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ESA 3 - Respiratory System > Acid-Base Balance > Flashcards

Acid-Base Balance Flashcards

1
Q

What should the pH of plasma be?

A

7.35-7.45

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2
Q

What happens if a patient has alkalaemia?

A

Alkalaemia lowers free calcium by causing Ca2+ ions to come out of solution.
It can lead to paraesthesia and tetany (seizures)
It is worse than acidaemia: if pH is above 7.55 then mortality is 45%, if pH is over 7.65 then 80% mortality!

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3
Q

What is acidaemia?

A

Increases in plasma potassium ion concentration effects excitability of cells - particularly cardiac muscle which can lead to arrhythmias.

Also, increasing [H+] affects many enzymes —> denatures proteins —> effects muscle contractility, glycolysis, hepatic function

The effects are severe it pH falls below 7.1 and life threatening if below 7.0

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4
Q

Describe the buffer system

A

Dissolved CO2 reacts with water to form H+ and HCO3- (reversible)

Net direction depends on the concentrations of reactants and products

pH depends on how much CO2 reacts to form H+

  • [CO2] dissolved pushes reaction to the right
  • [HCO3-] pushes reaction to the left
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5
Q

What does plasma pH depend on?

A

The ratio of [HCO3-]:pCO2

PCO2 is determined by respiration

  • Controlled by chemoreceptros
  • Disturbed by respiratory disease

[HCO3-] determined by the kidneys

  • Controlled by the kidneys
  • Disturbed by metabolic and renal disease
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6
Q

How do the kidneys control pH?

A

They can vary the recovery of HCO3- and the active secretion of H+

(Kidneys take time to adjust to change in pH)

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7
Q

How do the lungs control pH?

A

Alveolar ventilation allows diffusion of oxygen into blood and CO2 out of blood - controls pO2 and pCO2

Rate of ventilation is controlled by chemoreceptors

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8
Q

What is the pH of arterial blood determined by?

A

Ratio of pCO2:[HCO3-]

HCO3- made in RBCs BUT the concentration present is controlled by the kidneys.

Normal [HCO3-] is 22-26mmol/l but it can be changed to maintain pH.

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9
Q

Why does the acid produced due to metabolism not deplete HCO3-?

A

The kidneys recover all filtered HCO3-

Proximal tubule makes HCO3- from amino acids, putting NH4+ into urine

Distal tubule makes HCO3- from CO2 and H2O; the H+ is buffered by phosphate and ammonia in the urine.

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10
Q

Describe the renal control of HCO3-

A

HCO3- filtered at the glomerulus

Mostly recovered in PCT

H+ excretion linked to Na+ entry in PCT

H+ reacts with HCO3- in the lumen to form CO2 which enters cell

Converted back to HCO3- which enters ECF

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11
Q

How do the proximal tubules make HCO3-?

A

Glutamine is broken down into a-ketoglutarate

  • Produces HCO3- and NH4+
  • HCO3- enters ECF
  • NH4+ enters lumen (urine)
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12
Q

How do the DCT and CD control HCO3-?

A

DCT and CD secrete H+ produced from reaction of CO2 with water.

H+ ions are actively secreted

H+ buffered by ammonia and phosphate to make products which are excreted

No CO2 is formed to re-enter the cell

Allows HCO3- to enter plasma.

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13
Q

How does ammonia help to control HCO3-?

A

Ammonia = important response to control acid load

Ammonium generation from glutamine in PCT can be increased in response to low pH.

  • NH3 freely moves into lumen and throughout the interstitium
  • H+ actively pumped into lumen in DCT and CD
  • H+ combines with ammonium to make ammonium
  • NH4+ can also be taken up in TAL and transported to insterstitium and dissociates to H+ and NH3 - lumen of collecting ducts.
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14
Q

What is the minimum pH of urine?

A

4.5

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15
Q

Why will there be no HCO3- in the urine?

A

It has all been recovered by the kidney. There is H+ though.

Some H+ buffered by phosphate.
Some has reacted with ammonia to form ammonium

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16
Q

Why does acidosis cause hyperkalaemia?

A

In response to inwards movement of H+, there is an outward movement of k+ ion.

This decreases the potassium excretion in the distal nephron.

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17
Q

Why does alkalosis cause hypokalaemia?

A

Potassium ions move into cells in repose to H+ moving out.

Enhanced excretion of K in the distal nephron.

(Ca is the main issue in alkalosis though)

18
Q

How can hyperkalaemia lead to alkalosis and hypokalaemia lead to acidosis?

A

Hyperkalaemia makes intracellular pH of tubular cells more alkaline as:

  • H+ ions move out of the cells
  • This favours HCO3- excretion therefore causing metabolic acidosis

Hypokalaemia makes the intracellular pH of tubular cells more acidic

  • H+ ions move into the cells.
  • This favours H+ excretion and HCO3- recovery therefore causing metabolic alkalosis.
19
Q

How does ventilation disrupt acid base balance?

A

Hypoventilation —> Hypercapnia (pCO2 rise) —> fall in plasma pH —> respiratory acidosis

OR

Hyperventilation —> hypoventilation (pCO2 fall) —> rise in pH —> respiratory alkalaemia

PH goes in opposite direction of pCO2

20
Q

List the Characteristics of hypercapnia

A

High pCO2
Normal HCO3-
Low pH

21
Q

List the characteristics of hypocapnia

A

Low pCO2
Normal HCO3-
Raise pH

22
Q

How does the kidney compensate for respiratory changes?

A

Kidneys increase [HCO3-] to compensate for respiratory acidosis and decrease for alkalosis to get the ratio of pCO2:[HCO3-] similar.

But, this can take 2-3 days.

23
Q

What are the characteristics of compensatory respiratory acidosis?

A

High pCO2
Raised [HCO3-]
Relatively normal pH

24
Q

What ate the characteristics of compensated respiratory alkalosis

A

Low pCO2
Lowered [HCO3-]
Relatively normal pH

25
Q

How does metabolic acid cause a metabolic acidosis?

A

Acid - reacts with and removes HCO3-

This results in a fall in [HCO3-] which causes a fall in pH

This metabolic acidosis

26
Q

What is the anion gap?

A

This is the difference between the measured cations and measured anions.

[Na+]+ [K+]) - ([Cl-] + [HCO3-])

Normal gap is 10-18mmol/L as not measuring all the anions.

This gap is increased in HCO3- is replaced by other anions.

If a metabolic acid (e..g lactic acid) reacts with HCO3-, the anion of the acid replaces HCO3-.

27
Q

What will renal / metabolic acidosis to do the anion gap?

A

Metabolic causes increase the anion gap as the HCO3- is replaced by another organic anion from an acid.

In renal causes of acidosis anion gap will be unchanged - as despite not making enough HCO3, it is replaced by Cl-.

28
Q

What are the characteristics of compensating metabolic acidosis

A

Peripheral chemoreceptors (carotid bodies) detect drop in pH. This stimulates ventilation which leads to a decrease in pCO2.

Therefore it is characterised by:

  • Low HCO3-
  • Lowered pCO2
  • Nearer to normal pH.
29
Q

What is metabolic alkalosis?

A

If [HCO3-] increases, this is metabolic alkalosis.

Therefore, will have:

  • Normal pCO2
  • Raised HCO3-
  • Increased pH

Cannot normally be compensated to a great extent by reducing breathing - need to maintain pO2

Should be easy for kidneys to correct

30
Q

What conditions lead to respiratory acidosis?

A

Type 2 respiratory failure:

  • Low pO2 and high PCO2
  • The alveoli cannot be properly ventilated
  • Severe COPD, Severe asthma, drug overdose, neuromuscular disease

It can be compensated for my increase in [HCO3-] - in chronic conditions, it is usually well compensated for so that pH is near normal.

31
Q

What conditions can lead to respiratory alkalosis?

A

Hyperventilation:

  • Anxiety / panic attacks - acute settings
  • Low pCO2, rise in pH

Hyperventilation in response to long-term Hypoxia - Type 1 respiratory failure.

  • Low pCO2 with initial rise in pH
  • Chronic hyperventilation can be compensated for by fall in [HCO3-]
  • Can restore pH to near normal.
32
Q

What condition can lead to metabolic acidosis with an increased anion gap?

A

If anion gap in INCREASED - indicates a metabolic production of an acid.

  • Ketoacidosis - diabetes
  • Lactic acidosis - exercising to exhaustion, poor tissue perfusion
  • Uraemic acidosis - advanced renal failure - reduced acid secretion, build up of phosphate, sulphate and urate in the blood.
33
Q

What conditions can lead to metabolic acidosis with a normal anion gap?

A

Normal - HCO3- is replaced by Cl-

Renal tubular acidosis (rare)

  • Problems with transport mechanisms in the tubules
  • Type 1 (distal) RTA - inability to pump out H+
  • Type 2 (proximal) RTA (very rare) - problems with HCO3- reabsorbtion.

Severe persistent diarrhoea can also lead to metabolic acidosis due to loss of HCO3-

  • Replaced by Cl-
  • Therefore anion gap unaltered.
34
Q

What effect does metabolic acidosis have on potassium?

A

Cause increased reabsorbtion of K+ by the kidneys and movement of K+ out of cells. (Hyperkalaemia)

However in diabetic ketoacidosis may be a total body depletion of K+.

  • K+ moves out of cells (due to acidosis and lack of insulin)
  • BUT osmotic diuresis means K+ lost in urine
35
Q

What conditions can lead to metabolic alkalosis?

A

In alkalosis - HCO3- is retained in place of Cl-

Stomach is a major site of HCO3- production

  • By-product of H+ secretion
  • Severe prolonged vomitting -loss of H+
  • Mechanical drainage of the stomach

Also:

  • Potassium depletion / mineralcoticoid excess
  • Certain diuretics (loop and thiazide)
36
Q

Summarise metabolic alkalosis

A

HCO3- increase e.g. after persistent vomiting -This should be very easy to correct. HCO3- can be excreted very rapidly following infusion of HCO3-

Corrected by:
-Rise in pH of tubular cells leads to fall in H+ excretion and reduction in HCO3- recovery

BUT
Problems if there is also volume depletion
-Capacity to loose HCO3- is reduced because of high rate of Na+ recovery
-Recovering Na+ favours excretion and HCO3- recovery.

37
Q

What effect does metabolic alkalosis have on potassium?

A

Less H+ excretion in nephron leads to more K+ excreted.

Alkalosis also causes movement of K+ ions into cells.

This leads to hypokalaemia

38
Q

What changes will you get in respiratory acidosis or alkalosis?

A

PCO2 not normal, [HCO3-] is normal and pH has changed in opposite direction to pCO2.

39
Q

What changes will you get in metabolic acidosis or alkalosis?

A

[HCO3-] is not normal, pCO2 is normal and pH has changed in the same direction as [HCO3-].

40
Q

What values do you get if it is compensatory respiratory acidosis?

A

High pCO2
[HCo3-] is raised
pH is relatively normal

41
Q

If [HCO3-] is low, pCO2 is low, pH is normal, what could be wrong?

A

Compensatory respiratory alkalosis or compensated metabolic acidosis

Check respiratory disease or altitude exposure and anion gap to work out what it is.

ESA 3 - Respiratory System (24 decks)

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