Oxygen Transport Blood Buffers  Elmhurst College
Carbon Dioxide Transport Respiratory Acidosis Metabolic Acidosis  Chemistry Department
Buffers in the Kidneys Respiratory Alkalosis Metabolic Alkalosis  Virtual ChemBook

 LABORATORY TESTS

The following lab tests can be used to interpret and explain acidosis and
alkalosis conditions. All are measured on blood samples.

1. pH: This measures hydrogen ions - Normal pH = 7.35-7.45

2. pCO2= Partial Pressure of Carbon Dioxide: Although this is a pressure measurement, it relates to the concentration of GASEOUS CO2 in the blood. A high pCO2 may indicate acidosis. A low pCO2 may indicate alkalosis.

3. HCO3- = Bicarbonate: This measures the concentration of HCO3- ion only. High values may indicate alkalosis since bicarbonate is a base. Low values may indicate acidosis.

4. CO2 = Carbon Dioxide Content: This is a measure of ALL CO2 liberated on adding acid to blood plasma. This measure both carbon dioxide dissolved and bicarbonate ions and is an older test. Do not confuse with pCO2

Typically, dissolved carbon dioxide = l.2-2.0 mmoles/L and
HCO3- = 22-28 mmoles/L
Therefore, although it is listed as CO2 content, the lab
test really reflects HCO3- concentration.

Respiratory Acidosis

.ABNORMAL pH IN THE BODY: ACIDOSIS AND ALKALOSIS:

INTRODUCTION:

Normal blood pH is maintained between 7.35 and 7.45 by the regulatory systems. The lungs regulate the amount of carbon dioxide in the blood and the kidneys regulate the bicarbonate. When the pH decreases to below 7.35 an acidosis condition is present. Acidosis means that the hydrogen ions are increased and that pH and bicarbonate ions are decreased. A greater number of hydrogen ions are present in the blood than can be absorbed by the buffer systems.

Alkalosis results when the pH is above 7.45. This condition results when the buffer base (bicarbonate ions) is greater than normal and the concentration of hydrogen ions are decreased.

Both acidosis and alkalosis can be of two different types: respiratory and metabolic. Respiratory acidosis or alkalosis is caused by various malfunctions of the lungs. Metabolic acidosis or alkalosis is caused by various metabolic disorders which result in an excessive build up or loss of acids or bases.


 ABNORMAL pH CASE STUDIES

IDENTIFICATION OF ACID-BASE PROBLEMS IN CASE STUDIES

The previous conditions of pH abnormality are summarized in Table 2. This gives a summary of causes, lab results, compensations, and treatments for the four types of acid-base problems. Figure l gives a pictorial summary of the same types of information. Both will be available for use on homework or examination problems.

Note: Uncompensated lab results would be reported shortly after the condition appeared. Compensated lab results will be reported after a longer time has elapsed from the onset of the condition. The compensations are usually sufficient to bring pH near normal but neither pCO2 or HCO3- will be normal.

In a case study, specific lab values will be given along with symptoms. The student will have to determine the specific problem and discuss lab results, compensation methods, and treatments using equilibrium principles.

Procedure for Determining the Specific Condition and Analyzing the Results using Equilibrium Principles:

l. To Determine Whether the Condition is Acidosis or Alkalosis:

First look at the pH value and compare it to the ranges given in Table 2. This should give whether the condition is acidosis or alkalosis.

2. To Determine Whether the Condition is Metabolic or Respiratory:

Concentrate on the two columns under the conditions determined in #1. Now look at the pCO2 value, if given, and compare it to the ranges under the metabolic or respiratory condition. One of these should match. It may also give whether compensated or uncompensated. H+ + HCO3- or CO2 could be used in place of pCO2 for this determination.

3. To Determine Whether the Condition is Uncompensated or Compensated:
Concentrate on the findings in Nos. 1 and 2. Now look at H+ + HCO3-and/or CO2 values. Try to get the best match of values for the above conditions.

4. Finally check all lab results to see if they are consistent.

Hemoglobin - Oxygen Equation:

HHgb + O2 <===> HgbO2 + H+

Blood Buffer Equation:

Blood Buffer Equilibrium:
CO2 + HOH <===> H2CO3 <===> H+ + HCO3-

QUES. 8: Case 4:
Uncompensated Laboratory Results: pH =7.2, pCO2 = 60, CO2 = 24.

a. List the condition - acidosis or alkalosis, metabolic or respiratory, compensated or uncompensated.

pH = 7.2 indicates acidosis, uncompensated
pCO2 = 60 = respiratory
CO2 = 24 = respiratory, uncompensated

b. What is the primary cause of the condition?

Respiratory acidosis is caused when carbon dioxide is not removed from the blood due to hypoventilation, (slow breathing) or obstructions in the exchange of gases. Such conditions are caused by emphysema, asthma, bronchitis, pneumonia and pulmonary edema. Carbon dioxide increases in the blood since it cannot effectively diffuse out of the lungs.

c. Explain the other lab results using the primary cause and equilibrium principles.

Equilibrium: CO2 + HOH === H2CO3 === H+ + HCO3-

Compare the lab results with the normal values. In respiratory acidosis, the increased carbon dioxide is the primary cause for the other lab results. Increased CO2 causes the equilibrium to shift right increasing H2CO3, H+ and HCO3-. Increased H+ ion leads to decreased pH, (7.l5) from normal 7.4. The HCO3- (30 mmoles) is slightly higher than normal.

d. State and explain how the compensation will return pH to normal using equilibrium principles. Look at the table 2 to find compensation mechanisms.

Kidney Compensation:

The body has its own methods to correct abnormalities in pH. These are referred to as "compensation" methods. If one remembers that there are two main acid-base regulatory mechanisms, then in a case of respiratory acidosis, the kidneys attempt to compensate for the low pH.

The compensation is to increase excretion of H+ ion and therefore increase the retention or reabsorption of HCO3- into the blood. An increase in HCO3- ions in the blood causes the buffer equation to shift left, thus H+ ions decrease and pH increases.

Typical compensated lab results are: pH = 7.35, HCO$b3$s$s-$b = 35, pCO$b2$s = 70. Note that the pH is normal, but other lab results are abnormal.

e. Explain how the treatment with __?___ will work using equilibrium principles.

The best solution to improve respiratory acidosis is to treat the cause for hypoventilation. This would decrease the pCO2, decrease H+ and increase pH.
An emergency treatment would be to administer NaHCO3-. This would have the effect of shifting the blood buffer equilibrium to the left to decrease hydrogen ions and increase the pH back toward normal.