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Physiology > Acid base physiology > Flashcards

Acid base physiology Flashcards

1
Q

Hydrogen ion

A

Single free proton released from a hydrogen atom

- H+ exists in solution bound to water (H2O)

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2
Q

Acid

A

Molecule that releases hydrogen ions in solution

- HCl

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3
Q

Base

A

Molecule that can accept H ions in solution

- calcium carbonate

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4
Q

Hydrogen ions are very _____

A

Reactive!!

- alter structure and function of virtually every protein and enzyme in the body

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5
Q

Homeostasis requires control of _____

A

H+

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6
Q

Metabolic acidosis causes

A
  • decreased myocardial contractility
  • vasodilation
  • reduced responsiveness to catecholamines
  • impaired coagulation
  • decreased cell function
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7
Q

Respiratory acidosis causes

A

Cerebral edema via vasodilation

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8
Q

H+ concentration is kept ____ relative to other ions

A

Very low

- 0.00004 mEq/L (pH 7.4)

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9
Q

pH formula

A

log (1/[H+]) = -log[H+]

  • high [H+] = low pH
  • low [H+] = high pH
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10
Q

Normal blood pH is ____

A
  1. 4 (7.35=7.45)
    - arterial blood is 7.4
    - venous blood is 7.35 (contains more CO2, which acts as an acid)
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11
Q

Physiologic limits of blood pH

A

6.8-8.0

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12
Q

Intracellular fluid pH is ____ than blood

A

Lower (6.0-7.4

- cellular metabolism produces H+

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13
Q

Urine pH

A

4.5-8.5

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14
Q

What are the 3 primary systems that regulate changes in [H+]?

A
  • buffers: seconds (weakest)
  • lungs (ventilation): minutes (stronger)
  • kidneys: hours to days (strongest)
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15
Q

Is the body more efficient at dealing with acids or bases?

A

Acids

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16
Q

Buffer

A

Substance that can reversibly bind with H+
- accept or donate H+ to minimize changes in [H+] until balance can be restored

buffer + H+ Hbuffer

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17
Q

What are the 4 main buffers in the body?

A
  • bicarb (extracellular)
  • proteins (intracellular)
  • phosphate (intracellular and renal tubules)
  • ammonia (renal tubules) most important in removing bicarb
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18
Q

Bicarb buffer system

A

Consists of carbonic acid (H2CO3) and bicarb salt (NaHCO3)

  • carbonic acid weakly dissociates
  • bicarb salt almost completely dissociates
  • carbonic anhydrase is abundant in lungs and kidney
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19
Q

Bicarb buffer system formula

A

CO2 + H2O H2CO3 H+ -HCO3

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20
Q

What happens if an acid is added to the bicarb buffer system?

A

Consumed by bicarbonate, driving the equation left

- carbon dioxide is produced (and exhaled)

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21
Q

What happens if a base is added to the system?

A

Consumed by carbonic acid, driving the equation to the right

- carbon dioxide is consume (and ventilation decreases)

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22
Q

The bicarb system is connected to the _____

A

Lungs

  • both are dependent on each other
  • ventilation (CO2) is required to keep the bicarb system working at max efficiency
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23
Q

Abnormalities in ventilation affect the _____

A

pH

- hypoventilation –> increased CO2 –> increased carbonic acid

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24
Q

The lungs thru ventilation, control extracellular [H+] and provides a second means to respond to changes in ____

A

[H+]

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25
Q

Minute ventilation

A

Tidal volume x respiratory rate

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26
Q

Ventilation has 2 main stimuli

A
  • changes in blood carbon dioxide

- changes in blood oxygen

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27
Q

The body favors ___ control over ___

A

CO2; O2 control

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28
Q

The lungs are very effective at compensating for ____

A

Acidemia

  • ventilation can be easily increased
  • overall buffering capacity is 1-2x that of all other chemical buffers combined
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29
Q

The lungs are less effective at compensating for _____

A

Alkalemia

  • if ventilation decreases too much, hypoxemia occurs
  • the body won’t starve itself of oxygen
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30
Q

Why are diseases that suppress ventilation life threatening?

A

The body has limited ways to immediately respond to the acid base effects of hypercapnea

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31
Q

What are the cons of the buffer and lung systems?

A
  • in capable of returning pH back to normal

- in capable of permanently changing the hydrogen ion or bicarbonate concentrations in the body

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32
Q

How to do the kidneys control the acid-base balance of the system?

A

Excrete acidic or basic urine

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33
Q

4 processes that allow urine and blood pH to be controlled

A
  • bicarb is filtered continuously and reabsorbed
  • hydrogen ions are secreted
  • new bicarbonate can be made
  • non-volatile acids are filtered (by products of protein metabolism)
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34
Q

How does the kidney respond to alkalosis?

A

Fails to reabsorb HCO3

- helps retain hydrogen ions due to a lack of buffering

35
Q

How does the kidney respond to acidosis?

A

Reabsorbs all bicarb, actively secretes H+, makes new bicarb

36
Q

Where does bicarb reabsorption and H secretion occur?

A

In all segments except the thin loop of Henle

37
Q

___ must be secreted to reabsorb ____

A

H+; HCO3

38
Q

How is H+ secreted?

A
  • Na/H counter transport
  • Na/K on basolateral membrane establishes concentration gradient
  • H combines with bicarb to form CO2
  • CO2 enters cell
  • CO2 combines with water to form H and bicarb
  • bicarb re-enters the blood
39
Q

Proximal tubule

A

Does not secrete much H, just uses it to reabsorb bicarb

- H and bicarb completely titrate each other

40
Q

Incomplete titration

A
  • acidosis: bicarb is completely reabsorbed

- alkalosis: excess bicarb is excreted because it doesn’t have a H to help it re-enter the cell

41
Q

Intercalated cells of the distal tubule and collecting ducts

A

H is actively secreted

  • only secretes 5% of total H
  • concentrates H 900x that of the proximal tubules
  • is the segment that maximally acidifies urine*
42
Q

____ is reciprocal to bicarb

A

Chloride

43
Q

Intercalated cells can not make urine pH below

A

4.5

44
Q

How does the body bind H+?

A

Utilizes buffers to trap H+ and excrete them in the bound form
- keeps concentration gradient low enough for active transport to keep working

45
Q

Buffering systems

A
  • phosphate
  • ammonium: most important quantitatively
  • urate and citrate (to a lesser extent)
46
Q

What are the 2 main jobs of the buffers?

A
  • keep hydrogen ion concentration in the urine low

- helps form new bicarb

47
Q

What happens to hydrogen once all of the bicarb is reabsorbed?

A

H+ is free to interact with other buffers

  • H+ is excreted with phosphate as a salt
  • net effect is generation of new bicarb, whenever H+ combines with a buffer other than bicarb
48
Q

Ammonia buffer system in the proximal tubules

A
  • glutamine is an amino acid by product made in the liver
  • when acidosis occurs, the kidney uses it to make ammonium (NH4) and bicarb = generates 2 new bicarb molecules
  • ammonium is excreted in exchange for sodium
49
Q

Ammonia buffer system in the collecting ducts

A
  • ammonia (NH3) is able to freely pass thru the membrane into the lumen
  • combines with H+to form ammonium ions, which are less permeable and excreted
  • one bicarb is made/kept for every H+ that is secreted
50
Q

Ammonia buffer system control

A

With chronic acidosis, this system becomes the dominant means by which acid is secreted

  • an increase in extracellular H+ concentration stimulates renal glutamine metabolism
  • generates more ammonium to act as a buffer
  • new bicarb is made
  • opposite occurs when H+ concentrations drop
51
Q

Factors that affect H+ secretion

A
  • hydrogen ion concentration
  • carbon dioxide concentration
  • processes that control sodium (indirectly)
52
Q

Factors that increase H+ secretion and bicarb reabsorption

A
  • respiratory acidosis: increases CO2
  • metabolic acidosis: increase H, decrease bicarb
  • decrease ECF
  • increase angiotensin 2
  • increase aldosterone
  • hypokalemia
53
Q

Factors that decrease H+ secretion and bicarb reabsorption

A
  • respiratory alkalosis: decrease CO2
  • metabolic alkalosis: decrease H, increase bicarb
  • increase ECF
  • decrease angiotensin 2
  • decrease aldosterone
  • hyperkalemia
54
Q

CO2

A

Behaves as an acid

  • forms H+
  • respiratory
55
Q

HCO3

A

Behaves as a base

  • binds H+
  • metabolic
56
Q

How to cause acidosis

A
  • increase CO2 (hypoventilation)
  • decrease bicarb
  • add acid
57
Q

How to cause alkalosis

A
  • decrease CO2 (hyperventilation)
  • increase bicarb
  • lose acid
58
Q

Traditional blood gas analysis

A

Measurement of blood gases (O2 and CO2) and other parameters to evaluate acid-base status

  • respiratory system (PCO2)
  • metabolic system (HCO3)
  • when an abnormality occurs in one system, the other compensates
59
Q

Arterial blood sampling

A
  • acid base (more accurate)
  • ventilation
  • oxygenation (lung function)
60
Q

Venous blood sampling

A
  • acid base
  • ventilation
  • oxygen extraction (oxygen use by tissues)
61
Q

Pre analytical errors

A
  • sampling site
  • inappropriate blood tube
  • inappropriate blood to anti-coagulant ratio
  • air bubbles or prolonged exposure to room air
  • delay in performing the analysis
62
Q

Traditional blood gas analysis

A

Based on bicarb buffer system

  • ratio between CO2 and HCO3 determines pH
  • alterations in pH are explained by an abnormality of CO2 or HCO3
63
Q

Acidosis involves an increase in ___ and a decrease in _____

A

CO2; HCO3

64
Q

Alkalosis involves a decrease in ____ and an increase in ______

A

CO2; HCO3

65
Q

Respiratory analysis

A 
Acidosis
- CO2 increases = acidifying
- HCO3 increases to compensate
Alkalosis
- CO2 decreases = alkalinizing
- HCO3 decreases to compensate
66
Q

Metabolic analysis

A 
Alkalosis
- HCO3 increases = alkalinizing
- Co2 increases to compensate
Acidosis
- HCO3 decreases = acidifying
- CO2 decreases to compensate
67
Q

Compensation

A

Secondary change in the opposite system to oppose the primary acid base disturbance

68
Q

Respiratory system compensation

A
  • responds to metabolic disturbance

- rapid time of onset and complete within hours

69
Q

Metabolic system compensation

A

Responds to respiratory disturbance

- slower time of onset (hours) and takes 2-5 days to complete

70
Q

Acute compensation will ____ return the pH to a normal value

A

NOT

- a normal pH with an abnormal CO2 and bicarb implies a mixed acid-base disturbance

71
Q

What 3 components are needed to evaluate the acid base status?

A
  • pH
  • respiratory: PCO2
  • metabolic: HCO3
    (anion gap is helpful)
72
Q

Base excess

A

The titratable acid or base in the blood
- the amount of acid or base that must be added to a sample of oxygenated whole blood to restore the pH to 7.4 at 37 C at a PCO2 of 40

73
Q

BE is the metabolic component _____ of CO2

A

Independent

  • when CO2 levels are normal, BE and HCO3 correlate well
  • when CO2 is high, use BE
74
Q

Total CO2

A

Evaluates metabolic component

  • has nothing to do with respiratory component
  • is not a blood gas!!
75
Q

____ of all the CO2 in the body exists as bicarb

A

99%

- total CO2 and HCO3 will always be similar

76
Q

Anion gap

A

Developed to better characterize metabolic acidosis

  • rule of electroneutrality means there is no anion gap within the body
  • AG represents anions that aren’t readily measured (mostly neg plasma proteins)
77
Q

Normal (decreased) AG

A
  • bicarb is lost via kidneys or GIT
  • HCO3 and Cl are reciprocal, so chloride increases as bicarb decreases
  • hyperchloremic metabolic acidosis
78
Q

Increased AG

A
  • acid (unmeasured anions) is added to the system

- AG increases because bicarb decreases as Cl stays the same

79
Q

What are the 4 main causes of an increased anion gap metabolic acidosis

A
  • ketones
  • uremic acids (azotemia)
  • lactic acidosis
  • ethylene glycol
80
Q

Criticisms of traditional blood gas

A
  • doesn’t fully explain complexity of patient’s blood gas abnormalities
  • fails to provide direction on how best to treat complex blood gas abnormalities
81
Q

Semi-quantitative approach

A

Recognizes:

  • strong ions that fully dissociate in water
  • weak acids that can buffer
82
Q

The semi-quantitative approach depends on:

A
  • law of electroneutrality

- law of conservation of mass

83
Q

What contributes to the metabolic portion of the patient’s pH?

A 
Strong ion
- changes in free water 
- changes in chloride concentration
- increase in unmeasured anions
Weak acid
- decrease in albumin concentration
- increase in phosphorous concentration

Physiology (33 decks)

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