Ep: 04: Carbon Dioxide (Serum CO2)
This week we are focused on Carbon Dioxide!
Carbon Dioxide is produced in the body from cellular respiration. As a biomarker that we measure on a standard chemistry screen, CO2 is really a measure of bicarbonate in the blood. The majority of CO2 (about 75%), produced from cellular respiration, is carried in the blood as the bicarbonate ion. 5% remains in solution as dissolved CO2 and the remaining 20% remains combined with hemoglobin and other plasma proteins. Dissolved CO2, formed in the lungs, contributes little to the CO2 value.
One of the reasons you would pay attention to CO2 or bicarbonate on a blood test is as a general measure of tissue acidity or alkalinity. However, one of the difficulties in understanding acid-base balance is confusion in the terminology, especially the term CO2. Ideally, we should refer to CO2 in terms of CO2 content and CO2 gas.
- CO2 content refers to bicarbonate, which is an alkaline or base molecule. It is a solution and is regulated primarily by the kidneys. As previously mentioned, this is the CO2 referred to on blood chemistry panels.
- CO2 gas, on the other hand, refers to the dissolved CO2 and is mainly acid. It is regulated by the lungs
Both have a powerful impact on acid-base regulation and are regulated by different organ systems.
Serum CO2, as bicarbonate acts as one of the reserve alkaline elements in the blood. Bicarbonate neutralizes metabolic acids, such as hydrochloric and lactic acids.
While not the most sensitive measurement of pH or carbon dioxide gas, we can look at serum CO2 to help evaluate a trend towards an alkalosis or acidosis in the body. For a more sensitive evaluation of respiratory and/or metabolic acidosis or alkalosis, which are often seen in hospital settings, conditions of primary CO2 increase or decrease are best evaluated and followed-up with other readings and studies, such as blood gases.
So, in summary, elevated levels of serum CO2, or bicarbonate, are associated with a trend towards Metabolic Alkalosis and decreased levels are associated with a trend towards Metabolic Acidosis, in this situation, the bicarbonate is being used-up to buffer the increasing levels of acidity or H+ in the body.