The pCO2 is the partial pressure of carbon dioxide (CO 2) in arterial blood.It is also represented as PaCO 2. Expresses the effectiveness of alveolar ventilation, given the diffusing capacity of the gas. It is a good indicator of respiratory function and reflects the amount of acid in the blood (without lactic acid).t is also represented as PaCO 2. Expresses the effectiveness of alveolar ventilation, given the diffusing capacity of the gas. It is a good indicator of respiratory function and reflects the amount of acid in the blood (without lactic acid).