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Carbon dioxide in the blood is a by-product of metabolism, in which oxygen inhaled from the air and delivered by the blood to the body’s tissues is used to break down nutrients like sugars and fats for energy. This process, which is known as cellular or aerobic respiration, explains how energy in the form of calories from food is converted into adenosine triphosphate (ATP), which transports that energy and makes it available for use by the cells, which then release carbon dioxide back into the bloodstream as a waste product. Excessive or inadequate levels of carbon dioxide in blood, known respectively as hypercapnia and hypocapnia, can be indicative of problems with lung or kidney function or of electrolyte imbalances.
Found in the veins, the blood vessels that return deoxygenated and carbon-dioxide-rich blood to the heart and lungs, carbon dioxide (CO₂) is stored mostly as bicarbonate (HCO₃). Bicarbonate is an alkaline substance that helps the body to maintain its pH levels, or the balance between acid and alkaline. A carbon dioxide blood test can determine the amount of bicarbonate in the bloodstream. Normal levels of carbon dioxide in the blood fall between 20 and 29 mEq/L (milliequivalent per liter); levels that are too high or too low may be indicative of a fluid retention and therefore electrolyte imbalance, a function maintained by the kidneys, or of a disruption of normal lung function.
Hypercapnia, or excessive carbon dioxide in the blood, can be caused temporarily by vomiting, when the body’s fluid levels are too low, or chronically by diseases of the kidneys like Cushing syndrome. Cushing, which is a hormone disorder, can cause excessive urination and subsequently low potassium and CO₂ levels. Likewise, hypocapnia is a state of inadequate carbon dioxide in a person's blood. It can also be caused by diseases of the kidneys like Addison’s disease. Electrolyte imbalances caused by diarrhea or a condition like acidosis, a build-up of acid in the blood and other bodily fluids, can also lead to hypocapnia.
Temporary or chronic conditions of the lungs can also lead to changes in levels of carbon dioxide in the blood. Hyper or hypoventilation can alter the rate at which carbon dioxide is released from the lungs and thereby induce hypocapnia or hypercapnia in the bloodstream. Activities like scuba diving, in which participants inhale previously exhaled air can also alter CO₂ levels. Lung diseases resulting in disordered breathing may also disrupt normal quantities of carbon dioxide in the blood.