According to Reference.com, pCO2 is defined as carbon dioxide concentration. It is further defined as carbon dioxide partial pressure, which is expressed in kilopascals in reference to blood, according to The Free Dictionary. It can also be defined as the negative decadic logarithm of CO2.
Lower levels of carbon dioxide in the arterial system are often given as carbon dioxide partial pressure or pCO2. The venous system carries carbon dioxide throughout a human body, and the carbon dioxide is breathed out through a person's lungs. Carbon dioxide content is therefore low in the respiratory system and high in the venous system, which results in the aforementioned low carbon dioxide levels in the arterial system.