The PaCO₂ is the partial pressure of carbon dioxide in the arterial blood. The PaCO₂ is often considered the respiratory parameter as it reflects the subject’s ventilatory ability – i.e their ability to ‘blow off’ carbon dioxide, which is a strong predictor of respiratory health and morbidity.

Carbon dioxide is produced as a result of all the metabolic processes, such as wiggling your toes, exercising your muscles during a leisurely jog, digesting a large meal, or simply sat quietly watching television. Carbon dioxide is at its highest concentration at the mitochondria where it is the end product of metabolism. Carbon dioxide follows a reverse path to oxygen down partial pressure gradients to the lungs.

Carbon Dioxide is a waste product, which must be removed from the body via the lungs during exhalation. It is transported to the lungs in a plasma solution in the form of a chemical called carbonic acid (H₂CO₃). This is an acidic solution; too much and the individual must breath harder to exhale more CO₂ to maintain a normal blood pH, too little and the breathing depth and rate may reduce.

The normal values for PaCO₂ are between 4.8kPa and 6.0 kPa.