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2.2 Clinical Chemistry > Acid Base > Flashcards

Acid Base Flashcards

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1
Q

Define pH

A

A measure of the molar concentration of hydrogen ions in solution

pH = -log10 [H+]

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2
Q

What is increasing H+ concentration responsible for

A

Increasing acidity

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3
Q

What is increasing OH- concentration responsible for?

A

Increasing basicity

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4
Q

What are the two ley properties of the pH system?

A

As pH decreases the [H+] increases

The pH scale is a log scale, it is not linear

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5
Q

What is the pH reference range for plasma?

A

7.35 to 7.45

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6
Q

What is the H+ reference range for plasma

A

35-45 nmol/L

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7
Q

Why is acid base balance needed?
(3)

A

There is always production of acid by the body’s metabolic processes

To maintain balance, these acids need to be excreted or metabolised

The various acids produced by the body are classified as respiratory/volatile acids and as metabolic/fixed acids

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8
Q

How are acids produced by the body classified

A

Respiratory/volatile acids

Metabolic/fixed acids

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9
Q

What is acidosis?
(3)

A

pH below the normal range

Trying to push your pH into an acid range

Less than 7.35

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10
Q

What is alkalosis
(3)

A

pH above the normal range

Trying to push your pH into alkaline range

pH greater than 7.45

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11
Q

What two organs have roles in maintaining acid-base balance

A

Kidneys
Lungs

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12
Q

How do the kidneys regulate acid base balance
(2)

A

They regulate hydrogen ion concentration

They generate and recycle buffers that enter the bloodstream

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13
Q

How do the lungs regulate acid base balance

A

Eliminate carbon dioxide which is acidic

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14
Q

What is considered acidaemia

A

Blood pH less than 7.35

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15
Q

What is considered alkalaemia?

A

Blood pH greater than 7.45

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16
Q

How is carbon dioxide broken down?
(3)

A

CO2 (acid) + H2O

Converted to H2CO3 (Carbonic Acid)

Converted to H+ + HCO3- (bicarbonate ion)(base)

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17
Q

What molecule is acidic?

A

CO2

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18
Q

What molecule is basic

A

HCO3-

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19
Q

Why is carbon dioxide so important in acid-base balance?
(3)

A

Regulation of the amount of CO2 in blood, or more precisely the ratio of bicarbonate to dissolved carbon dioxide concentration is essential for maintaining acid-base balance

CO2 is a major determinant of blood pH because of it’s conversion to carbonic acid

As CO2 concentration rises, so does H+ concentration

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20
Q

What is pCO2?

A

Partial pressure of Carbon dioxide

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21
Q

What controls respiration rate?
(2)

A

pCO2 sensitive chemoreceptors in the brain stem and carotid artery

Respiration rate is increased if pCO2 is rising and decreased if PCO2 is declining

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22
Q

How does increased respiratory rate affect CO2 concentration

A

This increases the rate of CO2 elimination

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23
Q

How does decreased respiratory rate affect CO2

A

This promotes CO2 retention

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24
Q

What might a low CO2 be associated with
(2)

A

Metabolic acidosis

Compensated respiratory alkalosis

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25
Q

What might a high CO2 level be associated with?
(2)

A

Metabolic alkalosis

Compensated respiratory acidosis

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26
Q

What happens to CO2 in red blood cells when they reach the lungs
(3)

A

When the rbcs reach the pulmonary circulation, CO2 diffuses from the blood cells to the alveoli for it to be exhaled

As the rbc is oxygenated, bicarbonate passes from plasma to the red blood cells, buffering hydrogen ions released from haemoglobin

Reversal of the carbonic anhydrase reaction results in production of CO2, this then diffuses from red cells to plasma and ultimately to alveoli

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27
Q

Explain in your own words CO2 diffusion in blood
(3)

A

Two forms of CO2 present, CO2 in red blood cells and bicarbonate in plasma.

CO2 must move from red blood cells to alveoli to be exhaled, therefore CO2 in the form of bicarbonate must first be converted back to CO2

Therefore, bicarbonate pass from the plasma to the red blood cell where it is converted back to CO2 through the reverse of the carbonic anhydrase reaction

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28
Q

Compare the CO2 content of mixed venous blood arriving at the lung vs blood leaving the lungs

A

23.5 mmol/L upon entering

21.5 mmol/L when leaving

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29
Q

What are the five different ways carbon dioxide can be transported in the body?

A

90% is transported as bicarbonate
- 65% of this is in plasma
- 25% of this is in the red blood cells themselves

5% is physically dissolved in the plasma and red cell cytoplasm i.e. as CO2

5% is loosely bound to haemoglobin and plasma proteins

Less than 0.1% is transported as carbonic acid

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30
Q

What is arterial blood gas analysis?
(2)

A

This analysis includes measurement of parameters related to the carbon dioxide content of blood

Partial pressure of carbon dioxide (pCO2) and
Plasma bicarbonate concentration (HCO3-) is measured

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31
Q

What is the only concentration actually measured from arterial blood gas analysis?

A

pCO2

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32
Q

How is HCO3- determined

A

It is calculated from pCO2 and pH

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33
Q

When might CO2 be measured?
(2)

A

Can be measured from plasma or serum by chemical methods

It is included in all chemistry panels containing electrolytes

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34
Q

What is partial pressure of CO2?

A

A measure of the pressure exerted by that small portion (5%) of total carbon dioxide in blood that is dissolved in the aqueous phase of plasma and blood cell cytoplasm

Measure of the CO2 in plasma and rbcs

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35
Q

What happens when there is any deviation from normal pH range?
(4)

A

This alters protein structure and therefore function e.g it affect plasma membrane stability and enzyme activity

The central nervous system function deteriorates

Cardiac contractions become weak and irregular (heart failure)

Peripheral vasodilation (low blood pressure, circulation fails)

36
Q

When are acids produced by the body?
(5)

A

Amino acid breakdown

Release of fatty acids from triacylglycerols

Lactic acid

Ketone bodies

Carbon dioxide

37
Q

How many defence systems does the body have to respond to an acid-base alteration

A

Three, first, second and third

38
Q

What is the body’s first defence to an acid-base alteration?

A

Buffering [HCO3-, Hb, Phosphate]

39
Q

What is the body’s second defence to an acid-base alteration?

A

Respiratory: alteration in arterial pCO2 by increasing/decreasing respiration

40
Q

What is the body’s third defence to an acid-base alteration?
(2)

A

Renal: alteration in HCO3- excretion

Increase or decrease in HCO3- excretion

41
Q

Why must H+ be buffered in the blood

A

Buffers temporarily neutralise the acid in order to transport H+ to the kidneys and lungs

H+ combines with the base (B-) to form a weak acid HB

42
Q

What is a buffer

A

A substance which resists changes in pH

43
Q

What is a buffer system

A

A buffer system consists of a weak acid (which donate H+) and a weak base (that accept/absorb excess H+)

44
Q

What is the Henderson-Hasselbach equation and what is it used for?
(3)

A

Used to determine what the final pH will be

pH = PKa + log [B-]/[HB]

Ka is the acid dissociation constant

45
Q

What is the basis of the Henderson-Hasselbach equation?

A

pH depends on the ratio of the conjugate base (B-) to the undissociated acid (HB)

46
Q

What does a high Ka mean?

A

Mostly dissociated acid

47
Q

What does a low Ka mean

A

Partially dissociated acid

48
Q

What are the three main buffer systems in the body

A

Plasma proteins such as haemoglobin

Phosphate (HPO4-) and ammonia (NHs)

Bicarbonate

49
Q

Write about how bicarbonate acts as a buffer
(5)

A

It’s the principle buffer system in the blood

Based on carbonic acid and bicarbonate

Carbonic acid (H2CO3) forms spontaneously when CO2 dissolves in H2O

Gas exchange occurs at the lungs which removes CO2 (and thus acid)

Basis for how the body controls the pH of the extracellular fluid

50
Q

What does the concentration of a gas in a solution depend on?

A

The partial pressure of that gas (pCO2)

The solubility of the gas in the solvent (S)

51
Q

What in ratio in blood determines the acidity

A

The ratio of bicarbonate to pCO2

52
Q

What is the equation for the pH of blood

A

6.1 + log [HCO3-/(0.03) x (pCO2)]

6.1 = dissociation constant

53
Q

How is CO2 produced in the body

A

Aerobic metabolism

54
Q

What happens when CO2 is picked up by red blood cells?
(4)

A

CO2 + H2O is converted to H2CO3 (carbonic acid) by carbonic anhydrase

Carbonic acid then dissociates to H+ and HCO3- (bicarbonate)

The H+ produced is buffered by haemoglobin in the rbc which releases O2

In oxygenated tissue, O2 binds haemoglobin preferentially

55
Q

In oxygenated tissue, O2 binds haemoglobin preferentially, what does this do?
(3)

A

Prevents haemoglobin from buffering H+

Instead H+ must be buffered by bicarbonate (HCO3-)

Bicarbonate must be present for buffering to continue

56
Q

Why is recovery of bicarbonate from glomerular filtrate needed?
(2)

A

The glomerular filtrate will contain the same [HCO3-] as blood

Therefore this needs to be reabsorbed in the tubules

57
Q

How is HCO3- reabsorbed by the renal tubules
(4)

A

Renal tubular cells are impermeable to HCO30

Therefore bicarbonate combines with H+ to form carbonic acid which dissociates to CO2 and H2O

H2O is then excreted to get rid of excess H+

CO2 is taken up into the tubular cells

58
Q

What happens when CO2 is taken up into tubular cells?
(4)

A

It combines with H2O to form carbonic acid

Carbonic acid then dissociates to form HCO3- and H+

The H+ is then secreted into the lumen where it combines with HCO3-

Any HCO3- produced is reabsorbed back into the blood

59
Q

What are the five steps of bicarbonate filtration in renal tubular cells?

A

CO2 from the tubular blood supply diffuses into tubular cells

CO2 combines with H2O to form carbonic acid (H2CO3)

Carbonic acid dissociates to HCO3- and H+

HCO3- is taken up by the tubular cells (into blood supply)

H+ is actively secreted into urine

60
Q

What is the role of buffers in acid-base balance?
(5)

A

They provide a temporary solution to acid base imbalance

But these buffer systems are limited in the body

Ultimately excess H+ must be removed

Buffer systems must be linked with renal and respiratory mechanisms

Generation/recycling of buffers is necessary

61
Q

How are acids removed?

A

Excess H+ tied up in water molecules (H2O): excretion via the kidneys

Removal of CO2 by the lungs

62
Q

How does the respiratory system affect acid base balance?
(3)

A

When breathing is inadequate carbon dioxide (respiratory acid) accumulates

The extra CO2 molecules combine with water to form carbonic acid which contributes to an acid pH

The treatment, if all else fails is to lower PCO2 by breathing for the patient using a ventilator

63
Q

How does the metabolic system affect acid base balance?
(3)

A

When normal metabolism is impaired - acid forms, e.g. poor blood supply stops oxidative metabolism and lactic acid forms

This acid is not respiratory so, by definition, it is “metabolic acid”

If severe, the patient may be in shock and require treatment, possible by neutralising this excess acid with bicarbonate, possible by allowing time for excretion/metabolism.

64
Q

What is respiratory compensation

A

Lungs become involved to correct the pH

65
Q

What is metabolic compensation

A

Kidneys become involved to correct the pH

66
Q

What compensation is there if the primary problem is metabolic (renal)
(2)

A

There will be respiratory compensation

Compensation is immediate (within minutes)

Respiration rate is altered to retain or expire CO2

67
Q

What compensation is there if the primary problem is respiratory?
(3)

A

The compensation mechanism will be metabolic (renal)

Acute respiratory problem metabolic compensation is delayed (hours to days)

Chronic respiratory problem metabolic compensation will have kicked in (kidneys will regulate H+ and HCO3- retention and secretion)

68
Q

Write about respiratory compensation
(7)

A

Change in respiration to compensate for a change in plasma pH

The pH can be lowered or raised by changing the rate of respiration

Retention of CO2 will cause a decrease in pH (more acidic)

Therefore, a respiratory acidosis will compensate for a rise in plasma pH (alkalosis)

Increased expiration of CO2 will cause an increase in pH (more basic)

Therefore a respiratory alkalosis will compensate for a fall in plasma pH (acidosis)

This compensation allows plasma pH to recover to normal levels

69
Q

Write about metabolic (renal) compensation
(6)

A

Change in the rates of H+ and HCO3- secretion or reabsorption by the kidneys in response to changes in plasma pH

Decline in H+ secretion and HCO3- reabsorption

Therefore, a metabolic acidosis will compensate for a rise in plasma pH (alkalosis)

Secretion of H+ and reabsorption of HCO3-

Therefore, a metabolic alkalosis will compensate for a fall in plasma pH (acidosis)

This compensation allows plasma pH to recover to normal levels

70
Q

List four causes of metabolic acidosis

A

Increased formation of acids

Increased ingestion of acids

Decreased excretion of H+

Increased loss of bicarbonate

71
Q

What two processes may cause an increased formation of acids?

A

Increased ketone production (diabetic ketoacidosis)

Increased lactic acid formation

72
Q

The ingestion of what three things will cause metabolic acidosis?

A

Ethanol
Methanol
Ethylene glycol

73
Q

What may cause decreased excretion of H+

A

Renal failure

74
Q

What may cause increased loss of bicarbonate

A

Diarrhoea

Renal failure

75
Q

What are the two causes of metabolic alkalosis?

A

Loss of H+

Gain of HCO3-

76
Q

What might cause the loss of H+

A

Gastrointestinal loss via vomiting or diarrhoea

Renal loss via excess amounts of aldosterone or mediations such as diuretics

77
Q

What might cause the increased gain of HCO3-

A

Chronic alkali ingestion

Inappropriate correction of an acidosis

78
Q

What causes respiratory acidosis?

A

Retention of CO2

79
Q

What six things may cause the retention of CO2?

A

Airway obstruction

Pulmonary disease e.g. asthma, pneumonia

Thoracic musculoskeletal problems

Neuromuscular conditions or neurotoxins

Depression of respiration through medications

Choking

80
Q

What causes respiratory alkalosis?

A

Increased expiration of CO2

81
Q

What are the causes of increased expiration of CO2?
(8)

A

Low O2 (hypoxia)
- high altitude
- severe anaemia
- pulmonary disease

Increased respiratory drive
- Hyperventilation (artificial or panic)
- Medications e.g. Aspirin
- Trauma, infection

82
Q

How are blood gases measured

A

Using specific electrodes on blood gas analysers

83
Q

What blood gases are measured
(4)

A

[H+] measured and pH calculated
Partial pressure of CO2 (pCO2)
Partial pressure of O2 (pO2)
Bicarbonate (calculated from [H+] and pCO2)

84
Q

What are the six steps to acid-base analysis?

A

Is the pH normal?

Is the CO2 normal?

Is the HCO3 normal?

Match the CO2 or the HCO3 with the pH

Does the CO2 or the HCO3 go the opposite direction of the pH?

Are the pO2 and the O2 saturation normal?

85
Q

How would you investigate blood gas results?

A

Examine the [H+] or pH -> is there acidosis or alkalosis

Examine the pCO2 -> if abnormal it must be respiratory involvement

Examine the HCO3- -> if abnormal it must be metabolic involvement

Is there evidence of compensation?

86
Q

Define compensation

A

The attempt by the body to maintain homeostasis by correcting the pH

87
Q

Comment on compensation in a patient experiencing shock
(3)

A

A patient in shock will undergo anaerobic metabolism which produces lactic acid

The production of lactic acid will bind or use up available HCO3 and will be manifested by a decrease in the HCO3 level.

Therefore, the HCO3 level is an indicator of metabolic acid-base balance

2.2 Clinical Chemistry (28 decks)

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