Arterial blood gases

An arterial blood gas (ABG) is a blood test drawn from an artery — typically the radial artery — that measures pH, partial pressure of oxygen (PaO2), partial pressure of carbon dioxide (PaCO2), bicarbonate (HCO3−), and oxygen saturation. Together these values describe how well the lungs are oxygenating blood and clearing CO2, and whether the body's acid-base balance is intact.

Commonly cited normal adult ranges are pH 7.35–7.45, PaCO2 35–45 mmHg, HCO3− 22–26 mEq/L, and PaO2 roughly 80–100 mmHg. Interpretation follows a stepwise logic: a pH below 7.35 is acidemia and above 7.45 is alkalemia; the PaCO2 indicates whether the problem is respiratory (CO2 moves opposite to pH), while the bicarbonate indicates a metabolic cause (HCO3− moves with pH). The body then compensates — lungs adjust ventilation within minutes, kidneys adjust bicarbonate over days — pulling pH back toward normal.

ABGs guide real clinical decisions. A COPD patient retaining CO2 shows respiratory acidosis with renal compensation; a hyperventilating patient blows off CO2 into respiratory alkalosis; sepsis or diabetic ketoacidosis produces metabolic acidosis. For rehabilitation providers, gas values also flag whether a patient is stable enough to participate in therapy.

The NPTE-PT and NPTE-PTA exams test ABG interpretation within the pulmonary system — recognizing the four primary disturbances and their compensation patterns — and USMLE Step 1 ties ABGs to the physiology of breathing regulation, chemoreceptor drive, and ventilation-perfusion relationships.