Welcome to Part 54 of the ASCP MLS Exam Practice Series, focusing on Acid-Base Balance and Blood Gas Analysis. Maintaining the body’s acid-base equilibrium is critical for normal physiological function.
This section explores the biochemical principles of pH regulation, buffer systems, and interpretation of arterial blood gases (ABGs) in metabolic and respiratory disorders. Understanding these fundamentals helps medical laboratory professionals ensure accurate diagnosis and patient care in critical care settings.
📘 Key Topics Covered
- Concept of acids, bases, and buffers
- pH regulation and Henderson-Hasselbalch equation
- Major buffer systems: bicarbonate, phosphate, protein, hemoglobin
- Respiratory and renal mechanisms in acid-base balance
- Arterial blood gas components: pH, pCO₂, pO₂, HCO₃⁻, base excess
- Metabolic and respiratory acidosis/alkalosis
- Compensation mechanisms and mixed disorders
- Laboratory methods for ABG analysis (blood gas analyzer, electrodes)
- Quality control and preanalytical considerations in blood gas testing
- Clinical interpretation of acid-base disorders
🧠 Learning Objectives
By the end of this part, you should be able to:
- Explain the principles of acid-base balance and buffering.
- Interpret arterial blood gas (ABG) results accurately.
- Differentiate metabolic from respiratory acid-base disorders.
- Understand compensatory responses and clinical implications.
60 MCQs (4181-4240):
- The normal blood pH in a healthy adult is approximately:
a) 6.8
b) 7.0
c) 7.4
d) 7.8 - A blood pH below 7.35 indicates:
a) Acidosis
b) Alkalosis
c) Neutrality
d) Normal pH - A blood pH above 7.45 indicates:
a) Acidosis
b) Alkalosis
c) Hypoxia
d) Hypercapnia - The main buffer system of the blood is:
a) Hemoglobin buffer
b) Phosphate buffer
c) Bicarbonate–carbonic acid buffer
d) Protein buffer - The Henderson–Hasselbalch equation is used to calculate:
a) Blood glucose
b) pH
c) Hemoglobin concentration
d) Osmolality - In the Henderson–Hasselbalch equation, the ratio of bicarbonate to carbonic acid is normally:
a) 1:1
b) 10:1
c) 15:1
d) 20:1 - The organ primarily responsible for bicarbonate regulation is the:
a) Liver
b) Kidney
c) Lungs
d) Pancreas - The organ primarily responsible for carbon dioxide regulation is the:
a) Liver
b) Lungs
c) Kidney
d) Stomach - Respiratory acidosis results from:
a) Excess CO₂ retention
b) Loss of bicarbonate
c) Increased ventilation
d) Increased hydrogen ion excretion - Respiratory alkalosis results from:
a) CO₂ retention
b) Hyperventilation
c) Increased acid production
d) Bicarbonate loss - Metabolic acidosis is characterized by:
a) Increased bicarbonate
b) Decreased bicarbonate
c) Increased CO₂
d) Increased pH - Metabolic alkalosis is caused by:
a) Loss of acid or gain of bicarbonate
b) Gain of acid
c) Retention of CO₂
d) Decrease in bicarbonate - The most common cause of respiratory acidosis is:
a) Hyperventilation
b) Hypoventilation
c) Vomiting
d) Dehydration - Which of the following conditions commonly leads to metabolic acidosis?
a) Vomiting
b) Starvation or diabetic ketoacidosis
c) Anxiety
d) High altitude - Which of the following conditions causes metabolic alkalosis?
a) Severe diarrhea
b) Starvation
c) Prolonged vomiting
d) Diabetic ketoacidosis - The normal arterial bicarbonate (HCO₃⁻) concentration is:
a) 10–15 mmol/L
b) 22–26 mmol/L
c) 30–40 mmol/L
d) 45–55 mmol/L - The normal partial pressure of carbon dioxide (pCO₂) in arterial blood is:
a) 20–25 mmHg
b) 35–45 mmHg
c) 50–60 mmHg
d) 60–70 mmHg - The normal partial pressure of oxygen (pO₂) in arterial blood is:
a) 20–40 mmHg
b) 50–70 mmHg
c) 80–100 mmHg
d) 120–150 mmHg - The primary function of hemoglobin in acid–base balance is to:
a) Produce CO₂
b) Buffer hydrogen ions
c) Transport amino acids
d) Maintain osmotic pressure - The chloride shift (Hamburger phenomenon) refers to:
a) Movement of chloride into red blood cells in exchange for bicarbonate
b) Conversion of CO₂ into carbonic acid
c) Movement of sodium out of cells
d) Exchange of potassium for sodium - A decreased blood pH and increased pCO₂ indicate:
a) Respiratory acidosis
b) Metabolic acidosis
c) Respiratory alkalosis
d) Metabolic alkalosis - An increased pH and decreased pCO₂ indicate:
a) Respiratory acidosis
b) Respiratory alkalosis
c) Metabolic acidosis
d) Mixed disorder - An increased pH and increased bicarbonate level indicate:
a) Metabolic alkalosis
b) Metabolic acidosis
c) Respiratory acidosis
d) Respiratory alkalosis - A decreased pH and decreased bicarbonate indicate:
a) Metabolic acidosis
b) Respiratory acidosis
c) Metabolic alkalosis
d) Respiratory alkalosis - Compensation for respiratory acidosis involves:
a) Increased renal excretion of bicarbonate
b) Increased renal retention of bicarbonate
c) Increased ventilation
d) Decreased CO₂ production - Compensation for metabolic acidosis occurs by:
a) Decreasing ventilation
b) Increasing ventilation to blow off CO₂
c) Increasing bicarbonate excretion
d) Decreasing H⁺ excretion - In metabolic alkalosis, the lungs compensate by:
a) Increasing CO₂ retention
b) Increasing CO₂ elimination
c) Increasing bicarbonate excretion
d) Increasing hydrogen ion excretion - The measurement of blood gases is typically performed using:
a) Flame photometer
b) Ion-selective electrode
c) Blood gas analyzer
d) Spectrophotometer - The sample of choice for blood gas analysis is:
a) Venous blood
b) Arterial blood
c) Capillary plasma
d) Serum - The anticoagulant commonly used in arterial blood gas collection is:
a) EDTA
b) Heparin
c) Citrate
d) Oxalate - The expected blood gas results for a patient in chronic renal failure would match the pattern of:
a) Metabolic Acidosis
b) Respiratory Acidosis
c) Metabolic Alkalosis
d) Respiratory Alkalosis - Severe diarrhea typically causes which acid-base imbalance?
a) Metabolic Acidosis
b) Metabolic Alkalosis
c) Respiratory Acidosis
d) Respiratory Alkalosis - At a normal blood pH of 7.40, what is the approximate ratio of bicarbonate to carbonic acid?
a) 15:1
b) 20:1
c) 25:1
d) 30:1 - The reference range for the pH of arterial blood measured at 37°C is:
a) 7.28-7.34
b) 7.33-7.37
c) 7.35-7.45
d) 7.45-7.50 - An emphysema patient suffering from fluid accumulation in the alveolar spaces is likely to be in what state?
a) Respiratory Acidosis
b) Respiratory Alkalosis
c) Metabolic Acidosis
d) Metabolic Alkalosis - A patient is admitted to the emergency room in a state of metabolic alkalosis. Which of the following would be consistent with this diagnosis?
a) High TCO₂, increased HCO₃⁻
b) Low TCO₂, increased HCO₃⁻
c) High TCO₂, decreased H₂CO₃
d) Low TCO₂, decreased H₂CO₃ - A common cause of respiratory alkalosis is:
a) Vomiting
b) Starvation
c) Asthma
d) Hyperventilation - The most important buffer pair in plasma is:
a) Phosphate/Biphosphate
b) Hemoglobin/Imidazole
c) Bicarbonate/Carbonic Acid
d) Sulfate/Bisulfate - In respiratory acidosis, the primary compensatory mechanism is:
a) Increased respiratory rate
b) Decreased ammonia formation
c) Increased blood pCO₂
d) Increased plasma bicarbonate concentration - If an arterial blood gas sample is left exposed to room air, which of the following changes will occur?
a) pCO₂ increased, pO₂ decreased, pH decreased
b) pCO₂ decreased, pO₂ increased, pH increased
c) pCO₂ unchanged, pO₂ increased, pH unchanged
d) pCO₂ decreased, pO₂ decreased, pH decreased - Specimens for blood gas analysis should be drawn into a syringe containing which anticoagulant?
a) No preservative
b) Heparin
c) EDTA
d) Oxalate - The carbonic acid (H₂CO₃) concentration in blood can be calculated as:
a) HCO₃⁻ / 2.8
b) HCO₃⁻ / H₂CO₃
c) pCO₂ × 0.03
d) tCO₂ × 0.03 - A patient with diabetic ketoacidosis and a serum acetone of 4+ would be expected to have an arterial blood gas showing:
a) Low pH
b) High pH
c) Low pO₂
d) High pO₂ - Metabolic acidosis is best described as a(n):
a) Increase in CO₂ content and pCO₂ with a decreased pH
b) Decrease in CO₂ content with an increased pH
c) Increase in CO₂ content with an increased pH
d) Decrease in CO₂ content and pCO₂ with a decreased pH - The total CO₂ concentration in serum is primarily comprised of:
a) H₂CO₃ + HCO₃⁻
b) pCO₂ + HCO₃⁻
c) H₂CO₃ + pCO₂
d) tCO₂ + H₂CO₃ - A blood gas sample is not analyzed immediately and is not iced. In vitro glycolysis will cause a:
a) Rise in pH and pCO₂
b) Fall in pH and a rise in pO₂
c) Rise in pH and a fall in pO₂
d) Fall in pH and a rise in pCO₂ - The electrode used to measure pO₂ operates on the principle of:
a) Potentiometry
b) Amperometry
c) Coulometry
d) Conductometry - Which of the following is directly measured by modern blood gas analyzers?
a) pH, HCO₃⁻, and % O₂ saturation
b) pH, pCO₂, and pO₂
c) HCO₃⁻, pCO₂, and pO₂
d) pH, pO₂, and % O₂ saturation - The pH electrode uses a special glass membrane that is sensitive to which ion?
a) Sodium (Na⁺)
b) Hydrogen (H⁺)
c) Bicarbonate (HCO₃⁻)
d) Chloride (Cl⁻) - The pCO₂ electrode is essentially a modified version of which other electrode?
a) pO₂ electrode
b) pH electrode
c) Sodium ISE
d) Chloride electrode - Given a pCO₂ of 60 mm Hg and a bicarbonate of 18 mmol/L, calculate the pH. (pK = 6.1)
a) 6.89
b) 7.00
c) 7.10
d) 7.30 - A patient has the following results: Sodium 136 mEq/L, Potassium 4.4 mEq/L, Chloride 92 mEq/L, Bicarbonate 40 mEq/L, arterial pH 7.32, pCO₂ 79 mm Hg. This is most compatible with:
a) Respiratory Alkalosis
b) Respiratory Acidosis
c) Metabolic Alkalosis
d) Metabolic Acidosis - The proper handling of an arterial blood gas sample includes:
a) On ice, no clots, no air bubbles
b) On ice, thin fibrin strands only, no air bubbles
c) On ice, no clots, fewer than 4 air bubbles
d) Room temperature, no clots, no air bubbles - The Henderson-Hasselbalch equation for the bicarbonate buffer system is:
a) pH = pK + log([H₂CO₃] / [HCO₃⁻])
b) pH = pK + log([HCO₃⁻] / [H₂CO₃])
c) pH = pK – log([HCO₃⁻] / [H₂CO₃])
d) pH = pK – log([H₂CO₃] / [HCO₃⁻]) - Most of the carbon dioxide present in blood is in the form of:
a) Dissolved CO₂
b) Carbonate
c) Bicarbonate ion
d) Carbonic acid - An arterial blood gas specimen obtained at 8:30 AM is not received in the laboratory until 11:00 AM. The technologist should:
a) Perform the test immediately upon receipt
b) Perform the test only if the specimen is submitted in ice water
c) Request a venous blood specimen
d) Request a new arterial specimen be obtained - A patient with chronic lung disease has results showing pH 7.32, pCO₂ 78 mm Hg, and HCO₃⁻ 39 mEq/L. These results indicate:
a) Metabolic compensation by renal retention of bicarbonate
b) Metabolic compensation by renal excretion of bicarbonate
c) Respiratory compensation by retention of CO₂
d) Combined retention by excretion of CO₂ - The anion gap is useful for quality control of laboratory results for which set of analytes?
a) Amino acids and proteins
b) Blood gas analyses
c) Sodium, potassium, chloride, and total CO₂
d) Calcium, phosphorus and magnesium - The buffering capacity of blood is maintained by a reversible exchange process between bicarbonate and:
a) Sodium
b) Potassium
c) Calcium
d) Chloride - Calculate the blood pH given a bicarbonate concentration of 24 mEq/L and a carbonic acid concentration of 1.2 mEq/L. (pK = 6.1, log 20 = 1.3)
a) 7.28
b) 7.38
c) 7.40
d) 7.42
📌 How to Use This Practice Set
- Answer each question before checking the key.
- Focus on why the correct answer is right and the others are wrong.
- Use this set as timed practice to simulate the real exam environment.
Answer Key
Answer Key:
- c) 7.4
- a) Acidosis
- b) Alkalosis
- c) Bicarbonate–carbonic acid buffer
- b) pH
- d) 20:1
- b) Kidney
- b) Lungs
- a) Excess CO₂ retention
- b) Hyperventilation
- a) Respiratory acidosis
- b) Respiratory alkalosis
- a) Metabolic alkalosis
- a) Metabolic acidosis
- b) Increased renal retention of bicarbonate
- b) Increasing ventilation to blow off CO₂
- a) Increasing CO₂ retention
- c) Blood gas analyzer
- b) Arterial blood
- b) Heparin
- b) Heparin
- c) pCO₂ × 0.03
- a) Low pH
- d) Decrease in CO₂ content and pCO₂ with a decreased pH
- a) H₂CO₃ + HCO₃⁻
- d) Fall in pH and a rise in pCO₂
- b) Amperometry
- b) pH, pCO₂, and pO₂
- b) Hydrogen (H⁺)
- b) pH electrode
- b) Decreased bicarbonate
- a) Loss of acid or gain of bicarbonate
- b) Hypoventilation
- b) Starvation or diabetic ketoacidosis
- c) Prolonged vomiting
- b) 22–26 mmol/L
- b) 35–45 mmHg
- b) 50–70 mmHg
- b) Buffer hydrogen ions
- a) Movement of chloride into red blood cells in exchange for bicarbonate
- a) Metabolic Acidosis
- a) Metabolic Acidosis
- b) 20:1
- c) 7.35-7.45
- a) Respiratory Acidosis
- a) High TCO₂, increased HCO₃⁻
- d) Hyperventilation
- c) Bicarbonate/Carbonic Acid
- d) Increased plasma bicarbonate concentration
- b) pCO₂ decreased, pO₂ increased, pH increased
- c) 7.10
- b) Respiratory Acidosis
- a) On ice, no clots, no air bubbles
- b) pH = pK + log([HCO₃⁻] / [H₂CO₃])
- c) Bicarbonate ion
- d) Request a new arterial specimen be obtained
- a) Metabolic compensation by renal retention of bicarbonate
- c) Sodium, potassium, chloride, and total CO₂
- d) Chloride
- c) 7.40
Top 8 Medical Laboratory Scientist (MLS) Exams:
Top 8 Medical Laboratory Scientist (MLS) Exams that are recognized globally and can help professionals validate their credentials and enhance their career opportunities:
1. ASCP – American Society for Clinical Pathology (USA)
- Exam Name: MLS(ASCP)
- Eligibility: Bachelor’s degree with clinical laboratory experience.
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- Exam Name: MLT(AMT) or MT(AMT)
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- Exam Name: CSMLS General or Subject-specific Exams
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- Exam Name: DOH/HAAD License Exam
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