A normal arterial blood gas (ABG) measures five main values: pH between 7.35 and 7.45, PaCO2 between 35 and 45 mmHg, PaO2 between 80 and 100 mmHg, bicarbonate (HCO3) between 22 and 26 mEq/L, and oxygen saturation (SaO2) between 95% and 100%. A sixth value, base excess, normally falls between -4 and +2 mEq/L. Together, these numbers reveal how well your lungs exchange gases and how balanced your blood’s acid-base chemistry is.
What Each Value Tells You
The pH is the centerpiece of the ABG. It measures how acidic or alkaline your blood is on a tight scale. Even small shifts outside the 7.35 to 7.45 window signal a meaningful problem. A pH below 7.3 or above 7.6 is considered a critical value that requires immediate attention.
PaCO2 reflects how effectively your lungs are clearing carbon dioxide. Carbon dioxide is acidic, so when it builds up (above 45 mmHg), blood becomes more acidic. When you breathe too fast and blow off too much CO2 (below 35 mmHg), blood becomes more alkaline. Critical thresholds sit at below 20 or above 60 mmHg.
PaO2 measures the oxygen dissolved in arterial blood. Normal is 80 to 100 mmHg when breathing room air. A PaO2 of 60 to 79 mmHg is classified as mild hypoxemia, 40 to 59 mmHg as moderate, and anything below 40 mmHg as severe. SaO2 measures the percentage of hemoglobin actually carrying oxygen, and it should stay at 95% or above.
Bicarbonate (HCO3) reflects the metabolic side of acid-base balance, controlled mainly by the kidneys. It acts as a buffer, neutralizing excess acid. Base excess is a related number that summarizes whether the body has too much or too little buffering capacity overall.
How Your Body Keeps These Numbers in Range
Your lungs and kidneys work as a team to keep blood pH stable, but they operate on very different timelines. The lungs respond in minutes. If your blood becomes too acidic, your breathing rate increases to blow off more CO2, pulling pH back toward normal. If blood becomes too alkaline, breathing slows to retain CO2.
The kidneys take much longer. They adjust by holding onto or excreting bicarbonate and hydrogen ions. This process begins within hours but takes 2 to 3 days for a full response, sometimes up to 5 or 6 days for complete compensation. This is why a sudden respiratory problem can leave your pH low for days before the kidneys catch up.
When compensation is working but incomplete, the pH remains abnormal, and the ABG is described as “partially compensated.” When the pH has returned to the normal range, it’s “fully compensated,” even though the underlying problem may still exist. For example, someone with chronic lung disease might have a persistently high PaCO2, but their kidneys have raised bicarbonate levels enough to normalize the pH.
Respiratory vs. Metabolic Problems
ABG results point to one of four basic imbalances. A simple way to sort them out: look at whether the pH and PaCO2 move in opposite directions or whether the pH and bicarbonate move in the same direction.
- Respiratory acidosis: pH is low, PaCO2 is high. The lungs aren’t clearing enough carbon dioxide. This happens with conditions like COPD, severe asthma attacks, or anything that depresses breathing.
- Respiratory alkalosis: pH is high, PaCO2 is low. You’re breathing too fast and exhaling too much CO2. Anxiety, pain, fever, or being on a ventilator can cause this.
- Metabolic acidosis: pH is low, bicarbonate is low. The body is either producing too much acid (as in diabetic ketoacidosis or sepsis) or losing too much bicarbonate (as in severe diarrhea or kidney disease).
- Metabolic alkalosis: pH is high, bicarbonate is high. This often results from prolonged vomiting, which removes stomach acid, or from certain medications that shift the balance.
Why ABG Tests Are Ordered
ABGs are not routine blood work. They’re ordered when there’s a real concern about breathing, oxygen levels, or acid-base balance. Common reasons include monitoring lung diseases like COPD, asthma, or cystic fibrosis; evaluating suspected carbon monoxide poisoning or smoke inhalation injuries; and investigating kidney disorders that might be disrupting acid-base chemistry. Head or neck injuries that could affect breathing are another reason.
The test uses blood drawn from an artery, usually at the wrist. Before the draw, a circulation test is often performed: both arteries supplying the hand are compressed, then released one at a time to confirm that blood flow from the remaining artery is sufficient. Color should return to the palm within 5 to 15 seconds. This step matters because arterial puncture carries a small risk of clotting at the site, and you need to know the hand would still get adequate blood flow if that happened.
Factors That Shift Normal Ranges
Altitude is the biggest environmental factor. At 1,400 meters (about 4,600 feet), healthy nonsmokers have an average PaO2 around 70 to 79 mmHg, well below the sea-level norm of 80 to 100. PaCO2 also runs lower at altitude, averaging around 33 to 34 mmHg, because the body compensates for thinner air by breathing faster. The pH stays essentially the same because the kidneys adjust bicarbonate levels over time.
Age matters too. At sea level, healthy adults aged 18 to 24 average a PaO2 near 100 mmHg, while those over 64 average closer to 89 mmHg. This gradual decline reflects normal aging of the lungs and is not considered a disease state.
Sample handling can also affect results. If the blood sits at room temperature for even 30 minutes before analysis, cells continue consuming oxygen and producing CO2. This artificially lowers pH and raises PaCO2. Placing the sample on ice slows this process significantly. If your results seem inconsistent with how you feel, delayed processing is one possible explanation.