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How is carbon dioxide transported from body tissues to the lungs?
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human respiratory system
The lungs serve as the gas-exchanging organ for the process of respiration.
Carbon dioxide is transported from body tissues to the lungs through three primary mechanisms, as its transport is more complex than that of oxygen.1
- Bicarbonate Ions: Approximately 88 percent of carbon dioxide is carried as bicarbonate ions (HCO). Inside red blood cells, the enzyme carbonic anhydrase rapidly converts carbon dioxide and water into carbonic acid, which then dissociates into hydrogen and bicarbonate ions. To maintain electrical balance, a chloride shift occurs, where bicarbonate is moved out of the red blood cell into the plasma in exchange for chloride ions.12
- Carbamate: Some carbon dioxide binds directly to blood proteins, primarily hemoglobin, to form compounds known as carbamates. This process is enhanced by the release of oxygen in the tissues, as the resulting change in hemoglobin's configuration increases its affinity for carbon dioxide.1
- Dissolved Gas: About 5 percent of carbon dioxide remains unchanged and is transported dissolved in the blood.1
When blood reaches the lungs, where the partial pressure of carbon dioxide is lower, these reactions reverse, allowing the gas to be eliminated through expired air.13