Carbon dioxide (\(CO_2\)) is a natural byproduct of the body’s metabolism, created as cells convert nutrients into energy. \(CO_2\) plays a central part in maintaining the blood’s acid-base balance, or pH. Inside the bloodstream, \(CO_2\) combines with water to form carbonic acid, which quickly dissociates into bicarbonate and hydrogen ions. This chemical reaction is the primary buffer system that prevents the blood from becoming too acidic or too alkaline. Because \(CO_2\) levels directly influence the concentration of acid in the blood, its concentration is a regulated measure reflecting both metabolism and lung efficiency.
Understanding Carbon Dioxide Measurement in the Body
The specific measurement of \(CO_2\) in the blood that yields a value like 33 is the partial pressure of carbon dioxide, or \(PCO_2\). Clinicians obtain this value through an Arterial Blood Gas (ABG) test, which requires a small blood sample drawn from an artery, typically in the wrist. The resulting \(PCO_2\) is expressed in units called millimeters of mercury (mmHg).
This measurement represents the pressure exerted by \(CO_2\) gas dissolved in the arterial blood. It serves as a direct indicator of the respiratory system’s effectiveness, as the lungs are the primary mechanism for controlling \(CO_2\) levels. By adjusting the rate and depth of breathing, the respiratory system can rapidly regulate blood pH by increasing or decreasing the amount of \(CO_2\) exhaled.
Contextualizing a Value of 33
For a healthy adult, the normal range for the partial pressure of carbon dioxide (\(PCO_2\)) in the arterial blood is between 35 and 45 mmHg. A \(PCO_2\) reading of 33 mmHg is slightly below this lower limit. This low value indicates hypocarbia, or hypocapnia, meaning there is less \(CO_2\) dissolved in the blood.
Since \(CO_2\) acts as an acid, reducing its concentration causes the blood to become more alkaline, a condition defined as respiratory alkalosis. Therefore, a value of 33 mmHg suggests the person is hyperventilating, or breathing faster or deeper than required, resulting in the excessive expulsion of \(CO_2\).
Common Causes of Low Carbon Dioxide
The central mechanism behind a low \(CO_2\) level is alveolar hyperventilation, which is breathing that exceeds the body’s production of \(CO_2\). This hyperventilation stems from either a primary physiological drive or a secondary, compensatory response to a medical issue.
Primary Physiological Causes
Primary causes directly stimulate the respiratory drive. These include behavioral responses like anxiety or panic attacks, which cause rapid breathing due to stress. Other triggers include fever, pain, and high altitude. For instance, at high altitudes, the body breathes faster to take in more oxygen, which secondarily causes a loss of \(CO_2\). Certain lung conditions, such as an acute asthma exacerbation or pulmonary embolism, can also cause rapid, shallow breathing, resulting in a lower \(PCO_2\).
Compensatory Hyperventilation
Compensatory hyperventilation occurs when the body intentionally lowers \(CO_2\) to correct a metabolic problem. If the blood becomes too acidic from a non-respiratory cause, such as diabetic ketoacidosis (DKA) or kidney failure, the body attempts to restore balance. To counter the excess acid, the brain signals the lungs to expel more acidic \(CO_2\), thereby raising the blood pH. In these cases, the \(PCO_2\) of 33 mmHg is not the primary disorder but the body’s attempt to self-correct a serious underlying metabolic issue.
Implications of Persistent Hypocarbia
While a \(PCO_2\) of 33 mmHg is only slightly low, persistent or more severe hypocarbia can affect several body systems. Neurological effects are common, as low \(CO_2\) causes blood vessels in the brain to constrict. This reduced cerebral blood flow can lead to symptoms like dizziness, lightheadedness, and confusion.
The shift toward alkalinity (respiratory alkalosis) also alters the balance of electrolytes, particularly calcium. The higher pH causes calcium to bind more tightly to proteins, reducing the active calcium available. This functional hypocalcemia can lead to sensory disturbances, such as tingling or numbness (paresthesia), often felt in the hands, feet, and mouth. Treatment focuses on addressing the root cause, such as managing anxiety, treating a lung condition, or correcting underlying metabolic acidosis.