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Urinary System > Acid-Base Balance 2 > Flashcards

Acid-Base Balance 2 Flashcards

1
Q

What do respiratory disorders effect?

A

PCO2

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2
Q

What do renal disorders effect?

A

[HCO3-]

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3
Q

Name acid-base balance disorders

A
  1. Respiratory acidosis
  2. Respiratory alkalosis
  3. Metabolic acidosis
  4. Metabolic alkalosis
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4
Q

What occurs in respiratory acidosis?

A

pH has fallen and it is due to a respiratory change, so Pco2 must have increased. Respiratory acidosis results from reduced ventilation and therefore retention of CO2.

Decreased pH with increased HCO3-

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5
Q

What are the causes of respiratory acidosis?

A

Acute:
Drugs which depress medullary resp centres (barbiturates and opiates) and obstruction of major airways

Chronic:
Lung diseases i.e. bronchitis, emphysema, asthma

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6
Q

What is the compensatory mechanism for respiratory acidosis?

A

Increase [HCO3-]:
The ↑ Pco2 will → ↑ secretion of H+ and ↑ HCO3- . Acid conditions stimulate renal glutaminase so get more NH3 produced, BUT, it takes time.

Renal compensation increases generation of new HCO3- and ↑ reabsorption and ↑ excretion of H+

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7
Q

What is the problem of the renal compensatory mechanism for respiratory acidosis?

A

Although the renal compensation to ↑ HCO3- protects the pH, it does not correct the original disturbance. Only restoration of normal ventilation can remove the primary disturbance.

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8
Q

What occurs in chronic respiratory acidosis?

A

Blood gas values are never normalised. They may be eg pH 7.32, Pco2 65mmHg (8.67 kPa), [HCO3-] 38 mmoles/l. The underlying disease process prevents the correction of ventilation, but because the kidney maintains high [HCO3-], the pH is protected.

Lung disease always have aberrant PCO2 and [HCO3], but as long as kidney function isn’t impaired pH is maintained. Problems arise when lung disease develop renal dysfunction.

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9
Q

What occurs in metabolic acidosis?

A

An acidosis of metabolic origin must be due to a ↓ [HCO3-].

So, ↓ [HCO3-], either due to ↑ buffering of H+ or direct loss of HCO3-

This means that there is an increase in CO2, so to protect the pH, Pco2 must be decreased (by blowing off CO2 by increasing ventilation)

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10
Q

What are the causes of metabolic acidosis?

A
  1. ↑ H+ production, as in ketoacidosis of a diabetic or in lactic acidosis.
  2. Failure to excrete the normal dietary load of H+ as in renal failure.
  3. Loss of HCO3- as in diarrhoea ie. failure to reabsorb intestinal HCO3-.
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11
Q

What is the compensatory mechanism for metabolic acidosis?

A

Stimulates ventilation so that Pco2 falls. The ↑ in ventilation (kussmaul breathing) is in depth rather than rate, reaching a maximum of 30 l/min compared to normal 5-6 l/min when the arterial pH falls to 7.0.

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12
Q

What is Kussmaul breathing?

A

Degree of hyperventilation in metabolic acidosis which is an established clinical sign of renal failure or diabetic ketoacidosis.

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13
Q

What is the problem in the compensatory mechanism for metabolic acidosis?

A

Normally the kidneys would correct disturbance by restoring [HCO3-] and excrete H+ ions, but as PCO2 is decreases (to protect pH):
• Total amount of H+ secreted decreases
• Therefore decreases HCO3 reabsorption (joins to reform CO2)
• But increase HCO3 generation and H+ excreted as titratable acid and NH4

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14
Q

What disturbances stimulate ventilation?

A

Increase in CO2 or H+ ions

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15
Q

What is the mechanism to compensate for increase metabolic H+ within the body in metabolic acidosis?

A
  1. Immediate buffering in ECF and then ICF.
  2. Respiratory compensation within minutes.
  3. Renal correction of the disturbance takes longer to develop the full response to ↑ H+ excretion and generate new HCO3- because renal glutaminase takes 4-5 days to reach maximum.
  4. As HCO3- starts to ↑, respiratory compensation begins to wear off until eventually get rid of all excess H+ .
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16
Q

What occurs in respiratory alkalosis?

A

Due to a fall in Pco2 and this can only occur through increased ventilation and CO2 blow-off

17
Q

What are causes of respiratory alkalosis?

A

Acute:
Voluntary hyperventilation, aspirin, first ascent to altitude

Chronic:
Long term residence at altitude, decreased Po2 to < 60mmHg (8kPa) stimulates peripheral chemoreceptors to increase ventilation.

18
Q

What is the compensatory mechanism for respiratory alkalosis?

A

[HCO3] should ↓:
If ↓ Pco2, less H+ is available for secretion, therefore less of the filtered load of HCO3- is reabsorbed (as it’s unneeded) so HCO3- is lost in the urine.

Ventilation would need to be corrected.

19
Q

What occurs in metabolic alkalosis?

A

[HCO3] must have ↑ and Pco2 will ↑ to protect the pH

Increased HCO3 decreases H+ and so equation shifted to the right, causing CO2 to decrease (and therefore decrease in PCO2)

20
Q

What are the causes of alkalosis?

A
  1. ↑ H+ ion loss- vomiting loss of gastric secretions
  2. ↑ renal H+ loss- aldosterone excess, excess liquorice ingestion
  3. Excess administration of HCO3- if renal function impaired.
  4. Blood transfusions because bank blood contains citrate to prevent coagulation, which is converted to HCO3-, but need at least 8 units to have this effect.
21
Q

What is the compensatory mechanism for metabolic alkalosis?

A

Hypoventilation to increase Pco2 which corrects pH, but elevated HCO3.

Greatly ↑ filtered load of HCO3- exceeds the level of H+ secretion to reabsorb it, so the excess is lost in the urine and H+ is reabsorbed;.

22
Q

What is the summary of A/B disturbances is resp. acidosis?

A
  • ↑ H+
  • ↓ pH
  • Disturbance: ↑ [PCO2]
  • Compensation: ↑ [HCO3]
23
Q

What is the summary of A/B disturbances is resp. alkalosis?

A
  • ↓ H+
  • ↑ pH
  • Disturbance: ↓ [PCO2]
  • Compensation: ↓ [HCO3]
24
Q

What is the summary of A/B disturbances is metabolic. acidosis?

A
  • ↑ H+
  • ↓ pH
  • Disturbance: ↓ [HCO3]
  • Compensation: ↓ [PCO2]
25
Q

What is the summary of A/B disturbances is metabolic . alkalosis?

A
  • ↓ H+
  • ↑ pH
  • Disturbance: ↑ [HCO3]
  • Compensation: ↑ [PCO2]
26
Q

What are disturbances cause a decrease in pH (acidosis)?

A

Decreased HCO3 or increased PCO2

27
Q

What are disturbances cause an increase in pH (alkalosis)?

A

Increased HCO3 or decreased PCO2

28
Q

Describe the change in pH in chronic resp acidosis compared to acute

A

For a given increase in Pco2, there is a smaller decrease in pH in chronic respiratory acidosis than in acute respiratory acidosis.

This is because initially [HCO3] is raised only by titratable acid, but after 4-5 days, NH3 production allows raise in [HCO3] also

29
Q

Why does high acidity cause hyperkalaemia?

A

As H+ ions are buffered intracellularly in exchange for K+ ions. Danger of Ventricular Fibrillation if increase K+ in ECF.

30
Q

What is the treatment of hyperkalaemia?

A
  1. Insulin (+ glucose if non-diabetic), stimulates cellular uptake of K+
  2. Also for hyperkalaemia, calcium resonium - exchanges Ca ions or K ions (12-24hrs)
  3. Ca gluconate (IV) -> decrease excitability of heart, stabilises cardiac muscle cell membranes.
31
Q

What can vomiting cause?

A
  • Loss of NaCl and H2O -> hypovolaemia

* Loss of HCl -> metabolic alkalosis

32
Q

How is metabolic alkalosis exacerbated in vomiting?

A

The hypovolaemia will stimulate aldosterone to increase distal tubule Na+ reabsorption.

Under conditions of Na+ reabsorption, (and due to loss of Cl-), the main ion exchanged for Na+ is H+.

So metabolic alkalosis is exacerbated as H+ are excreted in order to pump Na into the capillary to draw water out to increase BV.

Also:
The respiratory compensation for the metabolic alkalosis ie ↑Pco2 helps drive the H+ secretion and exacerbates the metabolic alkalosis by adding yet more HCO3- to the plasma.

Shows that restoration of volume is more important than correction of metabolic alkalosis

33
Q

Why does alkalosis occur in vomiting and diarrhoea despite loss of acid and alkali?

A

Become alkalotic because ↓ ECF volume -> ↑ aldosterone -> “contraction alkalosis”

34
Q

Why does liquorice cause metabolic alkalosis?

A

Contains glycyrrhizic acid, which is very similar to aldosterone, so that excess ingestion -> metabolic alkalosis.

35
Q

Define the anion gap

A

The difference between the sum of the principal cations (Na+ and K+) and the principal anions in the plasma (Cl- and HCO3-)

36
Q

What is the normal anion gap value?

A

14-18mmole/L

37
Q

Why is the anion gap measured in metabolic acidosis?

A

2 patterns: either no change or gap increases

If the acidosis is due for example to a loss of bicarbonate from the gut, then the reduction of bicarbonate is compensated by an increase in chloride and so there is no change in anion gap.

However, in eg lactic or diabetic acidosis, the reduction in bicarbonate is made up by other anions such as lactate, acetoacetate, Beta-OH butyrate and so the anion gap is increased.

Urinary System (31 decks)

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