Physiology: Acidosis & Alkalosis Flashcards
If a person is of normal acid-base status, what will their values of plasma... - pH - [HCO3-] - PCO2 ... be?
- pH: ~7.4 (range 7.35 - 7.45)
- PCO2: ~40 mmHg (range 35 - 45)
Acidosis is defined as a pH of…?
Alkalosis is defined as a pH of…?
Acidosis: pH <7.35
Alkalosis: pH >7.45
Moving past what plasma pH levels will result in death?
pH <6.8 or >8
If normal acid-base balance is disrupted, what 2 processes occur?
Compensation, then correction
What is the difference between compensation and correction for acid-base disturbances?
Compensation = the restoration of pH to 7.4 as soon as possible, irrespective of what happens to [HCO3-] and PCO2
Correction = restoration of [HCO3-] and PCO2 to normal following restoration of pH
How does the body initially try to buffer a pH change? (2)
- The acid or base is immediately diluted in the ECF
- Blood and ECF buffers (e.g., HCO3-, Hb) minimise the pH change
Initial buffer stores are quickly depleted, so how does the kidney get involved? (3)
The kidneys rectify the buffer stores by H+ secretion which leads to…
- Reabsorption of HCO3-
- Secretion of H+ as TA (which generates a ‘new’ HCO3-)
- Secretion of H+ as NH4+ (which generates a ‘new’ HCO3-)
What is a Davenport diagram used for?
To show the relationship between plasma pH, [HCO3-] and PCO2 following a respiratory or metabolic acid-base disturbance
On a Davenport diagram, where would the dot for a ‘normal’ acid-base status lie:
x-axis: ?
y-axis: ?
x-axis: pH = 7.4
y-axis: [HCO3-] = 25 mmol/l
On a Davenport diagram, in what directions does the dot for a 'normal' acid-base status move in the event of an uncompensated... - Respiratory acidosis - Respiratory alkalosis - Metabolic acidosis - Metabolic alkalosis ...?
- Respiratory acidosis: to the left (pH lower) and up (HCO3- higher)
- Respiratory alkalosis: to the right (pH higher) and down (HCO3- lower)
- Metabolic acidosis: to the left (pH lower) and down (HCO3- lower)
- Metabolic alkalosis: to the right (pH higher) and up (HCO3- higher)
What causes respiratory acidosis? In what conditions may this occur?
Retention of CO2 by the body
This may occur in COPD, asthma, restrictive airway tumours, respiratory depression etc.
How does retention of CO2 cause respiratory acidosis?
Excess CO2 drives the CO2-HCO3- equilibrium to the right
CO2 + H2O – > H2CO3 – > H+ + HCO3-
This results in higher plasma conc. of H+ and HCO3- (acidosis occurs as H+ increase is greater than that of HCO3-)
What values of pH and PCO2 indicate uncompensated respiratory acidosis?
pH <7.35
PCO2 > 45 mmHg
(i.e., both pH and PCO2 outwith the normal ranges but in opposite directions)
The renal/respiratory system compensates for respiratory acidosis.
Why?
The renal system
Since the respiratory system is the underlying cause of the imbalance (through CO2 retention), it is unable to compensate for it
How does the renal system compensate for respiratory acidosis?
- CO2 retention stimulates H+ secretion
- This stimulates HCO3- reabsorption, H+ excretion as titratable acid, and H+ excretion as NH4+
- As a result, acid is removed from the body and ‘new’ HCO3- is added to the blood
What is achieved by renal compensation for the respiratory acidosis?
What now needs to be corrected?
pH is raised back to ~7.4
However, renal compensation further raises [HCO3-] and this needs to be corrected
Following restoration to pH 7.4, how are HCO3- and PCO2 corrected?
By restoring normal ventilation (e.g., by removing obstruction, managing asthma/COPD), PCO2 can be lowered to normal
This lowers HCO3- to normal too
What causes respiratory alkalosis? In what conditions may this occur?
Excessive removal of CO2 by the body through hyperventilation
This may occur in high altitudes, fever, brainstem damage, hysteria etc.
How does excess CO2 removal cause respiratory alkalosis?
CO2 removal drives the CO2-HCO3- equilibrium to the left
CO2 + H2O < – H2CO3 < – H+ + HCO3-
This results in lower plasma conc. of H+ and HCO3- (alkalosis occurs as H+ decrease is greater than that of HCO3-)
What values of pH and PCO2 indicate uncompensated respiratory alkalosis?
pH >7.45
PCO2 <35 mmHg
(i.e., both pH and PCO2 outwith the normal ranges but in opposite directions)
The renal/respiratory system compensates for respiratory acidosis.
Why?
The renal system
Since the respiratory system is the underlying cause of the imbalance (through excess CO2 removal), it is unable to compensate for it
How does the renal system compensate for respiratory alkalosis?
- Excessive CO2 removal reduces H+ secretion into the tubule
- H+ secretion is insufficient to reabsorb all of the filtered HCO3-
- As a result, HCO3- is excreted from the body and the urine is alkaline. No ‘new’ HCO3- is formed as no titratable acid and NH4+ are formed and excreted in the urine
What is achieved by renal compensation for the respiratory alkalosis?
What now needs to be corrected?
pH is lowered back to ~7.4
However, renal compensation further lowers [HCO3-] and this needs to be corrected
Following restoration to pH 7.4, how are HCO3- and PCO2 corrected?
By restoring normal ventilation (e.g., by stopping the hyperventilation), PCO2 can be raised back to normal
This raises HCO3- to normal too
What is the most common acid-base disturbance?
Metabolic acidosis
What causes metabolic acidosis? When might this occur? (3)
Excess H+ from any source other than CO2
This may occur due to…
- Ingestion of acids or acid-producing foodstuffs
- Excessive metabolic production of H+ e.g., lactic acid in exercise, ketoacidosis in DM
- Excessive loss of base e.g., diarrhoea
How does excess H+ cause metabolic acidosis?
Excess H+ drives the CO2-HCO3- equilibrium to the left due to HCO3- depletion
CO2 + H2O –> H2CO3 – > H+ + HCO3-
This increases H+ further which drives the metabolic acidosis
What values of pH and [HCO3-] indicate uncompensated metabolic acidosis?
pH <7.35
[HCO3-] <23 mmol/l
(i.e., both pH and [HCO3-] are outwith the normal ranges and in the same direction (both are lower))
Why can both the renal and respiratory systems help compensate for/correct a metabolic acidosis?
Because neither of them are the cause of the imbalance
How does the respiratory system compensate for metabolic acidosis?
- Decreased plasma pH stimulates peripheral chemoreceptors
- Ventilation is increased to blow off more CO2
- As a result, H+ levels are lowered
What is achieved by respiratory compensation for the metabolic acidosis?
What now needs to be corrected?
pH is raised back to ~7.4
However, respiratory compensation further lowers [HCO3-] and this needs to be corrected
Following restoration to pH 7.4, how are HCO3- and PCO2 corrected by…
- The renal system
- The respiratory system
…?
The renal system:
- As HCO3- is low, it is readily reabsorbed as there is less of it
- Normal H+ secretion continues, so more will produce titratable acid and NH4+, which leads to acid excretion in the urine and generation of more ‘new’ HCO3-
- These both increase plasma [HCO3-]
The respiratory system:
- Once [HCO3-] is restored, ventilation can normalise
Why is respiratory compensation essential in metabolic acidosis?
The kidneys cannot immediately excrete the acid load whereas respiratory compensation can kick in within a matter of minutes
What causes metabolic alkalosis? When might this occur? (3)
Excessive loss of H+ from the body
This may occur due to…
- Loss of HCl from the stomach
- Ingestion of alkali or alkali-producing foods
- Aldosterone hypersecretion (stimulates Na+/H+ exchange in the tubules, resulting in H+ secretion)
How does loss of H+ cause metabolic alkalosis?
[HCO3-] rises as a result of H+ loss or addition of a base
What values of pH and [HCO3-] indicate uncompensated metabolic alkalosis?
pH >7.45
[HCO3-] >27 mmol/l
(i.e., both pH and HCO3- outwith the normal ranges, and both in the same direction (both are too high))
Why can both the renal and respiratory systems help compensate for/correct a metabolic alkalosis?
Because neither of them are the cause of the imbalance
How does the respiratory system compensate for metabolic alkalosis?
- Increased plasma pH slows ventilation via peripheral chemoreceptors
- Slowed ventilation results in CO2 retention
- As a result, H+ levels rise
What is achieved by respiratory compensation for the metabolic alkalosis?
What now needs to be corrected?
The pH is lowered back down to ~pH 7.4
However, respiratory compensation further increases [HCO3-] and this needs to be corrected
Following restoration to pH 7.4, how are HCO3- and PCO2 corrected by…
- The renal system
- The respiratory system
…?
The renal system:
- Filtered HCO3- load is so large that not all of it can be reabsorbed, and so some is excreted in the urine
- All of the secreted H+ is used up for reabsorption of the HCO3-, and so no titratable acid or NH4+ is formed. This means that no acid is excreted in the urine
- These both reduce plasma [HCO3-]
The respiratory system:
- Once [HCO3-] is restored, ventilation can normalise
Reminder:
Look over how to draw a Davenport diagram for normal acid-base status, all of these acid-base disturbances, and all of the compensatory and corrective changes for each!
