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HLT-RESP80-02 · Lesson 8 · 38 clock hours · theory

Carbon Dioxide Transport and Acid-Base Balance

Objective. Students will explain the three mechanisms of carbon dioxide transport in blood, apply the Henderson-Hasselbalch equation to interpret pH, and classify arterial blood gas results as normal, respiratory acidosis, respiratory alkalosis, metabolic acidosis, or metabolic alkalosis with or without compensation.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

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Carbon Dioxide Transport in Blood

Carbon dioxide (CO₂) is a continuous byproduct of cellular metabolism. The blood must carry it from the tissues to the lungs for elimination. Unlike oxygen, CO₂ travels by three distinct mechanisms, and understanding each one is essential for interpreting arterial blood gases (ABGs) and managing patients safely.

Mechanism 1: Dissolved CO₂

About 5–10% of CO₂ is carried simply dissolved in plasma. This dissolved fraction is measured directly as the partial pressure of carbon dioxide (PaCO₂). Normal arterial PaCO₂ is 35–45 mmHg. Although this is a small fraction of total CO₂ transport, it is physiologically critical because dissolved CO₂ drives the chemical reactions that determine blood pH.

Mechanism 2: Carbamino Compounds

Approximately 5–10% of CO₂ binds directly to protein molecules, mostly hemoglobin, forming carbamino compounds. When CO₂ attaches to the amino groups of hemoglobin, the result is carbaminohemoglobin. This reaction is rapid and reversible. At the lungs, where CO₂ tension is lower, the CO₂ releases and is exhaled.

Mechanism 3: Bicarbonate (HCO₃⁻) — The Major Route

The largest fraction — roughly 70% — travels as bicarbonate ions. Inside the red blood cell, CO₂ combines with water in a reaction catalyzed by the enzyme carbonic anhydrase:

> CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

Bicarbonate diffuses out of the red cell into plasma; chloride ions shift inward to maintain electrical balance (the chloride shift). The hydrogen ions released are buffered by hemoglobin, preventing a dangerous drop in pH. At the lungs, the entire process reverses: bicarbonate re-enters the red cell, reforms CO₂, and CO₂ is exhaled.

The Henderson-Hasselbalch Equation

Blood pH is determined by the ratio of bicarbonate (the metabolic component, regulated by the kidneys) to dissolved CO₂ (the respiratory component, regulated by the lungs):

> pH = 6.1 + log ( [HCO₃⁻] / [0.0307 × PaCO₂] )

The constant 6.1 is the pKa of the carbonic acid–bicarbonate buffer system. The factor 0.0307 converts PaCO₂ (mmHg) to dissolved CO₂ concentration (mEq/L). Normal values: pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L. The key insight is that pH rises when HCO₃⁻ rises or PaCO₂ falls, and pH falls when HCO₃⁻ falls or PaCO₂ rises.

ABG Interpretation: The Four-Step Method

A systematic approach prevents errors in clinical practice.

Step 1 — Evaluate pH. Is it normal (7.35–7.45), acidotic (< 7.35), or alkalotic (> 7.45)? The pH tells you the net direction of the disturbance.

Step 2 — Evaluate PaCO₂. Is it normal (35–45 mmHg), elevated (> 45, respiratory acidosis), or low (< 35, respiratory alkalosis)? Ask: does the PaCO₂ change explain the pH change?

Step 3 — Evaluate HCO₃⁻. Is it normal (22–26 mEq/L), low (< 22, metabolic acidosis), or high (> 26, metabolic alkalosis)? Ask: does the HCO₃⁻ change explain the pH change?

Step 4 — Assess for compensation. If both PaCO₂ and HCO₃⁻ are abnormal, the system that did not cause the primary problem is compensating. Compensation moves pH back toward normal but rarely restores it fully. Label the result uncompensated (only one abnormality), partially compensated (both abnormal, pH still outside normal range), or fully compensated (both abnormal, pH within normal range but not 7.40).

The Five Primary ABG Patterns

| Disorder | pH | PaCO₂ | HCO₃⁻ |

|---|---|---|---|

| Respiratory Acidosis | < 7.35 | > 45 | Normal / ↑ (compensated) |

| Respiratory Alkalosis | > 7.45 | < 35 | Normal / ↓ (compensated) |

| Metabolic Acidosis | < 7.35 | Normal / ↓ (compensated) | < 22 |

| Metabolic Alkalosis | > 7.45 | Normal / ↑ (compensated) | > 26 |

| Normal | 7.35–7.45 | 35–45 | 22–26 |

Clinical and Safety Considerations

ABG interpretation directly guides life-sustaining interventions. An RT who misclassifies a respiratory acidosis may withhold ventilatory support a patient urgently needs. Conversely, over-ventilating a COPD patient who chronically retains CO₂ can produce a dangerous alkalosis. Always correlate ABG results with the patient's clinical picture — respiratory rate, level of consciousness, and history — before acting. Report critical values (pH < 7.20 or > 7.60, PaCO₂ > 70 mmHg in a non-chronic retainer) to the supervising clinician immediately. Acting outside your verified order set or scope of practice, even with correct ABG interpretation, violates both patient safety standards and your state RT license requirements.

Key terms8

PaCO₂Partial pressure of arterial carbon dioxide, measured in mmHg; reflects the respiratory component of acid-base balance. Normal range is 35–45 mmHg.
Bicarbonate (HCO₃⁻)The major form of CO₂ transport in blood and the metabolic component of acid-base balance, regulated by the kidneys. Normal arterial value is 22–26 mEq/L.
Carbonic anhydraseEnzyme found in red blood cells that catalyzes the rapid conversion of CO₂ and water into carbonic acid, enabling efficient CO₂ transport as bicarbonate.
Henderson-Hasselbalch equationMathematical expression relating blood pH to the ratio of bicarbonate to dissolved CO₂: pH = 6.1 + log([HCO₃⁻] / [0.0307 × PaCO₂]).
Chloride shiftThe exchange of bicarbonate ions out of the red blood cell for chloride ions into the red blood cell, which occurs to maintain electrical neutrality during CO₂ transport.
Respiratory acidosisAn acid-base disturbance characterized by pH < 7.35 and PaCO₂ > 45 mmHg, caused by alveolar hypoventilation and CO₂ retention.
Metabolic acidosisAn acid-base disturbance characterized by pH < 7.35 and HCO₃⁻ < 22 mEq/L, caused by excess acid production, bicarbonate loss, or renal failure.
CompensationThe physiologic response by the system not primarily responsible for an acid-base disorder — lungs compensate for metabolic disturbances; kidneys compensate for respiratory disturbances — acting to return pH toward normal.

Worked examples4

1. Apply the Henderson-Hasselbalch equation: A patient has PaCO₂ = 40 mmHg and HCO₃⁻ = 24 mEq/L. Calculate pH and verify it is normal.
  1. Calculate dissolved CO₂: 0.0307 × 40 = 1.228 mEq/L
  2. Calculate the ratio: HCO₃⁻ / dissolved CO₂ = 24 / 1.228 = 19.54
  3. Take the log: log(19.54) ≈ 1.291
  4. Add the pKa: pH = 6.1 + 1.291 = 7.391
Answer: pH ≈ 7.39 — within the normal range of 7.35–7.45, confirming a normal acid-base state.
2. Classify this ABG using the four-step method: pH 7.28, PaCO₂ 58 mmHg, HCO₃⁻ 25 mEq/L.
  1. Step 1 — pH: 7.28 is < 7.35 → acidosis.
  2. Step 2 — PaCO₂: 58 mmHg is > 45 mmHg → respiratory acidosis; the elevated CO₂ matches the acidotic pH, so the respiratory system is the primary cause.
  3. Step 3 — HCO₃⁻: 25 mEq/L is within the normal range (22–26 mEq/L) → no metabolic component.
  4. Step 4 — Compensation: Only one value (PaCO₂) is abnormal → uncompensated.
Answer: Uncompensated respiratory acidosis. Clinically, this patient has alveolar hypoventilation; the RT should assess airway, work of breathing, and be prepared to support ventilation per physician orders.
3. Classify this ABG: pH 7.38, PaCO₂ 50 mmHg, HCO₃⁻ 29 mEq/L.
  1. Step 1 — pH: 7.38 is within the normal range (7.35–7.45), but it is on the acidotic side of 7.40, suggesting the primary disorder is acidosis.
  2. Step 2 — PaCO₂: 50 mmHg is > 45 mmHg → points to respiratory acidosis as the primary problem.
  3. Step 3 — HCO₃⁻: 29 mEq/L is > 26 mEq/L → elevated, consistent with renal retention of bicarbonate as compensation.
  4. Step 4 — Compensation: Both PaCO₂ and HCO₃⁻ are abnormal, and pH has returned to the normal range → fully compensated respiratory acidosis. This pattern is classic for a chronic CO₂ retainer such as a stable COPD patient.
Answer: Fully compensated respiratory acidosis. The kidneys have retained bicarbonate over time to buffer the chronically elevated CO₂, returning pH to within normal limits.
4. Classify this ABG: pH 7.50, PaCO₂ 30 mmHg, HCO₃⁻ 23 mEq/L.
  1. Step 1 — pH: 7.50 is > 7.45 → alkalosis.
  2. Step 2 — PaCO₂: 30 mmHg is < 35 mmHg → respiratory alkalosis; the low CO₂ matches the alkalotic pH, confirming respiratory cause (hyperventilation).
  3. Step 3 — HCO₃⁻: 23 mEq/L is within normal range → no metabolic component.
  4. Step 4 — Compensation: Only PaCO₂ is abnormal → uncompensated.
Answer: Uncompensated respiratory alkalosis. Common causes include anxiety, pain, or excessive mechanical ventilation. The RT should identify and address the cause of hyperventilation and, if the patient is on a ventilator, evaluate ventilator settings with the care team.

The math in this lesson2

Learners compute dissolved CO₂ and the bicarbonate ratio before taking the log.
A-SSE.1F-LE.4
Brush up in the math halls at newSchoolWorks.com:
In class: Lecture on CO2 transport mechanisms (dissolved, carbamino, and bicarbonate) and the Henderson-Hasselbalch equation; students complete a guided note packet and work two example ABG problems with the instructor. · ABG interpretation drill: students individually classify a set of 10 pre-printed ABG values using the four-step method (pH → PaCO2 → HCO3 → compensation); pairs check each other's work against an answer key. · Group whiteboard challenge: instructor calls out clinical scenarios (e.g., hyperventilating post-surgical patient, patient with COPD retaining CO2) and student teams race to write the expected ABG pattern and the respiratory therapist's role in management.
Materials: Egan's Fundamentals of Respiratory Care; ABG interpretation worksheet set (10 cases); Henderson-Hasselbalch reference card; whiteboards or large paper pads; dry-erase markers.
Check: Ten-item ABG classification quiz; students must correctly classify at least nine of ten results, including correct identification of compensation, to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Applied Sciences — Cardiopulmonary Anatomy & Physiology
1. Approximately what percentage of carbon dioxide in the blood is transported as bicarbonate ions?
Applied Sciences — Cardiopulmonary Anatomy & Physiology
2. Which enzyme in the red blood cell catalyzes the conversion of carbon dioxide and water into carbonic acid?
Applied Sciences — Cardiopulmonary Anatomy & Physiology
3. Using the Henderson-Hasselbalch equation, if a patient's PaCO₂ rises while HCO₃⁻ remains unchanged, what happens to blood pH?
Clinical Data — ABG Interpretation
4. A patient's ABG results are: pH 7.22, PaCO₂ 68 mmHg, HCO₃⁻ 27 mEq/L. Which classification is correct?
Clinical Data — Patient Assessment
5. A post-surgical patient is anxious and breathing at 28 breaths per minute. Which ABG pattern would you most likely expect?
Clinical Data — ABG Interpretation
6. A stable COPD patient with chronic CO₂ retention has ABG results: pH 7.37, PaCO₂ 55 mmHg, HCO₃⁻ 30 mEq/L. What is the correct interpretation?
Clinical Data — ABG Interpretation
7. During ABG interpretation, you identify that only the PaCO₂ is abnormal and the HCO₃⁻ is normal. Which term best describes the compensation status?
Patient Safety and Professional Practice
8. A patient's pH is 7.18 and PaCO₂ is 72 mmHg. The respiratory therapist confirms this is a critical value. What is the most appropriate immediate action?
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