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LR Renal > Physiology: Acidosis & Alkalosis > Flashcards

Physiology: Acidosis & Alkalosis Flashcards

1
Q 
If a person is of normal acid-base status, what will their values of plasma...
- pH
- [HCO3-]
- PCO2
... be?
A
  • pH: ~7.4 (range 7.35 - 7.45)
  • PCO2: ~40 mmHg (range 35 - 45)
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2
Q

Acidosis is defined as a pH of…?

Alkalosis is defined as a pH of…?

A

Acidosis: pH <7.35

Alkalosis: pH >7.45

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3
Q

Moving past what plasma pH levels will result in death?

A

pH <6.8 or >8

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4
Q

If normal acid-base balance is disrupted, what 2 processes occur?

A

Compensation, then correction

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5
Q

What is the difference between compensation and correction for acid-base disturbances?

A

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

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6
Q

How does the body initially try to buffer a pH change? (2)

A
  • The acid or base is immediately diluted in the ECF

- Blood and ECF buffers (e.g., HCO3-, Hb) minimise the pH change

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7
Q

Initial buffer stores are quickly depleted, so how does the kidney get involved? (3)

A

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-)
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8
Q

What is a Davenport diagram used for?

A

To show the relationship between plasma pH, [HCO3-] and PCO2 following a respiratory or metabolic acid-base disturbance

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9
Q

On a Davenport diagram, where would the dot for a ‘normal’ acid-base status lie:
x-axis: ?
y-axis: ?

A

x-axis: pH = 7.4

y-axis: [HCO3-] = 25 mmol/l

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10
Q 
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
...?
A
  • 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)
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11
Q

What causes respiratory acidosis? In what conditions may this occur?

A

Retention of CO2 by the body

This may occur in COPD, asthma, restrictive airway tumours, respiratory depression etc.

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12
Q

How does retention of CO2 cause respiratory acidosis?

A

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-)

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13
Q

What values of pH and PCO2 indicate uncompensated respiratory acidosis?

A

pH <7.35
PCO2 > 45 mmHg

(i.e., both pH and PCO2 outwith the normal ranges but in opposite directions)

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14
Q

The renal/respiratory system compensates for respiratory acidosis.

Why?

A

The renal system

Since the respiratory system is the underlying cause of the imbalance (through CO2 retention), it is unable to compensate for it

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15
Q

How does the renal system compensate for respiratory acidosis?

A
  • 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
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16
Q

What is achieved by renal compensation for the respiratory acidosis?
What now needs to be corrected?

A

pH is raised back to ~7.4

However, renal compensation further raises [HCO3-] and this needs to be corrected

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17
Q

Following restoration to pH 7.4, how are HCO3- and PCO2 corrected?

A

By restoring normal ventilation (e.g., by removing obstruction, managing asthma/COPD), PCO2 can be lowered to normal

This lowers HCO3- to normal too

18
Q

What causes respiratory alkalosis? In what conditions may this occur?

A

Excessive removal of CO2 by the body through hyperventilation

This may occur in high altitudes, fever, brainstem damage, hysteria etc.

19
Q

How does excess CO2 removal cause respiratory alkalosis?

A

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-)

20
Q

What values of pH and PCO2 indicate uncompensated respiratory alkalosis?

A

pH >7.45

PCO2 <35 mmHg

(i.e., both pH and PCO2 outwith the normal ranges but in opposite directions)

21
Q

The renal/respiratory system compensates for respiratory acidosis.

Why?

A

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

22
Q

How does the renal system compensate for respiratory alkalosis?

A
  • 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
23
Q

What is achieved by renal compensation for the respiratory alkalosis?
What now needs to be corrected?

A

pH is lowered back to ~7.4

However, renal compensation further lowers [HCO3-] and this needs to be corrected

24
Q

Following restoration to pH 7.4, how are HCO3- and PCO2 corrected?

A

By restoring normal ventilation (e.g., by stopping the hyperventilation), PCO2 can be raised back to normal

This raises HCO3- to normal too

25
Q

What is the most common acid-base disturbance?

A

Metabolic acidosis

26
Q

What causes metabolic acidosis? When might this occur? (3)

A

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
27
Q

How does excess H+ cause metabolic acidosis?

A

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

28
Q

What values of pH and [HCO3-] indicate uncompensated metabolic acidosis?

A

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))

29
Q

Why can both the renal and respiratory systems help compensate for/correct a metabolic acidosis?

A

Because neither of them are the cause of the imbalance

30
Q

How does the respiratory system compensate for metabolic acidosis?

A
  • Decreased plasma pH stimulates peripheral chemoreceptors
  • Ventilation is increased to blow off more CO2
  • As a result, H+ levels are lowered
31
Q

What is achieved by respiratory compensation for the metabolic acidosis?
What now needs to be corrected?

A

pH is raised back to ~7.4

However, respiratory compensation further lowers [HCO3-] and this needs to be corrected

32
Q

Following restoration to pH 7.4, how are HCO3- and PCO2 corrected by…
- The renal system
- The respiratory system
…?

A

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

33
Q

Why is respiratory compensation essential in metabolic acidosis?

A

The kidneys cannot immediately excrete the acid load whereas respiratory compensation can kick in within a matter of minutes

34
Q

What causes metabolic alkalosis? When might this occur? (3)

A

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)
35
Q

How does loss of H+ cause metabolic alkalosis?

A

[HCO3-] rises as a result of H+ loss or addition of a base

36
Q

What values of pH and [HCO3-] indicate uncompensated metabolic alkalosis?

A

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))

37
Q

Why can both the renal and respiratory systems help compensate for/correct a metabolic alkalosis?

A

Because neither of them are the cause of the imbalance

38
Q

How does the respiratory system compensate for metabolic alkalosis?

A
  • Increased plasma pH slows ventilation via peripheral chemoreceptors
  • Slowed ventilation results in CO2 retention
  • As a result, H+ levels rise
39
Q

What is achieved by respiratory compensation for the metabolic alkalosis?
What now needs to be corrected?

A

The pH is lowered back down to ~pH 7.4

However, respiratory compensation further increases [HCO3-] and this needs to be corrected

40
Q

Following restoration to pH 7.4, how are HCO3- and PCO2 corrected by…
- The renal system
- The respiratory system
…?

A

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

41
Q

Reminder:

A

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!

LR Renal (16 decks)

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