Arterial blood gases (ABGs) have a reputation for reducing even confident nursing students into anxious test-takers. The moment you see a pH, PaCO₂, and HCO₃⁻ on an exam or clinical worksheet, it can feel like a foreign language—one spoken fluently only by critical care nurses and respiratory therapists.
The truth is far less intimidating.
Acid–base balance follows a predictable set of physiological rules. When those rules are understood in the right order, ABGs stop feeling like abstract chemistry and start reading like a story your body is telling. This guide breaks down ABGs step by step, without unnecessary jargon, complicated formulas, or memorization overload. By the end, you’ll be able to interpret acid–base disorders with confidence and apply them in real clinical and exam settings.
Why Acid–Base Balance Matters in Nursing
Acid–base balance refers to the body’s ability to maintain a stable pH, which is essential for cellular function, enzyme activity, and overall homeostasis. Even slight deviations in blood pH can impair oxygen delivery, alter heart rhythm, depress neurological function, or lead to life-threatening complications.
For nurses, ABG interpretation is not just a test topic—it’s a clinical skill. ABGs help assess respiratory function, metabolic status, kidney compensation, and the effectiveness of treatments such as oxygen therapy, mechanical ventilation, IV fluids, and medications.
On exams like the ATI TEAS, NCLEX, and nursing school unit tests, acid–base questions are designed to evaluate your critical thinking, not your ability to memorize values in isolation.
The Foundation: Understanding Normal ABG Values
Before interpretation comes orientation. Every ABG analysis begins with knowing what “normal” looks like.
Normal reference ranges are:
pH: 7.35–7.45
PaCO₂: 35–45 mmHg
HCO₃⁻: 22–26 mEq/L
The pH tells you whether the blood is acidic or alkaline. PaCO₂ reflects respiratory function and carbon dioxide retention. HCO₃⁻ represents metabolic control, largely regulated by the kidneys.
Rather than memorizing these values as separate facts, it helps to understand their roles. Carbon dioxide behaves as an acid in the body. Bicarbonate acts as a base. The lungs and kidneys work continuously—often in opposite directions—to keep pH within a very narrow, safe range.
Step One: Look at the pH First
ABG interpretation always starts with the pH. This single number sets the direction for everything that follows.
A pH below 7.35 indicates acidosis
A pH above 7.45 indicates alkalosis
At this stage, ignore PaCO₂ and HCO₃⁻. The goal is simply to determine whether the blood is acidic or alkaline. Many errors happen when students try to interpret everything at once. pH is the anchor.
If the pH is within the normal range but trending toward one side, compensation may already be occurring—a concept we’ll revisit later.
Step Two: Identify the Primary Problem (Respiratory vs. Metabolic)
Once you know whether the patient is acidotic or alkalotic, the next step is identifying the system responsible.
This is where understanding cause-and-effect relationships becomes more important than memorization.
PaCO₂ moves opposite the pH
HCO₃⁻ moves in the same direction as the pH
If the pH is low (acidic) and PaCO₂ is high, the problem is respiratory. Excess carbon dioxide is making the blood acidic, resulting in respiratory acidosis.
If the pH is low and HCO₃⁻ is low, the issue is metabolic. Loss of bicarbonate or accumulation of metabolic acids causes metabolic acidosis.
The same logic applies to alkalosis. High pH with low PaCO₂ indicates respiratory alkalosis, while high pH with elevated bicarbonate signals metabolic alkalosis.
This relational approach eliminates guesswork and allows you to reason through any ABG scenario logically.
Respiratory Disorders: When the Lungs Are the Problem
Respiratory acid–base disorders originate from changes in ventilation.
Respiratory Acidosis
Respiratory acidosis occurs when ventilation is inadequate, leading to carbon dioxide retention. Common causes include chronic obstructive pulmonary disease (COPD), hypoventilation, airway obstruction, oversedation, and respiratory muscle fatigue.
ABG findings typically show:
Low pH
High PaCO₂
Normal or elevated HCO₃⁻ (if compensation is present)
In chronic cases, the kidneys compensate by retaining bicarbonate to buffer excess acid.
Respiratory Alkalosis
Respiratory alkalosis results from excessive ventilation, causing carbon dioxide to be blown off too quickly. Anxiety-induced hyperventilation, fever, pain, early sepsis, and high altitude exposure are common triggers.
ABG findings include:
High pH
Low PaCO₂
Normal or decreased HCO₃⁻ with compensation
Understanding the clinical context helps solidify interpretation. Ask what the lungs are doing and why.
Metabolic Disorders: When the Kidneys or Chemistry Are at Fault
Metabolic disorders involve changes in bicarbonate levels due to kidney function or systemic metabolic processes.
Metabolic Acidosis
Metabolic acidosis occurs when bicarbonate is lost or acids accumulate. Diabetic ketoacidosis, lactic acidosis, severe diarrhea, renal failure, and toxin ingestion are frequent causes.
ABG patterns show:
Low pH
Low HCO₃⁻
Low PaCO₂ if respiratory compensation is occurring
The body compensates by increasing respiratory rate to expel carbon dioxide.
Metabolic Alkalosis
Metabolic alkalosis results from excess bicarbonate or acid loss. Vomiting, nasogastric suction, diuretic use, and excessive antacid intake are common contributors.
ABGs typically reveal:
High pH
High HCO₃⁻
Elevated PaCO₂ if the lungs compensate by hypoventilating
These conditions are often tested alongside electrolyte imbalances, especially potassium shifts.
Compensation: The Body’s Attempt to Restore Balance
Compensation occurs when the system not primarily affected attempts to correct the pH. The lungs respond quickly, within minutes. The kidneys respond more slowly, over hours to days.
Compensation can be:
Uncompensated: pH abnormal, one value abnormal
Partially compensated: pH abnormal, both PaCO₂ and HCO₃⁻ abnormal
Fully compensated: pH normal, PaCO₂ and HCO₃⁻ abnormal
It’s important to note that compensation never overcorrects. If the pH has crossed to the opposite side of normal, another primary disorder is present.
A Simple ABG Interpretation Framework
When faced with ABGs on exams or in practice, consistency matters more than speed. A structured approach reduces errors.
Check the pH
Determine acidosis or alkalosis
Identify whether PaCO₂ or HCO₃⁻ matches the pH change
Assess compensation
Connect findings to the clinical scenario
This framework works universally, whether interpreting a straightforward exam question or managing a critically ill patient.
Common ABG Mistakes Students Make
Many students struggle with ABGs not because the topic is inherently difficult, but because of avoidable errors.
Interpreting values out of order, memorizing patterns without understanding physiology, and ignoring the patient’s clinical picture are frequent pitfalls. Another common mistake is assuming normal pH means no problem exists. Fully compensated disorders still reflect underlying pathology and must be recognized.
Avoid rushing. ABGs reward deliberate, logical thinking.
How ABGs Are Tested on Nursing Exams
On standardized exams, ABG questions rarely appear in isolation. They are often embedded in scenarios involving respiratory distress, renal dysfunction, medication effects, or acid–base-altering conditions like vomiting or sepsis.
Test writers are assessing your ability to synthesize data, prioritize interventions, and recognize instability. Understanding acid–base balance improves performance not only on ABG questions but across respiratory, cardiac, and metabolic topics.
Final Thoughts: ABGs Are a Skill, Not a Mystery
Acid–base balance is not about memorizing numbers or decoding cryptic lab values. It’s about understanding how the body maintains equilibrium and what happens when that balance is disrupted.
When approached step by step, ABGs become predictable, logical, and manageable. With practice, what once felt overwhelming becomes second nature.
Mastering ABGs strengthens your clinical reasoning, boosts exam confidence, and prepares you for real-world nursing care—where interpretation leads directly to intervention.
Acid–base balance doesn’t require tears. It requires clarity, structure, and a solid understanding of physiology. Once you have those, the numbers start telling a story—and you’ll know exactly how to read it.
If you found this guide helpful, consider sharing it with a fellow nursing student or bookmarking it for future review. Mastery comes from repetition, and ABGs are a skill worth practicing.
