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Respiratory > Acid Base > Flashcards

Acid Base Flashcards

1
Q

How do hospital laboratories report hydrogen ion status?

A 
UK= nmol/L rather than pH
USA= pH
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2
Q

List the units for H+ concentration

A 
millimoles per litre (mmol/L)
nanomoles per litre (nmol/L) etc.
or as pH, which is defined as
pH = - log10 [H+] 
In equation- mol/L
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3
Q

What is the relationship between pH and H+ concentration?

A 
Logarithmic and inverse
H+ conc increases, pH decreases
Lower pH= higher H+ conc
pH= -log (H+)
If (H+) is small- standard form
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4
Q

Describe two important points about pH

A

(H+)= 1/ 10^pH mol/L

Logarithm so no units

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5
Q

Why is an increase in pH by 0.3 a factor of 2 change in (H+)?

A 
log2= 0.30
antilog0.3= 2
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6
Q

What is an acid?

A

Substance that releases H+ ions on ionisation (dissociation)

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7
Q

What is the equation for acid dissociation?

A

HA= H+ + A-

Reversible reaction

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8
Q

What is a base?

A

Substance that accepts free H+; A- is called the ‘conjugate base’ derived from HA

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9
Q

What does strength of acid/ base depend on?

A

Degree of dissociation. Acetic acid is a weak acid, whatever its concentration, just as ammonia is a weak base, because in solution both exist largely in their un-ionized form

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10
Q

Describe a strong acid

A

exists in solution Predominantly in the dissociated form A-
Concentration of the free H+ is close to the total concentration of the acid. This is true for inorganic acids such as HCl and HNO3.

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11
Q

Describe a weak acid

A

Very little dissociation occurs. Acid is present mostly as its un-ionised form HA, while [A-] and free [H+] are relatively small.
This applies to most organic acids, such as acetic acid, lactic acid and long-chain fatty acids.

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12
Q

What equilibrium constant of the dissociation do we use for strengths of acids?

A

Ka (if we are working in [H+]), or the pKa (if we are working in pH)

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13
Q

What is the formula for Ka?

A

Ka= (H+)(A-)/ (HA)

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14
Q

What is the formula for pKa?

A

pKa= -logKa

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15
Q

What are the relative sizes of Ka and pKa for strong and weak acids?

A

Strong acid= large Ka, low pKa

Weak acid= small Ka, high pKa

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16
Q

What is the equation for pH with pKa (Henderson-Hasselbalch equation)?

A

pH= pKa + log(A-)/(HA)

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17
Q

What is a conjugate base?

A

Species that can accept a hydrogen ion

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18
Q

What are the roles of NH2 group and NH3- group?

A

NH2- base (can combine with H+)

NH3- = conjugate acid (dissociate to give H+)

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19
Q

What is the ratio change per pH unit difference?

A

Changes by a factor of 10

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20
Q

What is a zwitterion?

A

At least one has a positive and one has a negative electrical charge and the net charge of the entire molecule is zero

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21
Q

What group does histidine have in its sidechain?

A

Amino group/ imino group (in the imidazole ring)

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22
Q

What group does aspartic acid have in its sidechain?

A

Carboxyl group

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23
Q

What group does Arginine have in its sidechain?

A

Amino group/ guanidino group

24
Q

What are other amino acid sidechains that may ionize in the physiological range of [H+]?

A

Cysteine and tyrosine, although they have relatively high pKa values (8.3 and 10.1 respectively)

25
Q

Why can’t asparagine and glutamine by titrated?

A

Because they are amides, not primary amines, and they do not bind H+

26
Q

How does net charge on a protein depend on the contributions from all the ionising amino acid sidechains?

A

When [H+] is low= the carboxyl groups will all be ionised, and the amino acid groups mostly unprotonated and therefore uncharged - the overall charge on the protein will be negative
When [H+] is high= all of the amino groups will be protonated (positively charged) and the carboxyl groups will not be ionised - the overall charge will be positive.
If a protein is titrated from a low to a high pH, or vice versa, there will be a point at which it has equal positive and negative charges - the net charge is zero.

27
Q

What is the isoelectric point?

A

When a protein has equal positive and negative charges resulting a net charge of zero
Usually expressed as a pH value and denoted pl

28
Q

Why is the pl value important?

A

Because the solubility of a protein is lowest at this pH. The reason for this is that repulsion between protein molecules is least when the net charge is zero, so the protein can aggregate and precipitate, with physiological consequences

29
Q

Why are there different pl values for different proteins?

A

It depends on the amino acid composition. Proteins that contain a lot of basic amino acids have a high pl value whereas those that contain many acidic residues have a low pl.

30
Q

Examples of different pl values and why

A

Histones contain many basic (lysine and arginine) residues. At the pH within the cell they have strong positive charges and so interact with DNA, which is negatively charged.
Cytochrome c is also very basic, and its positive charge helps it bind to the inner membrane of mitochondria, which has a negative charge on its phospholipids.
Pepsin, the major digestive enzyme in the stomach, contains many acidic amino acids, and even some attached phosphate residues, giving it a very low pI (~1). It therefore won’t have much of a charge in the acidic conditions of the stomach, where its substrates will mostly have a positive charge - this minimizes nonspecific electrostatic effects on enzyme-substrate binding.

31
Q

What is isoelectric precipitation?

A

If a protein is brought to a pH equal or near to its pI, its overall charge is low, so inter-molecular repulsion is minimal, and protein molecules tend to clump together; in other words, the protein’s concentration exceeds its solubility, and so it precipitates

32
Q

Why is isoelectric precipitation important when it occurs with myoglobin?

A
  • Severe muscle damage – for example crushing, as may occur in a road traffic accident – releases a lot of myoglobin from the cytoplasm of skeletal muscle, and it enters the plasma.
  • Being a small protein (molecular weight 17 kDa) it is not filtered out in the kidney, and enters the glomerular filtrate.
  • The pH of the filtrate in the distal nephron is usually low (4 – 5), and the pI of myoglobin is 5.2. Isoelectric precipitation of myoglobin can occur there, leading to clogging of the nephron and eventually to renal failure
33
Q

What is the normal (H+) in health?

A

the plasma [H+] varies between 36 and 44 nmol/L.
If [H+] is outwith this reference range there is an acid-base disturbance.
[H+] < 20 or >120 nmol/L is considered to be life-threatening.
Low and precisely controlled within a relatively narrow range

34
Q

Why is accumulation of H+ dangerous?

A

Rapid build-up of any molecule or ion within a cell disrupts homeostasis and is therefore threatening.
Any change in [H+] alters the charges on the side-chains of many of the amino acids that constitute proteins.
Charge interactions are crucial to protein folding and therefore protein function - remember that enzymes are proteins. Consequently changes in [H+] seriously compromise the enzymic activities and structural functions of proteins in the cell.
Changes in [H+] can also disrupt the precisely controlled distribution of ions across the cell membrane, upon which the excitability of neurons and muscle cells is critically dependent.

35
Q

Where does the main threat to the stability of (H+) within the body come from?

A 
H+ production
Carbon dioxide (CO2) produced by oxidation of fuels (eg in the TCA cycle).
Metabolism of food constituents to other acidic products.
36
Q

Describe the production of CO2/ volatile acid?

A

20 moles every day. The CO2 dissolves in body fluids and liberates H+ by reacting with water to form carbonic acid, which ionizes to bicarbonate

37
Q

How does metabolism yield H+?

A

The complete breakdown of carbohydrate or fat neither consumes nor produces H+: the carbon is oxidised to CO2 and the hydrogen to water. However when glucose is incompletely degraded to lactic acid, H+ is liberated.
For fats, incomplete oxidation to ‘ketone bodies’ (acetoacetic and 3-hydroxybutyric acids) yields H+.
For protein the situation is a little more complex. The carbon in aminoacids is oxidised to CO2 and the hydrogen to water, but the nitrogen isn’t oxidised to nitrogen dioxide (NO2): instead it is converted to ammonia (NH3), which is of course a base, and then on to urea, which is neutral. Some amino acids (cysteine and methionine) contain sulphur, which is oxidised to sulphuric acid. On balance, protein metabolism results in net H+ production because of the formation of sulphuric acid.

38
Q

What happens to anions in fruit and vegetables?

A

Potassium citrate, sodium malate, and sodium lactate are all salts found in quite high concentrations in vegetables and fruit juices- will be metabolised to CO2 and H20

39
Q

How does anion metabolism lead to a net lowering of H+?

A

In order for these anions to be metabolised to CO2 and H2O, H+ must come from the cytoplasm of the metabolically active cells, and so the metabolism of these anions consumes H+. The net effect is a lowering of body [H+]. The stoichiometry of lactate oxidation is as follows:
CH3.CHOH.COO– + 3O2 + H+ = 3CO2 + 3H2O

40
Q

How is a deficit of bicarbonate in plasma treated?

A

Direct infusion of bicarbonate

41
Q

How does a fasting patient have a high rate of metabolic acid production?

A

During starvation there is increased production of acetoacetic and 3-hydroxybutyric acids (the so-called ketone bodies) from the breakdown of fat and proteins.
These are relatively strong acids, resulting in net H+ production.
net protein degradation also results in the production of sulphuric acid.

42
Q

What is a buffer?

A

Chemical that can resist a change in the free concentration of an ion in a solution

43
Q

What is the short term solution to buffer use?

A

Instantaneously removes free H+ from body solutions, but this is only a holding operation

44
Q

What is the long term solution to buffer use?

A

The body must get rid of these H+ ions before it runs out of the A- ions that are needed to buffer them. It does this by urinary excretion.

45
Q

When do buffers work best?

A

Within plus/minus 1 pH unit of their pK

46
Q

Name the important physiological buffers

A

Phosphate- urinary, intracellular
Bicarbonate- primary in extracellular fluid
Ammonia- formed in proximal tubule from glutamine- urinary, allows excretion of H+ as NH4
Protein- in cells and extracellular fluid, erythrocyte haemoglobin

47
Q

Describe the bicarbonate buffer system

A

Because the H2CO3 is in equilibrium with CO2 and H2O as well as with H+ and HCO3–. However under physiological conditions the concentration of dissolved CO2 is about 200 times that of the H2CO3 so in practice we can ignore the latter and write the equilibrium as:
CO2 + H20= (reversible) H+ + HCO3-

48
Q

What is carbonic anhydrase?

A

Enzyme catalyses spontaneous reaction between CO2 and water so always close to equilibrium

49
Q

Where is carbonic anhydrase found in the body?

A

Red blood cells (role in transport of CO2 in the form of HCO3-)
Cells of the kidney for H+ secretion (distal tubule- H+ secretion, proximal tubules- HCO3- resorption)
Gastric mucosa H+ secretion- parietal (oxyntic cells)

50
Q

What is the equation for (H+) in the bicarbonate buffer system?

A 
(H+) = Ka x (CO2)/(HCO3-)
(H+) = const. x (Pco2)/(HCO3-)
const= 180, Pco2 in kiloPascals
51
Q

What organs are involved in the bicarbonate buffer system?

A

HCO3- controlled by kidneys
Pco2 controlled by lungs
Symbolic relationship

52
Q

Why is the blood buffer vulnerable to acid-base disturbances?

A

pK of bicarbonate buffer (6.10) over one unit pH away from physiological pH (7.4) so outside effective buffering range of bicarbonate
There will be very little H2CO3 to deprotonate

53
Q

We do we ignore the second dissociation in a dibasic acid?

A

Dissociation constant very small

54
Q

How does a decrease in H+ affect calcium ions?

A

Decrease in H+= fall in free Ca2+= intracellular Ca2+ tightly controlled by various calcium pumps, change can affect peripheral nerves and common symptom is tingling of extremities circumoral paraesthesia

55
Q

How is H+ and CO2 produced by living cells?

A

Oxidative metabolism produces large amounts of H+ (for example through formation of sulphuric acid from proteins, phosphoric acid from DNA and RNA, and of organic acids such as lactic, acetoacetic and 3-hydroxybutyric acids from the breakdown of carbohydrate and fat).
The operation of the TCA cycle (Krebs’ cycle) also increases [H+] since it releases CO2, which dissolves in water to produce carbonic acid, a weak acid that ionizes, releasing [H+].

Respiratory (41 decks)

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