Transcript Acid-Base Biochemistry

Acid-Base Biochemistry
Dr. Catherine Street
Consultant Clinical Biochemist
Colchester Hospital university NHS
Foundation Trust
Acid-Base Biochemistry
► Definitions
► Methods
► Physiology
► Pathology
Acid-Base Biochemistry
Definitions
What is an acid?
What is a base?
Acid-Base Biochemistry
Definitions
►
Definitions of an acid
1. Taste
2. Boyle
3. Arrhenius
4. Bronsted-Lowry
5. Lewis
Acid-Base Biochemistry
Definitions
► Taste
Acere – tasting sour
Lemon juice
Vinegar
Definition - Thousands of years old
Acid-Base Biochemistry
Definitions
► Robert
Boyle 17th century
► Acids taste sour, are corrosive to metals,
change litmus (a dye extracted from
lichens) red, and become less acidic when
mixed with bases (Alkali).
► Bases (Alkali) feel slippery, change litmus
blue, and become less basic (alkaline) when
mixed with acids.
Acid-Base Biochemistry
Definitions
Arrhenius
► Arrhenius suggested that acids are
compounds that contain hydrogen and can
dissolve in water to release hydrogen ions
into solution. For example, hydrochloric acid
(HCl) dissolves in water as follows:
HCl
H2O
+
→
H
+
Cl
(g)
(aq)
(aq)
Acid-Base Biochemistry
Definitions
► Arrhenius
defined bases as substances that
dissolve in water to release hydroxide ions
(OH-) into solution. For example, a typical
base according to the Arrhenius definition is
sodium hydroxide (NaOH):
H2O
NaOH
(s)
→ Na+ (aq) + OH-(aq)
Acid-Base Biochemistry
Definitions
► The
Arrhenius definition of acids and bases
explains a number of things. Arrhenius's
theory explains why all acids have similar
properties to each other (and, conversely,
why all bases are similar): because all acids
release H+ into solution (and all bases
release OH-).
Acid-Base Biochemistry
Definitions
► The
Arrhenius definition also explains
Boyle's observation that acids and bases
counteract each other. This idea, that a
base can make an acid weaker, and vice
versa, is called neutralization.
Acid-Base Biochemistry
Definitions
► Neutralization:
As you can see from the
equations, acids release H+ into solution
and bases release OH-. If we were to mix an
acid and base together, the H+ ion would
combine with the OH- ion to make the
molecule H2O, or plain water:
► H+ (aq) + OH-(aq) → H2O
Acid-Base Biochemistry
Definitions
► The
neutralization reaction of an acid with a
base will always produce water and a salt,
as shown below:
► Acid Base
Water Salt
► HCl + NaOH → H2O + NaCl
► HBr + KOH → H2O + KBr
Acid-Base Biochemistry
Definitions
► Limitations
of Arrhenius
► The Arrhenius definition does not explain
why some substances, such as common
baking soda (NaHCO3), can act like a base
even though they do not contain hydroxide
ions.
Acid-Base Biochemistry
Definitions
Brǿnsted-Lowry 1923
An acid is any chemical species that
donates a proton to another chemical
species (proton donor)
A base is any chemical species that
accepts a proton from another chemical
species (Proton acceptor)
Acid-Base Biochemistry
Definitions
► The
Brønsted-Lowry definition of acids is
very similar to the Arrhenius definition, any
substance that can donate a hydrogen ion is
an acid (under the Brønsted definition, acids
are often referred to as proton donors
because an H+ ion, hydrogen minus its
electron, is simply a proton).
Acid-Base Biochemistry
Definitions
► The
Brønsted definition of bases is,
however, quite different from the Arrhenius
definition. Arrhenius base releases hydroxyl
ions whereas the Brønsted base is defined
as any substance that can accept a
hydrogen ion.
Acid-Base Biochemistry
Definitions
► The
Brønsted-Lowry definition includes the
Arrhenius bases so
► NaOH and KOH, as we saw above, would still be
considered bases because they can accept an H+
from an acid to form water.
► But it extends the concept of a base and
introduces the concept of conjugate acid-base
pairs
Acid-Base Biochemistry
Definitions
The removal of a proton (hydrogen ion) from
an acid produces its conjugate base, which
is the acid with a hydrogen ion removed,
and the reception of a proton by a base
produces its conjugate acid, which is the
base with a hydrogen ion added
Acid-Base Biochemistry
Definitions
► The
Brønsted-Lowry definition also explains
why substances that do not contain OHions can act like bases.
► Baking soda (NaHCO3), for example, acts
like a base by accepting a hydrogen ion
from an acid as illustrated below:
► Acid
Base
Salt
► HCl + NaHCO3 → H2CO3 + NaCl
Acid-Base Biochemistry
Definitions
► Lewis
definition 1923
► A substance that can accept an electron pair from
a base; thus, AlCl3, BF3, and SO3 are acids.
► The Lewis theory defines an acid as a species that
can accept an electron pair from another atom,
and a base as a species that can donate an
electron pair to complete the valence shell of
another atom
Acid-Base Biochemistry
Definitions
pH
Under the Brønsted-Lowry definition, both
acids and bases are related to the
concentration of hydrogen ions
present. Acids increase the concentration of
hydrogen ions, while bases decrease the
concentration of hydrogen ions (by
accepting them). The acidity or basicity of
something therefore can be measured by its
hydrogen ion concentration.
Acid-Base Biochemistry
Definitions
► In
1909, the Danish biochemist Sören Sörensen
invented the pH scale for measuring acidity. The
pH scale is described by the formula:
► pH = -log [H+]
► Note: concentration is commonly abbreviated by
using square brackets, thus [H+] = hydrogen ion
concentration. When measuring pH, [H+] is in
units of moles of H+ per litre of solution.
Acid-Base Biochemistry
Methods
pH electrode
Acid-Base Biochemistry
Methods
► pH
electrode
Acid-Base Biochemistry
Methods
How the pH Electrode works
► As the pH Glass comes into contact with an
aqueous substance to measure, a gel layer
forms on the membrane. This also happens
on the inside of the glass layer.
.
Acid-Base Biochemistry
Methods
How the pH Electrode works
►The pH value of the aqueous solution
will either force Hydrogen Ions out of
the gel layer or into this layer. The
Internal buffer in the glass electrode
has a constant pH value and this keeps
the potential at the inner surface of the
membrane constant.
Acid-Base Biochemistry
Methods
How the pH Electrode works
►The membrane potential is therefore
the difference between the inner and
outer charge. If you then factor in the
stable potential of reference electrode,
you have a voltage proportional to the
pH value of the solution being
measured, this being approximately
58mV/pH unit @ 20ºC
Acid-Base Biochemistry
Methods
Other methods you need to know and
understand
► Carbon dioxide electrode
► Oxygen electrode
► Laboratory measurement of bicarbonate
► Ion selective electrodes for K+ Na+ Cl-
Acid-Base Biochemistry
Physiology
► What
is Physiological pH range?
Acid-Base Biochemistry
Physiology
► Extracellular
fluid
pH 7.35 – 7.46 (35-45 nmol/L)
Does this apply to whole body
?any different pH ranges elsewhere
Acid-Base Biochemistry
Physiology
More extreme/variable pH range
Digestive tract
Gastric Juice 1.0-3.0
Pancreatic Juice 8.0-8.3
Intercellular organelles
Lysosomal pH 4-5
Digestive and lysosomal enzymes
function optimally at these pH ranges
Acid-Base Biochemistry
Physiology
Traditionally use pH to measure acidity
Problem
1. direction of pH change is
opposite to increase/decrease of Hydrogen
ion concentration
2. Use of log scale ‘masks’ the extent of the change
-change of 0.3 in pH represents doubling/halving of
hydrogen ion concentration
Acid-Base Biochemistry
Physiology
► More
recently – use Hydrogen ion
concentration [H+]
► Traditionalists and older equipment use pH
► For large pH changes may not register
change in units eg nmole/L to moles/L
► Most practical - give both
Acid-Base Biochemistry
Physiology
► WHAT
BODY?
THE SOURCES OF ACID IN THE
Acid-Base Biochemistry
Physiology
► Sources
of acid
 Metabolism of food
 Metabolism of drugs
 Inborn errors of metabolism
Acid-Base Biochemistry
Physiology
► Acid
production from metabolism of food
 Sulphuric acid from metabolism of sulphurcontaining amino acids of proteins
 Lactic acid from sugars
 Ketoacids from fats
Acid-Base Biochemistry
Physiology
► Acid
production from metabolism of drugs
 Direct metabolism of drug to more acidic
compound eg salicylates urates etc
 Induction of enzymes which metabolise other
compounds (endogenous or exogenous) to
acids
Acid-Base Biochemistry
Physiology
► Inborn
errors of metabolism
 Organic acid disorders
 Lactic acidosis
Acid-Base Biochemistry
Physiology
Greatest potential source of acid
Carbon dioxide
(1) CO2 + H2O <=> H2CO3
(2) H2CO3 <=> H+ + HCO3-
Potentially 15,000 mmol/24 hours
Acid-Base Biochemistry
Physiology
► Hydrogen
ion homeostasis
► 1. buffering
► 2. excretion
Acid-Base Biochemistry
Physiology
Buffering of hydrogen ions
In health as hydrogen ions are produced
they are buffered – limiting the rise in [H+]
Acid-Base Biochemistry
Physiology
Buffer solutions consist of a weak acid and its
conjugate base
As hydrogen ions are added some will
combine with the conjugate base and
convert it to undissociated acid
Acid-Base Biochemistry
Physiology
Bicarbonate – carbonic acid buffer system
H+ + HCO3- <=> H2CO3
► Addition
of H+ drives reaction to the right
Conversely
► Fall in H+ drives reaction to the left as
carbonic acid dissociates producing more H+
Acid-Base Biochemistry
Physiology
► Buffering
systems in blood
 Bicarbonate ions-most important
 Proteins including intracellular proteins
 Haemoglobin
Acid-Base Biochemistry
Physiology
► Buffer
solutions operate most efficiently at
[H+] that result in approximately equal
concentration of undissociated acid and
conjugate base
► But at normal extracellular fluid pH
[H2CO3]  1.2 mmol
whereas [HCO3-] is twenty times greater
Acid-Base Biochemistry
Physiology
►
The bicarbonate system is enhanced by
the fact that carbonic acid can be formed
from CO2 or disposed of by conversion to
CO2
CO2 + H2O <=> H2CO3
Acid-Base Biochemistry
Physiology
► For
every hydrogen ion buffered by
bicarbonate – a bicarbonate ion is
consumed.
► To maintain the capacity of the buffer
system, the bicarbonate must be
regenerated
► However, when bicarbonate is formed from
carbonic acid (CO2 and H2O) equimolar
amounts of [H+] are formed
Acid-Base Biochemistry
Physiology
► Bicarbonate
formation can only continue if
these hydrogen ions are removed
► This process occurs in the cells of the renal
tubules where hydrogen ions are secreted
into the urine and where bicarbonate is
generated and retained in the body
Acid-Base Biochemistry
Physiology
►2
different processes
► Bicarbonate regeneration (incorrectly
reabsorption)
► Hydrogen ion excretion
Acid-Base Biochemistry
Physiology
Importance of Renal Bicarbonate Regeneration
► Bicarbonate is freely filtered through the glomerulus so
plasma and glomerular filtrate have the same
bicarbonate concentration
► At normal GFR approx 4300 mmol of bicarbonate would
be filtered in 24 hr
► Without re-generation of bicarbonate the buffering
capacity of the body would be depleted causing acidotic
state
► In health virtually all the filtered bicarbonate is
recovered
Acid-Base Biochemistry
Physiology
► Renal
Bicarbonate Regeneration involves the
enzyme carbonate dehydratase (carbonic
anhydrase)
► Luminal side of the renal tubular cells
impermeable to bicarbonate ions
► Carbonate dehydratase catalyses the formation of
CO2 and H2O from carbonic acid (H2CO3) in the
renal tubular lumen
► CO2 diffuses across the luminal membrane into
the tubular cells
Acid-Base Biochemistry
Physiology
►
►
►
►
►
within the renal tubular cells carbonate dehydratase
catalyses the formation of carbonic acid (H2CO3) from CO2
and H2O
Carbonic acid then dissociates into H+ and HCO3The bicarbonate ions pass into the extracellular fluid and
the hydrogen ions are secreted back into the lumen in
exchange for sodium ions which pass into the extracellular
fluid
Exchange of sodium and hydrogen ions an active process
involving Na+/K+/H+ ATP pump
K+ important in electrolyte disturbances of acid-base
Acid-Base Biochemistry
Physiology
►
►
►
►
►
►
Regeneration of bicarbonate does not involve net excretion
of hydrogen ions
Hydrogen ion excretion requires the same reactions
occurring in the renal tubular cells but also requires a
suitable buffer in urine
Principal buffer system in urine is phosphate
80% of phosphate in glomerular filtrate is in the form of
the divalent anion HPO42This combines with hydrogen ions
HPO42- + H+ ↔ H2PO4-
Acid-Base Biochemistry
Physiology
► Hydrogen
ion excretion capacity
► The minimum urine pH that Can be
generated is 4.6 ( 25µmol/L)
► Normal urine output is 1.5L
► Without the phosphate buffer system the
free excretion of Hydrogen ions is less than
1/1000 of the acid produced by normal
metabolism
Acid-Base Biochemistry
Physiology
► The
phosphate buffer system increases
hydrogen ion excretion capacity to 30-40
mmol/24 hours
► In times of chronic overproduction of acid
another urine buffer system
► Ammonia
Acid-Base Biochemistry
Physiology
► Ammonia
produced by deamination of
glutamine in renal tubular cells
► Catalysed by glutaminase which is induced
by chronic acidosis
► Allows increased ammonia production and
hence increased hydrogen ion excretion via
ammonium ions
► NH3 + H+ ↔ NH4+
Acid-Base Biochemistry
Physiology
►
►
►
►
At normal intracellular pH most ammonia is present as
ammonium ions which can’t diffuse out of the cell
Diffusion of ammonia out of the cell disturbs the
equilibrium between ammonia and ammonium ions causing
more ammonia to be formed
Hydrogen ions formed at the same time!
These are used up by the deamination of glutamine to
glutamate during gluconeogenesis
Acid-Base Biochemistry
Physiology
► Carbon
dioxide transport
► Carbon dioxide produced by aerobic respiration
diffuses out of cells and into the ECF
► A small amount combines with water to form
carbonic acid decreasing the pH of ECF
► In red blood cells metabolism is anaerobic and
very little CO2 is produced hence it diffuses into
red cells down a concentration gradient to form
carbonic acid (carbonate dehydratase) buffered by
haemoglobin .
Acid-Base Biochemistry
Physiology
► Haemoglobin
has greatest buffering capacity when
it is dexoygenated hence the buffering capacity
increases as oxygen is lost to the tissues
► Net effect is that carbon dioxide is converted to
bicarbonate in red cells
► Bicarbonate diffuses out of red cells down
concentration gradient and chloride ions diffuse in
to maintain electrochemical neutrality (chloride
shift)
•Acid-Base Biochemistry
Physiology
► In
the lungs this process is reversed
► Haemoglobin is oxygenated reducing its
buffering capacity and generating hydrogen
ions
► These combine with bicarbonate to form
CO2 which diffuses into the alveoli
► Bicarbonate diffuses into the cells from the
plasma
Acid-Base Biochemistry
Physiology
► Most
of the carbon dioxide in the blood is present
as bicarbonate
► Carbon dioxide, carbonic acid and carbamino
compounds less than 1/10 th of the total
► Bicarbonate /total CO2 used interchangeably
though not strictly the same
► Most analytical methods actually measure total
CO2 as bicarbonate difficult to measure
Acid-Base Biochemistry
Physiology
The hydrogen ion concentration of plasma is
directly proportional to the PCO2 and
inversely proportional to bicarbonate
[H+] = k pCO2/[HCO3-]
[H+] in nmoles/L, [HCO3-] in mmoles/L
pCO2 in kPa k = 180
pCO2 in mm Hg k= 24
Acid-Base Biochemistry
Physiology
► Derived
bicarbonate
► Possible to use the equation to calculate the
bicarbonate concentration from the pCO2
and pH (blood gas analysers)
► ?how valid in non-ideal solutions
► Auto analysers – measured bicarbonate
Acid-Base Biochemistry
Physiology
► The
relationship between [H+], pCO2 and
bicarbonate fundamental to understanding
pathophysiology of hydrogen ion
homeostasis
Acid-Base Biochemistry
Pathology
►4
Components to acid-base disorders
 Generation
 Buffering
 Compensation
 Correction
Occurring concurrently
Acid-Base Biochemistry
Pathology
► Classification
of acid-base disorders
► Acidosis
► [H+]
above normal, pH below normal
► Alkalosis
► [H+] below normal, pH above normal
Acid-Base Biochemistry
Pathology
► Further
classified as
 Respiratory
 Non-respiratory (metabolic)
 Mixed – difficult to distinguish between primary
mixed condition and compensated disorder
Acid-Base Biochemistry
Pathology
► Respiratory
disorders involve a change in
pCO2
► Metabolic disorders involve change in
production or excretion of hydrogen ions or
both
Acid-Base Biochemistry
Pathology
► Non-respiratory
acidosis
► Increased production/reduced excretion of
acid
► ?causes
Acid-Base Biochemistry
Pathology
► Non-respiratory
acidosis
► Overproduction of acid
 Keto acidosis (diabetes, starvation, alcohol)
 Lactic acidosis (inherited metabolic defect or
drugs)
 Inherited organic acidoses
 Poisoning (salicylate, ethylene glycol, alcohol)
 Excessive parenteral amino acids
Acid-Base Biochemistry
Pathology
► Non-respiratory
acidosis
► Reduced excretion of acid
 Generalised renal failure
 Renal tubular acidoses
 Carbonate dehydratase inhibitors
Acid-Base Biochemistry
Pathology
► Non-respiratory
acidosis
► Loss of Bicarbonate
 Diarrhoea
 Pancreatic, intestinal, biliary fistula or drainage
Acid-Base Biochemistry
Pathology
► Compensation
of non-respiratory acidosis
Excess hydrogen ions are buffered by
bicarbonate forming carbonic acid which
dissociates to carbon dioxide to be lost in
expired air
The buffering limits the rise in [H+] at the
expense of reduction in bicarbonate
Acid-Base Biochemistry
Pathology
► Compensation
of non-respiratory acidosis
► Hyperventilation increases removal of CO2
lowering pCO2
► PCO2 / [HCO3-] ratio falls reducing [H+]
► Hyperventilation is the direct result of increased
[H+] stimulating the respiratory centre (Kussmaul
respiration)
Respiratory compensation of non-respiratory
acidosis
Acid-Base Biochemistry
Pathology
► Compensation
of non-respiratory acidosis
► Limitations
► Respiratory
compensation cannot completely
normalise the [H+] because the hyperventilation is
stimulated by the increase in [H+] and as this falls
the drive on the respiratory centre is reduced
► Increased work of respiratory muscles during
hyperventilation produces CO2 limiting the degree
to which PCO2 can be lowered
Acid-Base Biochemistry
Pathology
► The
degree of compensation may be limited
further if respiratory function is
compromised
► If it is not possible to correct the cause of
the acidosis may get a new steady state of
chronic acidosis
 [H+] [HCO3-] and ↓PCO2
Acid-Base Biochemistry
Pathology
► In
the absence of acidosis - hyperventilation would
normally generate a respiratory alkalosis
► Compensatory mechanisms usually involve
generation of a second opposing disturbance
► In non-respiratory acidosis the hyperventilation
limits the severity of the acidosis but is not great
enough to cause alkalosis in the patient
Acid-Base Biochemistry
Pathology
► Non-respiratory
compensation of nonrespiratory acidosis
► If renal function is normal excess [H+] can
be excreted by the kidneys
► But renal function is often impaired even if
not the primary cause of the acidosis
Acid-Base Biochemistry
Pathology
► Correction
of acidosis
► Complete correction requires reversal or
removal of the underlying cause
► Ethylene glycol poisoning – slow the rate of
metabolism with ethanol
► Diabetes – rehydration and insulin
Acid-Base Biochemistry
Pathology
► Summary
of non-respiratory acidosis

► [H+] 
► PCO2 
► pH
► [HCO3-]

Acid-Base Biochemistry
Pathology
► Management
of non-respiratory acidosis
► 1. Removal of cause
► 2. Administration of Bicarbonate – only in
severe cases pH <7.0 and where 1 is not
possible
► Must be given in small quantities with
constant monitoring of pH
Acid-Base Biochemistry
Pathology
► Respiratory
acidosis
► Primarily an increase in PCO2
► Number of different causes
Acid-Base Biochemistry
Pathology
► Retention
of CO2
► Production of carbonic acid
► For every hydrogen ion produced a
bicarbonate ion is generated
► Most of the [H+] is buffered by intracellular
buffers (haemoglobin)
► Development of renal compensation if renal
function is normal
Acid-Base Biochemistry
Pathology
► Acute
respiratory acidosis
For every KPa increase in PCO2
 increase in bicarbonate < 1 mmole
 Increase in [H+] 5.5 nmol/L
► Chronic
For every KPa increase in PCO2
 increase in bicarbonate 2-3 mmole
 Increase in [H+] 2.5 nmol/L
Acid-Base Biochemistry
Pathology
► Compensation
of respiratory acidosis
► Increased renal excretion of hydrogen ions
Acid-Base Biochemistry
Pathology
► Management
of respiratory acidosis
► With reduced ventilation it is usually the
hypoxaemia that is life threatening 4 mins if
ventilation ceases
► Improve alveolar ventilation bronchodilators
and antibiotics
► Artificial ventilation close monitoring
required to avoid over correction esp in
chronic acidosis
Acid-Base Biochemistry
Pathology
Summary of respiratory acidosis
Acute
Chronic
pH

[H+]

PCO2

Slight  or low
normal
Slight  or high
normal

[HCO3-]
Slight 

Acid-Base Biochemistry
Pathology
► Non
respiratory alkalosis
► Loss of un-buffered hydrogen ions
Gastrointestinal
- vomiting with pyloric stenosis
- diarrhoea
- nasogastric aspiration
Acid-Base Biochemistry
Pathology
Causes of non respiratory alkalosis
Renal
Mineralo-corticoid excess
Conn’s syndrome
Cushings syndrome
Drugs with mineralocorticoid activity
Diuretic therapy (not K+ sparing)
Acid-Base Biochemistry
Pathology
Causes of non respiratory alkalosis
Administration of alkali
 Over-treatment of acidosis
 Chronic alkali ingestion (antacids)
Acid-Base Biochemistry
Pathology
► Non
respiratory alkalosis
► Characterised by primary increase in ECF
bicarbonate
► Consequent reduction in [H+]
► Normally increase in bicarbonate causes
reduction in renal bicarbonate regeneration
and increased urinary excretion of
bicarbonate
Acid-Base Biochemistry
Pathology
► non
respiratory alkalosis
► Maintenance requires inappropriate renal
bicarbonate reabsorption/regeneration
- decrease in ECF volume (hypovolaemia)
- mineralocorticoid excess
- potassium depletion
Acid-Base Biochemistry
Pathology
► non
respiratory alkalosis
► Hypovolaemia
 Increased stimulus to sodium reabsorption
 Dependant on adequate anions
 If chloride deficient (GI losses) electrochemical
neutrality during Na+ absorption maintained by
increased bicarbonate absorption and by H+ and
K+ excretion
Acid-Base Biochemistry
Pathology
► non
respiratory alkalosis
► Mineralocorticoid excess
 Alkalosis perpetuated by increased hydrogen ion
excretion secondary to increased sodium
reabsorption
Potassium depletion
Potassium and hydrogen ion excretion compete
for exchange with sodium so depletion of
potassium causes increased H+ excretion
Acid-Base Biochemistry
Pathology
non respiratory alkalosis
► Compensation
► Low H+ inhibits the respiratory centre causing
hypoventilation and increase in PCO2
► Self- limiting as increase in PCO2 increases drive on
respiratory centre
► In chronic state development of reduced sensitivity to PCO2
– more significant compensation BUT
► Hypoventilation causing hypoxaemia will provide
stimulation of RC and prevent further compensation
►
Acid-Base Biochemistry
Pathology
► non
respiratory alkalosis
► Management
► Dependent on severity and cause
► - severe hypovolaemia /hypochloraemia
correct with saline infusion
► - potassium supplements/removal of
diuretics
Acid-Base Biochemistry
Pathology
► Summary
of non respiratory alkalosis

► pH

► PCO2

► [HCO3-]  
► [H+]
Acid-Base Biochemistry
Pathology
► Respiratory
alkalosis
► Causes
► Hypoxia
 High altitude
 Severe anaemia
 Pulmonary disease
Acid-Base Biochemistry
Pathology
► Respiratory
alkalosis
► Causes
► Increased
respiratory drive
 Stimulants eg salicylates
 Cerebral – trauma, infection, tumours
 Hepatic failure
Acid-Base Biochemistry
Pathology
► Respiratory
alkalosis
► Causes
Pulmonary disease
- Pulmonary oedema
- Pulmonary embolism
Mechanical over-ventilation
Acid-Base Biochemistry
Pathology
► Respiratory
alkalosis
► Characterised by reduction in PCO2
► Reduces the PCO2/ [HCO3-] ratio
For every KPa decrease in PCO2
 decrease in [H+] 5.5 nmol/L
 Small decrease in bicarbonate
Acid-Base Biochemistry
Pathology
► Respiratory
alkalosis
► Compensation
-reduction in renal hydrogen ion excretion
Develops slowly maximal in 36-72 hours
Acid-Base Biochemistry
Pathology
► Respiratory
alkalosis management
► Mainly removal of underlying cause
► Increasing inspired PCO2 by rebreathing of
expired air for temporary measure
- Prolonged – risk of hypoxia
Acid-Base Biochemistry
Pathology
► Summary
of respiratory alkalosis
►
Acute
Chronic
► pH 
Slight  or low normal
► [H+]

Slight  or high normal
► PCO2


► [HCO3-] Slight 

Acid-Base Biochemistry
Pathology
► Mixed
acid base disorders
respiratory alkalosis with metabolic acidosis
e.g. salicylate poisoning causes respiratory
alkalosis by directly stimulating the
hypothalamic respiratory centre causing
over-breathing and increased excretion of
CO2
Salicylate metabolised to acids
Interpretation of results
► Reference
ranges
► pH 7.35 – 7.46
► [H+] 35-45 nmol/L
► pCO2 4.5-6.0 kPa (35-46 mm Hg)
► pO2 11-15 kPa (85-105 mm Hg)
► Total Bicarbonate (CO2) 22-30 mmol/L
Further information
A further algorithm for interpretation of
acid-base data and a number of clinical
cases were provided as hard-copy.
These can be found in Marshall (see
recommended reading)
Acid-Base Biochemistry
Methods
Acid-Base Biochemistry
Methods
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The polarographic (Clark) oxygen electrode measures the
oxygen partial pressure in a blood or gas sample. A platinum cathode
and a silver/silver chloride anode are placed in a sodium chloride
electrolyte solution, and a voltage of 700 mv is applied (Figure 1). The
following reactions occur.
At the cathode: O2 + 2H2O + 4e– = 4OH–.
In the electrolyte: NaCl + OH– = NaOH + Cl–.
At the anode: Ag + Cl– = AgCl + e–.
Electrons are taken up at the cathode and the current generated is
proportional to oxygen tension. A membrane separates the electrode
from blood, preventing deposition of protein but allowing the oxygen
tension in the blood to equilibrate with the electrolyte solution. The
electrode is kept at a constant temperature of 37°C and regular checks
of the membrane are required to ensure it is not perforated or coated
in proteins. Sampling two gas mixtures of known oxygen tension
allows calibration.
Acid-Base Biochemistry
Methods
►
The Severinghaus or carbon dioxide electrode is a modified pH
electrode in contact with sodium bicarbonate solution and separated
from the blood specimen by a rubber or Teflon semipermeable
membrane. Carbon dioxide, but not hydrogen ions, diffuses from the
blood sample across the membrane into the sodium bicarbonate
solution, producing hydrogen ions and a change in pH.
►
Hydrogen ions are produced in proportion to the pCO2 and are
measured by the pH-sensitive glass electrode. As with the pH
electrode, the Severinghaus electrode must be maintained at 37°C, be
calibrated with gases of known pCO2 and the integrity of the
membrane is essential. Because diffusion of the CO2 into the
electrolyte solution is required the response time is slow at 2–3
minutes.
Acid-Base Biochemistry
Methods
ION SELECTIVE ELECTRODE
Acid-Base Biochemistry
RECOMMENDED READING
► Analytical/methods
 Tietz Textbook of Clinical Chemistry by
Carl A. Burtis (Author), Edward R. Ashwood
(Author)
► Clinical
 Clinical Biochemistry by William J. Marshall and
Stephen Bangert