The concentration of carbon dioxide (CO2) measured in the bloodstream is an important indicator of the body’s overall acid-base balance. This measurement primarily reflects the level of bicarbonate (HCO3-), which is the alkaline component of the body’s main buffer system. Bicarbonate acts as a transport mechanism, carrying most of the metabolically produced CO2 from the tissues back to the lungs for exhalation. By maintaining this balance, the body ensures the blood pH stays within a narrow, healthy range. Monitoring this level is therefore a direct way to assess how effectively the respiratory and metabolic systems are managing the body’s internal chemistry.
The Difference Between Blood Gas and Metabolic Panel CO2
A common point of confusion arises because CO2 is measured in two distinct ways, each providing a different view of the body’s function. The CO2 level included in a routine Basic Metabolic Panel (BMP) or Comprehensive Metabolic Panel (CMP) is referred to as Total CO2 or Bicarbonate. This venous blood test measures all forms of carbon dioxide in the blood, about 95% of which is the bicarbonate ion (HCO3-). This measurement is a reflection of the metabolic component of the acid-base system, which is largely controlled by the kidneys.
The other measurement is the partial pressure of carbon dioxide, known as PaCO2, which is obtained through an Arterial Blood Gas (ABG) test. This specialized test draws blood from an artery, not a vein, and directly measures the amount of gaseous CO2 dissolved in the blood plasma. PaCO2 is a direct reflection of lung function, showing how effectively the body is ventilating and “blowing off” carbon dioxide. A PaCO2 measurement is typically ordered in acute care settings to assess immediate respiratory status, while the more common Total CO2 from a metabolic panel is used to screen for chronic metabolic or kidney conditions.
Defining the Standard Normal Range
For non-pregnant adult women, the standard clinical reference ranges for CO2 levels are generally the same as those for men, as the underlying physiology of the acid-base buffer system is universal. The level most often reported on a routine blood test is the Bicarbonate (HCO3-), or Total CO2, from the metabolic panel. The normal range for bicarbonate is typically between 22 and 29 milliequivalents per liter (mEq/L).
The more specialized PaCO2 measurement, which reflects respiratory function, has a normal range of 35 to 45 millimeters of mercury (mmHg). While the baseline ranges are consistent across non-pregnant adults, pregnancy does introduce a physiological adaptation that can slightly shift these values.
Due to hormonal changes, particularly the effect of progesterone, pregnant women increase their minute ventilation, meaning they breathe more deeply and frequently. This process causes them to exhale more carbon dioxide, which can lead to a slightly lower PaCO2 and a compensatory, slightly lower bicarbonate level compared to the standard non-pregnant range.
Implications of Abnormal Levels
When a woman’s CO2 level, specifically the bicarbonate level, falls below the normal range of 22 to 29 mEq/L, it suggests a state of metabolic acidosis, where the blood has too much acid. This condition can arise from the body producing excessive acid, such as in diabetic ketoacidosis, or from the loss of bicarbonate, which often occurs with severe or chronic diarrhea. Low bicarbonate levels can also be an indicator of kidney dysfunction, as the kidneys are responsible for regulating and reabsorbing bicarbonate into the bloodstream.
Conversely, an elevated bicarbonate level, referred to as metabolic alkalosis, suggests a deficit of acid or an excess of base in the blood. This imbalance is commonly seen after prolonged, severe vomiting, where the body loses a significant amount of stomach acid. Certain diuretic medications can also cause the kidneys to retain bicarbonate, leading to an increased concentration in the blood.
Abnormal PaCO2 levels, reflecting respiratory issues, have a different set of implications. A high PaCO2 above 45 mmHg indicates hypoventilation, or insufficient breathing, which causes the body to retain carbon dioxide. This condition is called respiratory acidosis and can be a sign of lung diseases like Chronic Obstructive Pulmonary Disease (COPD) or respiratory depression caused by certain medications.
A low PaCO2, below 35 mmHg, suggests hyperventilation, where the body is exhaling too much carbon dioxide, resulting in respiratory alkalosis. While often caused by anxiety or panic, it can also be a compensatory mechanism, such as when the body attempts to correct an existing metabolic acidosis by increasing ventilation. In both metabolic and respiratory acid-base disorders, the body attempts to naturally compensate using the opposing system, highlighting the interconnected roles of the lungs and kidneys in maintaining a healthy CO2 balance.