Respiratory gases and the blood pH; the hemogas analysis


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      Notice: this lecture requires previous attendance to the lecture on water and electrolyte balance. and renal excretion.


      The function of the lung is gas exchange; respiratory gases are O2 and CO2. N2 is inhaled and expired with air, and is dissolved in all body fluid, but it does not take part in cellular respiration and is completely inhert in our body. H2O in the vapour phase is inhaled and expired, and respiration causes water loss (approx. 200-300 mL/day). The study of respiratory gases in the expired air and dissolved in the blood is important to assess respiratory function. Moreover, CO2, together with bicarbonate, is the most important blood buffer and determines the blood pH; thus, the study of respiratory gases is essential for the diagnosis of acid-base imbalances. The concentration of bicarbonate in our blood is controlled by: (i) conversion to and into CO2 and lung excretion of the latter; and (ii) direct urinary excretion. As a consequence a complete study of the acid-base balance requires also the assessment of kidney excretion of bicarbonate.


      The essential laboratory test for respiratory gases is the hemogas analysis, carried out on a sample of arterial blood taken in a gas tight syringe to avoid contamination with air. In babies venous blood may be used becauze of the difficulty of puncturing an artery. The hemogas analysis evaluates the respiratory function of lungs and blood and the blood's pH. The respiratory and buffer activities of blood and lung are strictly correlated, but it is important to discuss them separately, to improve clarity-

Hemogas anaysis: reference values for arterial blood
PO2 75-100 mmHg (age dependent)
Hemoglobin O2 saturation 96-100%
Total O2 content 20 mL/dL (depends on Hb content; reduced under anemia)
PCO2 35-45 mmHg (1.1-1.3 mM)
HCO3- 24-27 mM (plasma)
total CO2 (bicarbonate+CO2+carbamates) 26-30 mM (plasma)

      The main function of the lungs is the exchange of respiratory gases O2 and CO2. Since CO2 is also a blood buffer, the lung is the principal organ controlling the blood pH (but kidney is also extremely important because of the excretion of bicarbonate). An accessory function of the lung is the participation to the regulation of blood pressure via the renin-angiotensin system. Lung diseases are often revealed by altered blood gases in the hemogas analysis (reduced PO2 and increased PCO2, with respiratory acidosis). Diseases of the lung may compromise its mechanical ventilatory capacities (e.g. emphysema) or its ability to exchange gases (e.g. interstitial pneumonia), or both.
      During respiration O2 and CO2 are exchanged in comparable amounts; the ratio CO2 excreted : O2 absorbed is called the respiratory quotient and varies between 0.7 and 1.0 depending on the nutrient we are utilizing, 0.7 being the respiratory quotient for triglycerides and 1.0 for glucose.
      The two respiratory gases have very different solubility in water and this has profound effects on the mechanisms of their transport; some relevant differences are reported in the table below.
total content in arterial blood20 mL (gas)/dL or 8.5 mMoles/L20.5 mMoles/L (serum concentration, 26 mM, is higher than erythrocyte concentration)
Partial pressure in arterial blood95-100 mmHg40 mmHg
total content in venous blood15 mL (gas)/dL or 6.5 mMoles/L22.5 mMoles/L (serum concentration, 26 mM, is higher than erythrocyte concentration)
Partial pressure in venous blood40 mmHg45 mmHg
physical status97% bound to hemoglobin in the erythrocytes; 3% physically dissolved in erythrocytes and plasma90% as bicarbonate; 5% as CO2; 5% as protein carbamates
artero-venous difference2 mMoles/L ; 60 mmHg1.7 mMoles/L ; 5 mmHg

Three important differences between the two gases are:
(i) O2 is mostly Hb-bound, CO2 is mostly present as dissolved bicarbonate. Anemias may cause a very significant reduction of O2 content, while affecting CO2 content to a negligible extent (the amount of Hb-carbamates accounting for only 5% of the total CO2). Severe anemias may cause hyperpnea and hypocapnic respiratory alcalosis.
(ii) O2 has higher artero-venous differential pressures than CO2 and higher diffusion coefficients; thus pulmonary lesions that cause impaired gas diffusion may affect the CO2 content to a greater extent than the O2 content.
(iii) The O2 capacity of blood is saturable and follows the O2 binding curve of Hb; the capacity of CO2 is non-saturable (except that at very high levels the patients enters a comatous state).
      Alterations of blood gases and other parameters of the hemogas analysis may be observed under conditions where the airways present partial obstruction (e.g. severe asthma; bronchitis) or the gas exchange in the lung alveoli is compromised (e.g. pneumonia, interstitial pneumonia, emphysema). Usually alterations of PCO2 are observed at earlier stages of the disease than alterations of PO2 because the arterovenous gradient of PCO2 (normal values under rest for arterial and venous PCO2, respectively: 45 mmHg - 40 mmHg) is lower than that of PO2 (normal values under rest: 100 mmHg - 40 mmHg), as shown in the graph below. Remember that, according to Fick's law of diffusion, the gas flow is proportional to the pressure gradient, which is clearly greater for O2 than for CO2.

Reduced PO2 may cause reduction of Hb O2 saturation (in arterial blood Hb is almost completely O2 saturated and close to the upper asymptote of the O2 binding isotherm; thus a significant decrease of PO2 is required to observe desaturation). Low Hb O2 saturation causes cyanosis, i.e. a purplish o bluish discoloration of the skin. Cyanosis in the absence of reduced PO2 is suggestive of diseases that do not involve directly the lung (e.g. hemoglobinopathies, heart failure). Signficant alterations of both PO2 and Hb O2 saturation in the absence of increased PCO2 may be observed in congenital artero-venous shunts (e.g. atrial septum defects, Fallot tetralogy). Indeed an important consideration is that a cause of the incomplete O2 saturation of Hb is the (minor) physiological mixing of arterial and venous blood occurring in the anastomoses between pulmonary and bronchial veins.
      A reduction of the O2 content of blood (normal value: 20 mL O2 measured in the gas phase per dL of blood), may be due to reduced O2 of hemoglobin, associated to reduced PO2, in which case the disease affects the lungs, or to anemias or hemoglobinopathies, associated to normal PO2, in which case the lung is not affected.
      An important complement to hemogas analysis is the determination of respiratory volumes by spirometry.
      Pulse oximetry can be carried out at the patient's side and may be very useful. A pulse oximeter uses red and infrared light to measure the arterial Hb O2 saturation, thus it provides only one of the different parameters of the hemogas analysis; however the simplicity and affordability of pulse oximeters allows the physician to carry out this single measurement very efficiently.

      The condition in which the O2 content of arterial blood is reduced is called hypoxia (or hypoxemia). It may have several causes and the laboratory analyses are essential to establish a correct diagnosis. The principal symptoms of hypoxia are hyperpnea (increased respiration rate), and tachycardia; cyanosis may be present. Myocardial infarction, arrhythmias and ischemic damage of tissues are possible complications.
diseaselaboratory data
anemiasreduced O2 content parallels reduced Hb concentration; normal PO2
low affinity Hb mutants; metHbnormal Hb content; reduced Hb saturation; normal PO2
ventilatory insufficiencyreduced inspiratory volume; reduced PO2; reduced Hb saturation
reduced gas exchange (e.g. interstitial pneumonia)reduced PO2; reduced Hb saturation


      The alterations of the blood pH are called acidoses (if arterial pH < 7.35) and alkaloses (if arterial pH > 7.45). They are due to abnormal production or excretion of acidic or basic solutes in the serum, and of course are counteracted by the blood buffers. An interesting and well-written tutorial may be found at
this link; a very useful article on the subject may be read at this link.

BufferConcentration (ionizable group equivalents)pKProduction/excretion
CO2 / bicarbonate CO2:1.2 mM - 40 mmHg
bicarbonate: 26-28 mM
6.1 Krebs cycle; excreted by the lung (CO2) and the kidney (bicarbonate)
Hemoglobin 5 mM 7.0 (HbO2) - 7.8 (Hb) produced during the red blood cell differentiation; degraded by macrophages
Phosphate 1.2 mM 7.0 in equilibrium with calcium phosphate in the bone matrix; excreted in the urine

Audio: Blood buffers

      Even though the blood buffers behave in quite a complicated manner, over the physiological range their titration curve can be roughly approximated to that of a single weak acid with pK approx. 6.5. The lung and the kidney are the principal organs involved in the regulation of buffer concentration and blood pH, given that they excrete CO2 and bicarbonate, the components of the principal buffer (and the kidney excretes phosphate as well). The lung alone eliminates some 20 moles CO2 / day, a massive amount. A simplified scheme of the gas exchanges in relation to blood pH is depicted in the following figure.

Audio: Bicarbonate buffer

      The most concentrated buffer of blood is bicarbonate; the Henderson Hasselbalch equation for this buffer is as follows
pH = pKa + log ([HCO3-]/0.031 PCO2)
where pKa = 6.1, [HCO3-] is measured in mMol/L and PCO2 is measured in mmHg; 0.031 mM/mmHg is the solubility coefficient of CO2.

Audio: The Henderson and Hasselbalch equation of bicarbonate buffer

      Disturbances of the blood pH and buffer concentrations are called respiratory if caused by altered functioning of the lungs, metabolic otherwise. It is important to stress that disfunction of the lung can be to some extent compensated by the kidney and vice versa; thus each organ tends to oppose the disfunction of the other. The key features of the different forms of acidosis and alkalosis are as follows:

AbnormalityBlood pH (normal values: venous 7.36; arterial 7.4)Pressure of blood CO2 (normal values: venous 43 mmHg; arterial 40 mmHg) total CO2 (i.e. CO2 + bicarbonate; normal values 21-28 mM)
Metabolic alkalosis increased increased increased
Metabolic acidosis decreased decreased (compensatory) decreased
Respiratory alkalosis increased decreased (primary) decreased (compensatory)
Acute (uncompensated) respiratory acidosis strongly decreased increased (primary) slightly increased
Chronic (compensated) respiratory acidosis slightly decreased increased (primary) strongly increased (compensatory)

Audio: Acidoses and alkaloses

      The following remarks will help explaining the above table:
(i) pressure of CO2 in the blood indicates the concentration of pure CO2, i.e. it does not include bicarbonate; total CO2 is dominated essentially by bicarbonate ion concentration (the ratio [HCO3-] / [CO2] being approximately 20).
(ii) The metabolism produces mainly acids (CO2 and lactic acid, acetoacetic acid, etc.); however our organism is also better equipped to eliminate acids (CO2 by respiration; organic acids by liver conversion to other products, e.g. lactic acid to glucose) than bases. Acidosis is frequent, varied and may be severe. Respiratory alkalosis is rare, whereas metabolic alkalosis due excess loss of acidic fluids (e.g. severe vomiting) may be frequent especially in the elderly.
(iii) Acids can be volatile (CO2), excreted by the lung at a very fast rate, and non-volatile (lactic acid, acetoacetic acid), metabolized by the liver or excreted by the kidney at a slower rate.
(iv) Metabolic alkalosis is most often due to loss of acids (e.g. vomiting forces the gastric mucosa to replace the gastric juice, whose HCl content is obtained by a mnechanism that increases the serum bicarbonate and causes alkalosis), excess intake of alkaline substances (e.g. gastric antiacids, bicarbonate), and diuretics; it is corrected mainly by the kidney that excretes the excess bicarbonate.

(v) Metabolic acidosis, caused by overproduction of non-volatile acids (e.g. diabetic ketoacidosis) or by their impaired renal excretion, stimulates respiration that excretes the volatile acid (CO2): hence compensatory hyperpnea and hypocapnia (reduced P CO2). It is interesting to remark that pulmonary correction of metabolic acidosis is more effective than of metabolic alkalosis because respiration frequency can be increased to a more significant extent than it can be decreased.
(vi) Respiratory alkalosis is a consequence of hyperventilation (loss of CO2), but this only occurs in some types of CNS disturbances or under unusual environmental conditions (e.g. muscular effort at high altitude, where atmospheric P O2 is decreased - air hunger).
(vii) Respiratory acidosis is a common consequence of impaired gas exchanges (e.g. depression of respiratory centers in the CNS, insufficient mechanical ventilation in polyomyelitis or tuberculosis, or ventilation perfusion imbalance in chronic obstructive pulmonary disease, emphysema, etc.).

Audio: The hemogas analysis

      The hemogas analysis is the measurement of the pH and the concentrations and partial pressures of O2 and CO2 in a sample of the patient's blood drawn in a gas tight syringe. The measure is usually effected by means of potentiometric methods, used gas-specific electrodes.
      As a general rule, a hemogas analysis will indicate if an abnormality is present and will give some indication of its possible cause; the fundamental indications are as follows:
Diminished pH and diminished total CO2 = acute or chronic metabolic acidosis with respiratory compensation (e.g. diabetic ketoacidosis).
Strongly diminished pH, strongly increased PCO2 and slightly increased total CO2 = acute respiratory acidosis. The exchange of CO2 in the lung is impaired (e.g. because of acute viral pneumonia); this leads to increased arterial PCO2 and decreased arterial pH. Metabolic compensation is scarce or absent because it requires several days to become operative; this causes bicarbonate and total CO2 to increase only slightly.
Normal to diminished pH and strongly increased total CO2 = chronic respiratory acidosis. If the impaired gas exchange in the lung lasts long enough for the kidney to retain bicarbonate, metabolic compensation occurs (e.g. chronic obstructive pulmonary disease). Retention of bicarbonate (partially) restores the arterial pH but causes a strong increase in bicarbonate concentration and total CO2.
Increased pH and increased total CO2 = metabolic alkalosis with respiratory compensation (e.g. vomiting).
Increased pH and decreased PCO2 with moderately increased bicarbonate = acute respiratory alkalosis with minimal metabolic compensation (uncommon; e.g. neurological hyperpnea).
Normal to increased pH and decreased total CO2 = chronic respiratory alkalosis with metabolic compensation (e.g. life at high altitude).
      The above set of rules allows one to interpret simple deviations from the healthy conditions, i.e. those conditions where one disease (either respiratory or metabolic) is present and compensation is respiratory. These conditions are typical of young patients suffering of acute acid-base imbalance (notice that respiratory compensation of metabolic conditions is almost immediate, whereas metabolic compensation requires time). Notice that two parameters (pH and total CO2) are necessary even in the least complicated cases.

Audio: Classification of blood pH disturbances

Audio: Acute respiratory acidosis

Audio: Chronic respiratory acidosis

Audio: Metabolic acidosis

Audio: Alkaloses

      The anion gap, is the difference between the concentrations of (sodium + potassium) and (chloride + bicarbonate). The blood, as any other mixture, has zero net charge; thus the anion gap estimates the amount of non-measured negative charges (e.g. proteins). The normal value is around 15 mM. An increased anion gap may provide a gross indication of the presence of excess unmeasured negatively charged ions derived from carboxylic acids (e.g. lactate or acetoacetate). Accordingly there are two types of metabolic acidosis, either with normal anion gap (NAGMA: Normal Anion Gap Metabolic Acidosis) or with increased anion gap (HAGMA: High Anion Gap Metabolic Acidosis). The expected anion gap is calculated if albumin concentration is lower than normal. This correction is important in hypoalbuminemia because albumin provides an important contribution to the non-measured anions. The correction is as follows:
expected anion gap = 15 - 2.5 x (normal albumin - actual albumin)
where albumin is expressed in g/dL and normal albumin = 4 g/dL. If the patient has hypoalbuminemia his/her anion gap is to be interpreted with reference to the expected value, rather than the usual normal value.

common causes of HAGMA common causes of NAGMA
Renal failure
Lactic acidosis
Several types of poisoning
Some inherited defect of metabolism
Gastrointestinal loss of bicarbonate
Renal tubular acidosis
Carbonic anhidrase inhibitors

      A very useful representation of the results of a hemogas analysis is a plot of bicarbonate concentration versus PCO2. Because of the Henderson-Hasselbalch equation each couple of these parameters identifies a pH value, and couples corresponding to the same pH appears as lines in this graph.

      The graph identifies regions of the (PCO2,[HCO3-]) space corresponding to the main categories listed above: acute respiratory acidosis and alkalosis, chronic respiratory acidosis and alkalosis, metabolic acidosis and alkalosis. The main reason why respiratory conditions require the distinction between acute and chronic is that renal compensation requires several days, thus acute respiratory conditions are poorly compensated or uncompensated, whereas chronic respiratory conditions are usually well compensated. Metabolic conditions require respiratory compensation, which is established rapidly: thus they are usually well compensated irrespective of their onset being acute or chronic.
      We remark that chronic, compensated respiratory conditions may be compatible with a normal blood pH of 7.4, but are revealed by grossly altered values of PCO2 and bicarbonate.

Basic 4-parameter diagnostic interpretation of the arterial hemogas analysis.
      1) The first parameter to analyze is the blood's pH. This defines the conditions of acidosis and alkalosis, even though in the presence of effective compensation the alteration of the blood's pH may be minimal or even absent.
      2) PCO2 is the second most important parameter, which distinguishes hypocapnic from hypercapnic conditions. The general rule for interpreting this parameter in conjunction with the blood's pH is reported in the above Table "Types of acidosis and alkalosis". Since the PCO2 is primarily controlled by respiration, important changes of this parameter reflect either pulmonary disease or respiratory compensation. Even though PCO2 and bicarbonate are correlated by the Henderson and Hasselbalch equation, changes in PCO2 due to pulmonary disease affect the pH more strongly than the bicarbonate concentration, unless metabolic compensation intervenes.
      3) Total CO2 is essentially determined by bicarbonate concentration. Bicarbonate is primarily controlled by the kidney and and important changes of its concentration reflect metabolic disease or compensation.
      4) The anion gap is required to distinguish HAGMA from NAGMA.

Audio: Chronic respiratory acidosis

Some typical examples.
      1) Chronic obstructive pulmonary disease (COPD), diffuse interstitial pneumonia, etc. reduce the efficiency of gas exchanges. A typical hemogas analysis may be as follows:
oxygen saturation 78% on room air (normal value > 90%)
arterial pH 7.25 (normal value 7.44)
PCO2 70 mmHg (normal value 40-44 mmHg)
PO2 50 mmHg (normal value > 80 mmHg)
plasma bicarbonate concentration 35 mM (normal value 26 mM)
Anion gap 12 mEq/L
4-parameters analysis of this case is as follows:
1) pH is decreased, thus this condition is an acidosis.
2) PCO2 is significantly increased, thus this condition is a respiratory acidosis.
3) Bicarbonate and total CO2 are increased, thus metabolic compensation is present. Since metabolic (renal) compensation requires several days, this condition is a chronic respiratory acidosis.
4) The anion gap is normal; this occurs because the increase in bicarbonate is associated to renal excretion of chloride.
Description: chronic hypercapnic acidosis, associated (in this case) to reduced oxygen content and oxygen saturation.

      2) Adaptation to high altitude (4.000 m above sea level or higher). A typical hemogas analysis may be as follows:
oxygen saturation 75% on room air
arterial pH 7.48
PCO2 20 mmHg
PO2 60 mmHg
plasma bicarbonate concentration 16 mM
Anion gap 13 mEq/L
4-parameters analysis of this case is as follows:
1) pH is increased, thus this condition is an alkalosis.
2) PCO2 is significantly decreased, thus this condition is a respiratory alkalosis.
3 and 4) Bicarbonate and total CO2 are decreased, thus metabolic compensation is present, suggesting that the condition is chronic. The normal anion gap is normal because increased urinary excretion of bicarbonate is associated to chloride retention.
Diagnosis: chronic respiratory hypocapnic alkalosis due to hyperpnea (this is an attempt to compensate for the reduced atmospheric PO2)

      3) Type I diabetes mellitus:
plasma glucose > 250 mg/dL
arterial pH < 7.25
PCO2 20 mmHg
PO2 normal
serum bicarbonate 10-20 mM
anion gap 25 mM
4-parameters analysis of this case is as follows:
1) pH is decreased, thus this condition is an acidosis.
2 and 3) PCO2 and bicarbonate are both significantly decreased, thus this condition is a metabolic acidosis with respiratory compensation.
4) The anion gap is increased, i.e. the plasma contains an excess of non-measured anions: HAGMA.
Diagnosis: the most important causes of HAGMA are:renal failure, ketoacidosis, lactic acidosis, inherited defects of metabolism (e.g. methylmalonic aciduria), and several types of poisoning. In the present case the increased glycemia suggests diabetic metabolic acidosis due to ketone bodies (acetoacetic acid and 3-hydroxy butanoic acid). The anion gap is increased because of the presence of the non-measured anions acetoacetate and 3-hydroxy butanoate.


      In the presence of an acid-base imbalance whose diagnosis is not obvious, more refined measurements are indicated in order to separate the respiratory and metabolic cotributions. Special attention is required in the elderly given that metabolic and respiratory conditions of equal or opposite sign may coexist (e.g. pulmonary emphysema, causing chronic respiratory acidosis, may be present together with vomiting, causing acute metabolic alkalosis, or with diabetes, causing chronic metabolic acidosis). Several clinical concepts (and measurements) have been developed to discriminate the metabolic and respiratory components of blood buffers inbalance, as listed below.

      Measurement of the "standard" parameters
Standard pH, historically the first concept introduced to rationalize complex deviations from the healthy conditions of blood buffers balance was introduced into the clinical practice by Hasselbalch in 1916. It is the pH of the patient's arterial blood measured under standard conditions (P CO2=40 mmHg, hemoglobin fully saturated with O2, T = 37 C). Essentially, the use of standard conditions has the effect of reversing the compensatory effect of respiration and thus to make more evident the eventual presence of a metabolic component in the pH unbalance.
      In order to gain an understanding of the concept of standard pH and those that derived from it (standard bicarbonate and base excess) one should consider that CO2 behaves as an acid because of the reaction CO2 + 2 H2O <==> HCO3- + H3O+. Thus, if P CO2 < 40 mmHg, CO2 is absorbed during equilibration, and standard pH < pH. On the contrary, if P CO2 > 40 mmHg, CO2 is released during equilibration, and standard pH > pH.
      With the use of three parameters (total CO2, pH, and standard pH), one obtains a better description of the underlying clinical condition, according to the following table:
Typical laboratory data for acidoses and alkaloses
  pH    P CO2     total CO2 (or bicarbonate)     standard pH  
  (Base Excess)
<< 7.4 (e.g.: 7.2) >> 40 mmHg (e.g.: 60) > 26 mM (e.g.: 30) 7.4
(BE = 0)
uncompensated respiratory acidosis
(e.g. acute interstitial pneumonia)
< 7.4 (e.g.: 7.34)> 40 mmHg (e.g.: 50) >> 26 mM (e.g.: 50) > 7.4
(BE > 0)
respiratory acidosis with metabolic compensation
(e.g. chronic obstructive pulmonary disease, COPD)
< 7.4 > 40 mmHg  < 7.4, > pH
(BE < 0)
combined respiratory acidosis and metabolic acidosis
(e.g. COPD and diabets mellitus)
<< 7.4 (e.g.: 7.2)<< 40 mmHg (e.g.: 20) << 26 mM (e.g.: 10)< pH
(BE < 0)
metabolic acidosis with respiratory compensation
(e.g. type I diabetes mellitus)
> 7.4<< 40 mmHg (e.g.: 20)  < 26 mM (e.g.: 20) 7.4
(BE = 0)
uncompensated respiratory alkalosis
(e.g. acute adaptation to high altitude)
> 7.4  < 40 mmHg << 26 mM (e.g.: 15 mM) < 7.4
  (BE < 0)
respiratory alkalosis with metabolic compensation
(e.g. chronic adaptation to high altitude)
> 7.4 > 40 mmHg >> 26 mM (e.g.40 mM) > 7.4, < pH
(BE > 0)
metabolic alkalosis with respiratory compensation
(e.g. severe vomiting)

Standard bicarbonate, a concept developed by Astrup and Siggaard Andersen in 1957, is the concentration of bicarbonate one measures when a sample of arterial blood is equilibrated under standard conditions. The rationale of this procedure is that of setting the concentration of one of the components of the major blood buffer (CO2) and determinining that of the other. The number one measures is not the same one would obtain in a standard hemogas analysis, given that part of the bicarbonate originally present in the blood may be lost as CO2 (if the P CO2 was higher than 40 mmHg) or part of the gas may be absorbed and converted to bicarbonate. The standard bicarbonate measures the metabolic component of the acid-base balance of the blood and corrects for respiratory compensation. It can substitute for standard pH.

Base excess, again by Astrup and Siggaard Andersen, is the amount of strong acid (or base, in which case the resulting value is negative) required to restore the normal pH of 1 L of blood sample equilibrated under standard conditions. Standard base excess is the same concept except that erythrocytes are partially removed, to a final Hb content of 5 g/dL. BE (or SBE) is another indicator of the metabolic component of the disturbance: acidosis in the presence of positive base excess is an indication of respiratory acidosis. Notice that if acidosis is present (i.e. pH<7.4) and the base excess is positive (i.e. acid is required to restore the pH to 7.4), this implies that the process of equilibrating blood under standard conditions causes the pH to raise above 7.4 (in respiratory acidosis P CO2 > 40 mmHg; thus equilibration with standard P CO2 removes CO2 and bicarbonate). By contrast, acidosis in the presence of a negative base excess (base deficit) indicates an important metabolic component, with respiratory compensation. Base excess provides information analogous to, but more quantitative than, standard pH and standard bicarbonate (see the above Table "Typical laboratory data for acidoses and alkaloses").

      Calculated parameters and nomograms
      An alternative to the measurement of standard parameters makes use of calculated (approximated) parameters. The principle is as follows: given the usual parameters measured in a hemogas analysis one can infer which would be the "expected" parameter in the presence of optimal compensation.
      Winter's formula for PCO2. Winter's formula is used to calculate the PCO2 one would record under metabolic acidosis or alkalosis, in the presence of optimal pulmnary compensation:
PCO2, theor. in mmHg = 1.5 x [HCO3-] + 8
(allow an uncertainty margin of + or - 2 mmHg). If PCO2, theor < < PCO2 then respiratory alkalosis is present, in addition to the metabolic disturbance. If PCO2, theor > > PCO2 then respiratory acidosis is present, in addition to the metabolic disturbance.
- Compensated metabolic acidosis: pH=7.25; [HCO3-] = 18 mM; PCO2 =36 mmHg
Winter formula: PCO2, theor. in mmHg = 1.5 x 18 + 8 = 35 mmHg
Measured PCO2 is within + or - 2 mmHg from calculated one: optimal respiratory compensation is present.
- Insufficiently compensated metabolic acidosis: pH=7.25; [HCO3-] = 18 mM; PCO2 = 44 mmHg
Winter formula: PCO2, theor. in mmHg = 1.5 x 18 + 8 = 35 mmHg
Measured PCO2 is significantly higher than calculated one: insufficient respiratory compensation. Most probably in this patient we have the contemporaneous presence of a metabolic acidosis (bicarbonate is low) and a respiratory acidosis (PCO2 is higher than expected).

      Nomograms are graphs in which the measured parameters are reported, together with the indication of statistically determined areas or regions corresponding to single disease conditions. The use of nomograms is equivalent and alternative to the use of empirical formulas such as Winter's; as a general rule, however, nomograms are preferable because they provide a visualization of usual ranges, rather than single numbers, from which the estimation of a range is not obvious. Two such nomograms are reported below, the topmost by Siggaard-Andersen, the bottom one by Arbus.

      To use the nomogram we report the hemogas analysis parameters of our patient on the nomogram: if the patient falls in a shadowed area we can assume that he/she has a single acid-base defect, with the appropriate compensation (The areas usually include 95% of the "pure" cases); if the patient falls between two areas, then we assume that two concomitant diseases are present.

      Advanced, 5-parameters interpretation of the arterial hemogas analysis.
      1) The usual 4-parameters evaluation is carried out as above.
      2) The standard pH, or standard bicarbonate, or base excess is added to the picture and interpreted as in the Table above.
      3) The above parameters are located on specifically designed nomograms to help differential diagnosis.
      Example: comatous patient. His parameters are as follows: pH= 7.0; bicarbonate = 10 mM; PCO2 = 40 mmHg; anion gap = 25 mEq/L. Our standard four parameters interpretation is metabolic acidosis, becase the pH is low and bicarbonate is low. The anion gap is high, thus this is a case of HAGMA. PCO2 is "normal", but this contradicts our expectation that it should be reduced because of respiratory compensation. We locate the data of this patient in the Arbus' nomogram and we find that he falls in a white region, located between the metabolic acidosis and the acute respiratory acidosis. Thus the nomogram suggests that the patient has actually an acute respiratory acidosis superimposed over a (presumably chronic) metabolic acidosis.
      Diagnosis: the patient had diabetic ketoacidosis. Initially his condition was compensated (low PCO2); when the pH fell at 7.0 he entered a comatous state and the respiratory centers in the brain were depressed. Hypoventilation reduced the compensation and added the acute respiratory component. Glycemia was 300 mg/dL. Insulin and potassium therapy were promptly instituted. Note: the patient condition was a medical emergency, and was rapidly progressive, because the increase of PCO2 further depressed the respiratory centers, creating a vicious circle.


      Acidoses and alkaloses appearing around or after puberty are usually acquired (see below for the congenital acidoses of the newborn and infant), and may be due to several possible causes. Some common examples are listed in the Table below:
pH disturbancepossible diseases
acute respiratory acidosisinterstitial pneumonia
lung intoxication by fumes or other substances
depression of respiratory centers (e.g. eroine abuse, brainstem infarction, coma)
chronic respiratory acidosischronic obstructive pulmonary disease (in heavy smokers!)
lung emphisema
impaired ventilation
lung fibrosis and penumoconioses
metabolic acidosisdiabetic ketoacidosis
lactic acidosis (sepsis!)
kidney failure and several kidney diseases (acquired Fanconi's syndrome)
respiratory alkalosishyperventilation due to neurological damage
mountain sickness
metabolic alkalosisprolonged vomiting
diuretic therapy, some defects of tubular function
      In the pH disturbances of the adult and the elderly it is important to pay attention to the possible coexistence of multiple pathological conditions: e.g. diabetic ketoacidosis is of the metabolic type, but may cause coma, which in turn depresses the respiratory centers and cause respiratory acidosis: thus two causes of acidosis are present at the same time, one metabolic and one respiratory. In an elderly patient suffering of BPCO and chronic respiratory acidosis, an episode of acute vomiting may superimpose a metabolic alkalosis; thus two opposite alterations of the blood pH may be present at the same time (and the pH may appear almost normal, but with severely altered bicarbonate and total CO2).

      Several inherited metabolic disorders may cause metabolic acidosis because of the accumulation of metabolytes whose elimination is impaired. These must be diagnosed early because appropriate treatment or diet may prevent brain damage and progression of the disease. Acidosis may not be the most prominent symptom, and may actually be mild (in many cases neurological abnormalities may be prominent); however it provides an important diagnostic clue. Acidosis is of the metabolic type, with increased anion gap. Aciduria is usually present (the urine is acidic). Each of these defects is uncommon, but they are many and thus their cumulative incidence is significant, possibly as high as 1 every 100 births.
      The nutrients of animals are compounds containing reduced carbon (sugars, aminoacids, fatty acids). Carbon is oxidized using air O2, and since it is a non-metal its oxides are acidic (CO2, carboxylic acids). Blockade of metabolism because of the inherited deficiency of an enzyme may cause carboxylic acid intermediates to accumulate in the blood and in the urine. Each specific defect is discussed in the appropriate lecture or chapter, but it is useful to collect at least the most common defects in a cumulative Table.
diseasemetabolyte and pathway
orotic aciduriaorotic acid (pyrimidine biosynthesis)
methylmalonic aciduriamethylmalonic acid (catabolism of propionyl-CoA)
maple sirup urine diseasebranched chain α-ketoacids (catabolism of branched chain aminoacids)
phenylketonuriaphenylpiruvic acid (catabolism of Phe and Tyr)
alkaptonuriahomogentisic acid (catabolism of Phe and Tyr)
isovaleric aciduriaisovaleric acid (catabolism of Leu)
argininosuccinic aciduriaargininosuccinic acid (urea cycle)
mevalonic aciduriamevalonic acid (biosynthesis of cholesterol)
lactic acidurialactic acid (several causes, genetic or acquired)
Fanconi's syndromedefect of tubular reabsorption of bicabonate and other solutes (genetic or acquired)

Further readings
Paul K. Hamilton, Neal A. Morgan, Grainne M. Connolly and Alexander P. Maxwell: Understanding Acid-Base Disorders.
Alan W. Grogono Acid-base tutorial
HK Walker, WD Hall, and JW Hurst,Clinical Methods Butterworths, Boston, MA, USA.
JH Galla Metabolic alkalosis JASN
JW Severinghaus The invention and development of the blood gas analysis apparatus Anesthesiology 2002; 97: 253-56.
Jubran A Pulse oximetry Crit Care. 1999; 3: R11–R17.
Arbus GSAn in vivo acid-base nomogram for clinical use. Can. Med. Assoc. J. 1973; 109: 291.

Questions and exercises:
1) The normal parameters of arterial O2 are:
PO2 > 110 mmHg; O2 content 20 mL/dL; O2 saturation > 95%
PO2 > 90 mmHg; O2 content 15 mL/dL; O2 saturation > 95%
PO2 > 90 mmHg; O2 content 20 mL/dL; O2 saturation > 95%

2) Normal values for serum CO2 and bicarbonate are:
PCO2=40 mmHg (corresponding to 1,3 mM); bicarbonate=26 mM
PCO2=26 mmHg (corresponding to 0,8 mM); bicarbonate=26 mM
PCO2=40 mmHg (corresponding to 1,3 mM); bicarbonate=40 mM

3) The characteristic features of acute respiratory acidosis are
markedly increased PCO2, markedly decreased pH, normal or moderately increased bicarbonate
markedly increased PCO2, moderately decreased pH, markedly increased bicarbonate
reduced PCO2, markedly decreased pH, reduced bicarbonate

4) The anion gap is increased in
acute respiratory acidosis
diabetic ketoacidosis
adaptation to high altitude.

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Thank you Professor (lecture on bilirubin and jaundice).

The fourth recorded part, the one on hyper and hypoglycemias is not working.
Bellelli: I checked and in my computer it seems to work. Can you better specify
the problem you observe?

This Presentation (electrolytes and blood pH) feels longer than previous lectures
Bellelli: it is indeed. Some subjects require more information than others. I was
thinking of splitting it in two nest year.

Bellelli in response to a question raised by email: when we compare the blood pH
with the standard pH we do not mean to compare the "normal" blood pH (7.4)
with the standard pH. Rather we compare the actual blood pH of the patient, with
the pH of the same blood sample equilibrated under standard conditions.
Thus, if we say that standard pH is lower than pH we mean that equilibriation with
40 mmHg CO2 has caused absorption of CO2 and has lowered the pH with respect
to its value before equilibration.

(Lipoproteins) Is the production of leptin an indirect cause of type 2 diabetes since
it works as a stimulus to have more adipose tissue that produces hormones?
Bellelli: in a sense yes, sustained increase of leptin causes the hypothalamus to adapt
and to stop responding. Obesity ensues and this in turn may cause an increase in the
production of resistin and other insulin-suppressing protein hormones produced by the
adipose tissue. However, this is quite an indirect link, and most probably other factors
contribute as well.

(Urea cycle) what is the meaning of "dissimilatory pathway"?
Bellelli: a dissimilatory pathway is a catabolic pathway whose function is not to produce
energy, but to produce some terminal metabolyte that must be excreted. Dissimilatory
pathways are necessary for those metabolytes that cannot be excreted as such by the
kidney or the liver because they are toxic or poorly soluble. Examples of metabolytes
that require transformation before being eliminated are heme-bilirubin, ammonia,
sulfur and nitrogen oxides, etc.

Talking about IDDM linked neuropathy can be the C peptide absence considered a cause of it??
Bellelli: The C peptide released during the maturation of insulin, besides being an indicator
of the severity of diabetes, plays some incompletely understood physiological roles. For
example it has been hypothesized that it may play a role in the reparation of the
atherosclerotic damage of the small arteries. Thus said, I am not aware that it plays a direct
role in preventing diabetic polyneuropathy. Diabetic neuropathy has at least two causes: the
microvascular damage of the arteries of the nerve (the vasa nervorum), and a direct
effect of hyperglycemia and decreased and irregular insulin supply on the nerve metabolism.
Diabetic neuropathy is observed in both IDDM and NIDDM, and requires several years to
develop. Since the levels of the C peptide differ in IDDM and NIDDM, this would suggest
that the role of the C peptide in diabetic neuropathy is not a major one. If you do have
better information please share it on this site!

In acute intermitted porphyria and congenital erythropoietic porphyria why do the end product
of the affected enzymes accumulate instead of their substrate??
Bellelli: First of all, congratulations! This is an excellent question.
Remember that a condition is which the heme is not produced is lethal in the foetus; thus
the affected enzyme(s) must maintain some functionality for the patient
to be born and to come to medical attention. All known genetic defects of heme
biosynthesis derange but do not block this metabolic pathway.
Congenital Erythropoietc Porphyria (CEP) is a genetic defect of uroporphyrinogen
III cosynthase. This protein associates to uroporphyrinogen synthase (which is present
and functional in CEP) and guarantees that the appropriate uroporphyrinogen isomer is produced
(i.e. uroporphyrinogen III). In the absence of a functional uroporphyrinogen III
cosynthase other possible isomers of uroporphyrinogen are produced together with
uroporpyrinogen III, mostly uroporphyrinogen I. The isomers of uroporphyrinogen
that are produced differ because of the positions of propionate and acetate side chains,
and this in turn is due to the pseudo symmetric structure of porphobilinogen. Only
isomer III can be further used to produce protoporphyrin IX. Thus in the
case of CEP we observe accumulation of abnormal uroporphyrinogen derivatives, which, as
you correctly observed are the products of the enzymatic synthesis operated by
uroporphyrinogen synthase.
The case of Acute Intermittent Porphyria (AIP) is similar, although there may be variants
of this disease. What happens is that either the affected enzyme is a variant that does not
properly associate with uroporphyrinogen III cosynthase or presents active site mutations
that impair the proper alignement of the phoprphobilinogen substrates. In either case
abnormal isomers of uroporphyrinogen are produced, as in CEP.
Also remark that in both AIP and CEP we observe accumulation of the porphobilinogen
precursor: this is because the overall efficiency of the biosynthesis of uroporphyrinogens is
reduced. Thus: (i) less uroporphyrinogen is produced, and (ii) only a fraction of the
uroporphyrinogen that is produced is the correct isomer (uroporphyrinogen III).

is it possible to take gulonolactone oxidase to synthesize vitamin C
instead of vitamin C supplement?
Bellelli: no, this approach does not work. The main reason is that
the biosynthesis of vitamin C, as almost all other metabolic processes, occurs intracellularly.
If you administer the enzyme it will at most reach the extracellular fluid but will not be
transported inside the cells to any significant extent. Besides, there are other problems
in this type of therapy (e.g. the enzyme if administered orally, may be degraded by digestive
proteases; if administered parenterally, may cause the immune system to react against a
non-self protein). In theory one could think of a genetic modification of the inactive human
gene of gulonolactone oxidase, but the risk and cost of this intervention would not be
justified. In addition to these considerations, except for cases of shipwreckage or
other catastrophes, a proper diet or administration of tablets of vitamin C is effective,
risk-free and unexpensive, thus no alternative therapy is reasonable. However, I express my
congratulations for your search on the biosynthesis pathway of ascorbic acid.

Resorption and not reabsorption would lead to hypercalcemia ie bone matrix being broken down.
Bellelli: I am not sure to interpret your question correctly. Resorption indicates destruction of the bone matrix and release of calcium and
phosphate in the blood, thus it causes an increase of calcemia. Reabsorption usually means active transport of calcium from the renal tubuli to the blood, thus
it prevents calcium loss. It prevents hypocalcemia, and thus complement bone resorption. To avoid confusion it is better use the terms "bone resorption" and "
renal reabsorption of calcium". If you have a defect in renal reabsorption, parthyroid hormone will be released to maintain a normal calcium level by means of
bone resorption; the drawback is osteoporosis.

In Reed and Frost model: I haven't understood what is the relationship
between K and R reproductive index. Thank you Professor!
Bellelli: in the Reed and Frost model K is the theoretical upper limit of
R0. R the reproductive index is the ratio (new cases)/(old cases) measured after
one serial generation time. R0 is the value of R one measures at the beginning
of the epidemics, when in principle all the population is susceptible.

What is the link between nucleotide metabolism and immunodeficiencies and mental retardation?
Bellelli: the links may be quite complex, but the principal ones are as follows:
1) the immune response requires a replication burst of granulocytes and lymphocytes, which in turn requires
a sudden increase of nucleotide production, necessary for DNA replication. Defects of nucleotide metabolism
impair this phase of the immune defense. Notice that the mechanism is similar to the one responsible of
anemia which requires a sustained biosynthesis of nucleotides at a constant rate, rather than in a burst.
2) Mental retardation is mainly due to the accumulation of nulceotide precursors in the brain of the
newborn, due to the incompletely competent blood-brain barrier.

How can ornithine transaminase defects cause hyperammonemia? Is it due to the accumulation
of ornithine that blocks the urea cycle or for other reasons?
Bellelli: ornithine transaminase is required for the reversible interconversion of ornithine
and proline, and thus participates to both the biosynthesis and degradation of ornithine. The enzyme is
synthesized in the cytoplasm and imported in the mitochondrion. Depending on the metabolic conditions
the deficiency of this enzyme may cause both excess (when degradation would be necessary) or defect
(when biosynthesis would be necessary) of ornithine; in the latter case, the urea cycle slows down. Thus
there is the paradoxical condition in which alternation may occur between episodes of hyperammonemia
and of hyperornithinemia.

When we use the Berthelot's reaction to measure BUN do we also have to
measure the concentration of free ammonia before adding urease?
Bellelli: yes, in principle you should. Berthelot's reaction detects ammonia,
thus one should take two identical volumes of serum, use one to measure free ammonia,
the other to add urease and measure free ammonia plus ammonia released by urea. BUN is
obtained by difference. However, free ammonia in our blood is so much lower than urea that
you may omit the first sample, if you only want to measure BUN.

Why do we have abnormal electrolytes in hematological neoplasia e.g.
Bellelli: I do not have a good explanation for this effect, which may have
multiple causes. However, you should consider two factors: (i) acute leukemias cause a massive
proliferation of leukocytes (or lymphocytes depending on the cell type affected) with a very
shortened lifetime; thus you observe an excess death rate of the neoplastic cells. The dying
cells release in the bloodstream their content, which has an electrolyte composition different
from that of plasma: the cell cytoplasm is rich in K and poor in Na, thus causing hyperkalemia.
(ii) the kidney may be affected by the accumulation of neoplastic white cells or their lytic products.

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