCO3-]p rise?
- as a result of the disorder
- as a result of the renal compensation
What is correction process in respiratory acidosis?
Lowering PCO2 by restoration of normal ventilation
how does respiratory acidosis appear on davenport diagram?
pH too low and bicarbonate too high
What is respiratory alkalosis?
Excessive removal of CO2 by the body
When would respiratory alkalosis occur? 3 examples
- low inspired PO2 at altitude
- hyperventilation (causes include fever, brainstem damage)
- hysterical over breathing
What does respiratory alkalosis do to the buffer equation?
drives equilibrium to the left resulting in [H+]p and [HCO3-]p decrease
What indicates respiratory alkalosis?
pH> 7.45
PCO2< 35mmHg
what is the compensatory mechanism for respiratory alkalosis?
- excessive removal of CO2 reduces H+ secretion into the filtrate
- H+ secretion is insufficient to reabsorb the filtered bicarbonate
- bicarbonate is excreted into the urine
- no titratable acid or NH4+ is produced
What does renal compensation for respiratory alkalosis do to [HCO3-]p?
Lowers [HCO3-]p
What does correction require in respiratory alkalosis?
restoration of normal ventilation
how does respiratory acidosis appear on davenport diagram?
pH too high and bicarbonate too low
What is metabolic acidosis ?
Excess H+ from any source other than CO2
Give 3 examples of metabolic acidosis
- ingestion of acids or acid- producing foodstuffs
- excessive metabolic production of H+ (lactic acid during exercise or ketoacidosis)
- excessive loss of base from the body
In metabolic acidosis what is depleted as a result of buffering excess H+ or loss of HCO3- from the body?
[HCO3-] from the body
What is metabolic acidosis indicated by?
pH less than 7.35
[HCO3-]p is low
Explain respiratory compensation for metabolic acidosis
- decrease in plasma
- pH stimulates peripheral chemoreceptors
- ventilation is quickly increased and more carbon dioxide is blown off
- H+ and bicarbonate both lowered as there isn’t any carbon dioxide to make them
Explain correction for metabolic acidosis
- Filtered HCO3- is very low and very readily reabsorbed
- H+ secretion continues and produces TA & NH4+ to generate more “new” HCO3-
- The acid load is excreted (urine is acidic) and [HCO3-]p is restored
- Ventilation can then be normalised
how does metabolic acidosis appear on davenport diagram?
pH to low and bicarbonate too low
What does metabolic alkalosis result in?
Excessive loss of H+ from the body
Give 3 examples of metabolic alkalosis
- loss of HCL from the stomach (vomiting)
- ingestion of alkali or alkali- producing foods (ingestion of NaHCO3 as an antacid, though not a problem with modern antacids)
- aldosterone hypersecretion (causes stimulation of Na+/H+ exchange at the apical membrane of the tubule; acid secretion)
In metabolic alkalosis what happens to [HCO3-]p as a result of loss of H+ or addition of base?
[HCO3-]p rises
What is uncompensated metabolic alkalosis indicated by?
- pH > 7.45
- [HCO3-]p high
Explain Respiratory compensation for metabolic alkalosis
- Increased pH slows ventilation
- CO2 retained, PCO2 rises
- [H+]p rises, lowering pH
- [HCO3-]p also rises further
In metabolic alkalosis why is not all of the filtered HCO3- reabsorbed?
Because HCO3- is so large compared to normal
Explain the renal correction for metabolic alkalosis
- filtered bicarbonate load is so large compared to normal that not all the filtered bicarbonate is reabsorbed
- no titrateable acid or NH4+ is generated
- bicarbonate is excreted (alkaline urine)
- bicarbonate levels fall back to normal
