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Acid-base balance: a review of
normal physiology
Andrew J Kitching EDICM FRCA
Christopher J Edge MA PhD MB CChem
Regulation of hydrogen ion concentration is of
fundamental importance to the living organism
because of the effects on body proteins of
changes in acidity. The function of organs such as
the heart and brain is critically dependent on an
internal milieu in which the hydrogen ion content
is kept within carefully regulated limits. Both
volatile acids (i.e. carbonic acid) and non-volatile
acids (e.g. lactic acid) may contribute to the
hydrogen ion concentration in the cells. The acid
load produced each day is substantial. Physiological processes alter the acid-base composition
with the kidney excreting non-volatile acids, and
the lung volatile acid as carbon dioxide.
Blood pH in health varies between 7.35 and
7.45 (intracellular fluid pH is usually in the
range 7.0–7.3). Thus, hydrogen ion concentration in the blood is in the range 45–35 nmol l–1.
This means that hydrogen ion concentration is
about 106 times smaller than the serum sodium
ion concentration, yet it is kept within tight limits.
This is a remarkable achievement, made all the
more remarkable by the fact that hydrogen is a
very small ion with a very high charge density. It
is, therefore, capable of being extremely mobile
with hydrogen bonds being made and broken easily. Indeed, the concept of an isolated hydrogen
ion is a theoretical one; the high charge density of
a proton means that hydrogen ions exist as part of
bigger molecules, e.g. H9O4+.
Historical aspects
The history of acid-base measurement in patients
is closely linked to the history of artificial ventilation. The poliomyelitis epidemic in Copenhagen
in 1952 led to large numbers of patients receiving
artificial ventilation. It was, therefore, necessary
to develop a quick, accurate method of measuring
PCO2 in the blood in order to determine the adequacy of ventilation. Initially, this was done by
measuring the bicarbonate concentration and the
pH of the blood and then calculating PCO2 from
the Henderson-Hasselbalch equation (see
below). This was relatively inaccurate and timeconsuming. However, Astrup showed that there
was a linear relationship between pH and log
PCO2. By measuring the pH of a blood sample
and then re-measuring pH after equilibration
with two different CO2 mixtures, the PCO2 of the
patient could be interpolated.
From then on, the focus of acid-base physiology was concentrated on the carbon dioxide/
bicarbonate system. More sophisticated equipment became available which quickly and accurately determined blood pH and PCO2. As the
lungs excrete 13,000 mEq acid per day (as CO2),
there has been a tendency to regard the carbon
dioxide/bicarbonate system as the only one of
importance in the regulation of hydrogen ion concentration. The metabolic component of any acidosis was measured by eliminating the effects of
the respiratory component with a series of corrections. For example, standard bicarbonate is the
concentration of bicarbonate in the plasma when
the haemoglobin in the whole blood has been
fully oxygenated, at a temperature of 37°C and
corrected to a PCO2 of 40 mmHg (5.33 kPa).
Siggaard-Andersen’s nomogram was an attempt
to help clinicians work out the relative importance of a metabolic or respiratory component to
an acid-base problem. However, more recent
analyses of the theory of acid-base balance show
that the carbon dioxide/bicarbonate system is
only part of the picture (see later).
This article examines the role of the lung and
kidney in the regulation of hydrogen ion concentration. We introduce the basic concepts of
the physicochemical approach which, in recent
years, has contributed greatly to the debate on
the understanding of acid-base balance. Also,
we outline the role of the gut and liver in hydrogen ion homeostasis.
British Journal of Anaesthesia | CEPD Reviews | Volume 2 Number 1 2002
© The Board of Management and Trustees of the British Journal of Anaesthesia 2002
Key points
Regulation of hydrogen
ion concentration
depends upon homeostatic mechanisms in the lung,
kidney, liver and gut
The lungs excrete the
acid produced as a result
of metabolism as carbon
dioxide
The kidneys excrete
40–70 mmol per day of
acid produced from the
breakdown of inorganic
acids
The liver and the kidney
are both involved in
ammonium metabolism
which is central to the reabsorption of bicarbonate
Application of fundamental principles of physical
chemistry has recently
emphasised the importance of the chloride ion
in acid-base balance
Andrew J Kitching
EDICM FRCA
Consultant Anaesthetist,
Royal Berkshire and Battle Hospitals
NHS Trust, London Road,
Reading RG1 5AN
Christopher J Edge
MA PhD MB CChem
Specialist Registrar,
Nuffield Department
of Anaesthetics,
John Radcliffe Hospital,
Headley Way, Oxford OX3 9DU
3
Acid-base balance: a review of normal physiology
The lungs
Aerobic metabolism generates CO2 as a waste product. Acid, in
the form of hydrogen ions generated by metabolism, combines
with bicarbonate to form carbonic acid. This is then catalysed by
carbonic anhydrase to form CO2 and water. Water is excreted
mainly in the urine, with small amounts in the breath, sweat and
faeces: the lungs excrete CO2. This so-called ‘volatile acid’ load
amounts to approximately 13,000 mEq of acid per day. As CO2
is continuously excreted by the lungs, and water from the kidney,
there is no net gain in hydrogen or bicarbonate ions.
Nearly all CO2 is produced in the mitochondria. PCO2 values
are at their greatest in the mitochondria and they fall as a series
of tension gradients as the gas passes through cellular fluid and
membrane into extracellular fluid and then blood. In the lungs,
the PCO2 of the blood entering the pulmonary capillaries is normally higher than in the alveoli. Thus, CO2 diffuses from blood
to alveolar gas. The equilibrium concentration in the alveoli is
dependent on CO2 production, pulmonary circulation and alveolar ventilation.
Carbon dioxide is carried in the blood as dissolved CO2, bicarbonate ion, or as a carbamino group attached to a plasma protein
or haemoglobin. This binding of CO2 with amino groups takes
place on the N-terminal amino acid in each protein and aminoacids containing side chain amino groups such as lysine and arginine. Both hydrogen ion and CO2 compete for the binding site,
so the whole process is pH dependent:
–NH2 + CO2
–NH2 + H+
–NHCO2H
–NHCO2– + H+
–NH3+
Most CO2 is carried on haemoglobin, very little on plasma proteins. Reduced haemoglobin has an affinity 3.5 times greater for
CO2 than the oxygenated form. Carbon dioxide binds to the Nterminal α- and β-chain amino groups. However, there is competition with 2,3-DPG for the binding site. Carbamino formation
is not dependent on hydration of CO2. Therefore, it is independent of carbonic anhydrase activity.
In solution:
[CO2] = PCO2 × solubility coefficient of carbon dioxide
in plasma (Henry’s law)
Carbon dioxide in solution hydrates:
CO2 + H2O
H2CO3
HCO3– + H+
This reaction is non-ionic and slow, with an equilibration time
of several minutes. The whole process can be speeded up to fractions of seconds by the enzyme catalyst carbonic anhydrase. This
enzyme is present in erythrocytes (but not plasma), the nephron,
4
gut, pancreas, cardiac and skeletal muscle and pulmonary capillary endothelium. There are seven isoenzymes so far identified of
this zinc-containing protein.
The largest fraction of dissolved CO2 is in the form of bicarbonate ion, formed by the dissociation of carbonic acid:
H2CO3
HCO3– + H+
At a pH of 7.4, over 96% of H2CO3 is dissociated. This chemical
equilibrium can be written as:
[H+][HCO3–]
Ka =
[H2CO3]
And re-arranged as:
[H+] = Ka ×
[H2CO3]
[HCO3–]
which is the Henderson equation. Then, taking logarithms to base
10 of the reciprocal of each term in the equation we get:
[HCO3–]
pH = pKa + log
[H2CO3]
which is the Henderson-Hasselbalch equation.
With a pH of 7.4 and a pKa of 3.7 we get:
[HCO3–]
log
= 3.7
[H2CO3]
Therefore, the ratio of bicarbonate to carbonic acid which will
give a normal pH is about 5000 to 1. If bicarbonate falls in acidosis, carbonic acid and hence PCO2 will also fall to preserve the
ratio and stabilise the pH. H2CO3 cannot be measured, but, using
Henry’s law, we can replace it in the factor with αPCO2, which is
the total amount of CO2 dissolved in the blood (α = solubility of
CO2 in blood). Note that the usual form of the equation is:
pH = pK′a + log
[HCO3–]
αPCO2
where pK′a has the value 6.1. This takes into account the equilibrium constant for the reaction:
H2O + CO2
H2CO3
The kidney
The net excretion of acid by the kidney is approximately 75 mmol
per day. The principal buffers involved in acid excretion (and as a
consequence bicarbonate regeneration) are hydrogen phosphate
and ammonium ion.
The kidney recovers over 99% of the filtered load of bicarbonate and also generates further bicarbonate. At normal concentrations of serum bicarbonate (usually about 24 mmol l–1), the
British Journal of Anaesthesia | CEPD Reviews | Volume 2 Number 1 2002
Interstitium/
blood
Lumen
Acid-base balance: a review of normal physiology
Cell
3Na+
Na+
–
HCO3 + H+
H2CO3
CA IV
H2O + CO2
–
H+ + HCO3
2K+
+
H2CO3 Na
CA II
H2O + CO2
K+
Fig. 1 Proximal convoluted tubule. CA, carbonic anhydrase.
tubular transport mechanisms in the kidney are almost fully
utilised, i.e. they are close to their Tmax. Thus, if serum bicarbonate
increases much above 25 mmol l–1, more bicarbonate is lost in the
urine until the serum concentration drops below that associated
with Tmax. Tmax for bicarbonate also varies directly with the fractional sodium absorption and with hydrogen ion secretion.
In the proximal convoluted tubule (PCT), 90% of filtered bicarbonate is re-absorbed. Luminal bicarbonate picks up a hydrogen
ion to form carbonic acid (the hydrogen ion has been secreted in
exchange for sodium using a Na+/H+ antiporter). In the presence of
carbonic anhydrase (isoenzyme IV) present on the luminal brush
borders, CO2 and water are formed and pass into the cell by simple diffusion (Fig. 1). Within the luminal cell, again catalysed by
carbonic anhydrase (this time isoenzyme II), carbonic acid briefly
reforms and then dissociates to bicarbonate and hydrogen ion.
Hydrogen ion is then available for further secretion. On the blood
side of the luminal cell, sodium and bicarbonate are transported
across the cell membrane using a co-transporter. The rate of
absorption of bicarbonate is directly proportional to PCO2.
The distal convoluted tubule (DCT) re-absorbs 10% of the filtered load of bicarbonate. In this section of the nephron, bicarbonate absorption is dependent on the action of two transporters: an
H+/K+-ATPase and Na+/H+ transport exchange (aldosterone-dependant) in the principal cells. Again, secreted hydrogen ion combines
with bicarbonate in the urine to form carbonic acid and hence CO2
and water, which then passes into the cells as in the PCT.
For the generation of new bicarbonate, a buffer in the filtrate
is needed in order to mop-up hydrogen ion and maintain a
concentration gradient. Indeed, continuous unbuffered secretion
of hydrogen ion would quickly damage the nephron. This is where
ammonia and hydrogen phosphate act as urinary buffers, soaking
up hydrogen ion to maintain a concentration gradient and indirectly allowing net bicarbonate generation.
Ammonium ion is produced mainly in PCT cells by the de-amination of glutamic acid to 2-oxoglutarate and NH4+. The ammonium ion can then either dissociate into ammonia and a hydrogen ion
or be directly secreted via apical membrane Na+/NH4+ exchange.
The importance of this last process is still debated. Ammonia diffuses across the PCT membrane down a concentration gradient
(created when the secreted H+ combines with the ammonia to
reform ammonium ion) and H+ is transported out on the Na+/H+
antiporter. In all other parts of the nephron, ammonium is transported on a Na+/H+ ion transporter, with the ammonium taking the
place of hydrogen ion. The 2-oxoglutarate left behind in the tubule
cells reacts with H+ to form glucose or CO2, which forms bicarbonate. This bicarbonate is transported into the plasma via an electrogenic Na+/HCO3– co-transport mechanism. Thus, the de-amination of glutamate generates NH4+ which buffers the hydrogen ions
and produces bicarbonate for the plasma.
If NH4+ remains in the body, it combines with bicarbonate in the
liver to form urea, which means no net gain of bicarbonate. The
greater the secretion of NH4+, the greater the bicarbonate generation. Conversely, if excess bicarbonate and ammonium are generated (e.g. from increased dietary intake of protein), hepatic urea
synthesis increases from the excess ammonium and bicarbonate
present. A potent feedback circuit adjusts urea cycle flux in the liver
to the changes in acid-base balance. With more nitrogen ‘packaged’ as urea, less glutamate is available for the kidney to use for
the generation of ammonium ions. This illustrates the liver’s role in
bicarbonate homeostasis and in acid-base balance.
Some 50% of NH4+ produced in the PCT cells is re-absorbed by
the thick ascending loop of Henle and accumulates in the medullary
interstitium. This absorption is by the Na+/NH4+/2Cl– absorber
(NH4+ substitutes for K+ on the co-transporter). The NH4+ absorption
lowers the luminal concentration and hence also lowers the NH3 concentration as removal of NH4+ shifts the equilibrium towards NH4+.
The NH4+ from the medullary interstitium is then secreted as
NH3 by the cells of the collecting ducts (non-ionic diffusion) and
converted into NH4+ in the collecting tubule lumen by combination
with secreted H+ (Fig. 2). NH4+ excretion is increased in acidosis
because: (i) the de-amination enzymes are stimulated by acidosis;
and (ii) NH3 conversion to NH4+ in the collecting tubule is greater
if H+ secretion is greater as this maintains a gradient for NH3 secretion so that more NH3/NH4+ is removed from the renal medulla.
Urinary phosphate buffering takes place predominantly in the
DCT generating bicarbonate. Secreted hydrogen ion combines
British Journal of Anaesthesia | CEPD Reviews | Volume 2 Number 1 2002
5
Acid-base balance: a review of normal physiology
men
Cell
Interst
Interstitium/
blo
Lumen
blood
Glutamine
NH3 + H+
NH4+
HCO3–
–
H+ + HCO3
α-KG2–
H2CO3
CA
H2O + CO2
Fig. 2 Renal collecting ducts. α-KG = 2-oxoglutarate.
with phosphate filtered at the glomerulus to form hydrogen
phosphate. The H2PO4– constitutes the titratable acid of urine. This
process is facilitated in systemic acidosis because tubular resorption of filtered phosphate buffer is inhibited, making more phosphate buffer available in the urine to combine with hydrogen ion.
Titratable acid is only a fraction of total acid secretion because
total H+ secretion equals bicarbonate re-absorption plus acid phosphate excretion plus ammonium excretion, and only acid phosphate is titratable acid.
Recent developments in acid-base theory
The conventional model of renal and pulmonary regulation of
hydrogen ion concentration as outlined above was, until recently,
accepted as fundamental. However, there are problems with this
model. In order for bicarbonate to influence pH in accordance with
the Henderson-Hasselbalch equation, it would have to be independent of PCO2. But we know that HCO3– varies with CO2 which can
lead to confusion when measuring the metabolic component of
acid-base balance.
In 1983, Stewart published what has now become a landmark
paper in this field. He showed that the carbon dioxide/bicarbonate
system could not be viewed in isolation. By invoking fundamental
physicochemical laws such as the law of mass action and the law
of electroneutrality, he showed that complex solutions like biological fluids have their hydrogen ion concentration set by multiple
chemical equilibria reactions. None of these equilibria can be
viewed in isolation and all have to be satisfied simultaneously. The
Henderson-Hasselbalch equation is just one of these equations.
A mathematical model can be produced from these assumptions,
which accurately predicts the concentration of hydrogen ion even
in the critically ill patient with seriously disordered biochemistry.
6
The mathematical equation produced which predicts hydrogen ion
concentration is a higher order polynomial equation but only three
independent variables determine hydrogen ion concentration: (i)
PCO2; (ii) total weak acid concentration – mainly plasma proteins
(principally albumin) and phosphates; and (iii) strong ion difference (SID) – given by the total concentration (in mequiv/l) of fully
dissociated cations in solution minus the total concentration of fully
dissociated anions in solution. Thus, a normal SID would be
approximately 42–46 mmol l–1 and is obtained by adding together
the concentrations in milliequivalents per litre of the main cations
in solution (Na+, K+, Ca2+, Mg2+) and subtracting the concentrations of the main anions in solution (Cl–, lactate).
Note that bicarbonate is not an independent variable in this
analysis. Bicarbonate concentration changes only in response to
changes in PCO2, SID (if the SID narrows because Cl– is rising,
bicarbonate will fall), or total weak acid. Nothing else will affect
bicarbonate concentration.
From Stewart’s approach comes the realisation that acid-base
balance is not only affected by the lungs (by altering PCO2), and
the kidneys (by altering chloride and strong ion difference). The
liver and gut can have a major effect on acid-base balance too (by
altering the level of total weak acid). The metabolic alkalosis
associated with chronic hypoalbuminaemia occurrs because the
concentration of a weak acid-albumin- is reduced. We can also
explain how excessive infusions of saline result in metabolic acidosis. There is a greater rise in the plasma chloride concentration
than the plasma sodium concentration, and therefore a narrowing
of the strong ion difference (impurities from the infusion bag and
dissolved CO2 also have some effect, which is why an infusion
bag of 0.9% sodium chloride has a pH of 5.0–6.0).
The emphasis of the Stewart thesis is that to understand what is
happening pathophysiologically, we need to measure the changes in
these fundamental variables. Measurement of chloride concentration will thus become of greater importance in future. If chloride is
low or normal and there is acidosis, another strong ion must be present in the blood, for example lactate or a ketoacid.
Key references
Good DW, Knepper MA. Mechanisms of ammonium excretion: role of the
renal medulla. Semin Nephrol 1990; 10: 166–73
Lumb AB. Carbon dioxide. In: Nunn’s Applied Respiratory Physiology, 5th edn.
London: Butterworth-Heinemann, 2000
Stewart PA. Modern quantitative acid-base chemistry. Can J Physiol Pharmacol
1983; 61: 1444–61
Sykes MK,Vickers MD, Hull CJ. Principles of Measurement and Monitoring in
Anaesthesia and Intensive Care, 3rd edn. Oxford: Blackwell, 1991
Unwin RJ, Capasso G.The renal tubular acidoses. J R Soc Med 2001; 94: 221–5
See multiple choice questions 1–4.
British Journal of Anaesthesia | CEPD Reviews | Volume 2 Number 1 2002