The pCO2, PCO2, or is the partial pressure of carbon dioxide (CO2), often used in reference to blood, but also used in Oceanography to describe the partial pressure of CO2 in the Ocean and in life support systems engineering and underwater diving to describe the partial pressure in a breathing gas. Usually the arterial blood is the relevant context; the symbol for in arterial blood is . Measurement of in the systemic circulation indicates the effectiveness of ventilation at the lungs' alveoli, given the diffusing capacity of the gas. It is a good indicator of respiratory function and the closely related factor of acid–base homeostasis, reflecting the amount of acid in the blood (without lactic acid).
- Its normal values are in the range 35–45 mmHg.
- If the is less than 35 mmHg, the patient is hyperventilating, and if the pH (potential hydrogen) is greater than 7.45, corresponding to a respiratory alkalosis.
- If the is higher than 45 mmHg, the patient is hypoventilating, and if the pH is less than 7.35, is in respiratory acidosis.