List the three methods that carbon dioxide is carried in the bloodstream.
The correct answer and explanation is:
Carbon dioxide (CO₂) is carried in the bloodstream through three main methods: as bicarbonate ions (HCO₃⁻), bound to hemoglobin (carbaminohemoglobin), and dissolved in plasma.
- Bicarbonate Ions (HCO₃⁻): The majority of carbon dioxide (about 70%) is transported in the blood as bicarbonate ions. This process occurs in the red blood cells. CO₂ enters the cells and reacts with water, catalyzed by the enzyme carbonic anhydrase, to form carbonic acid (H₂CO₃). Carbonic acid then dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻). The bicarbonate ions are transported into the plasma in exchange for chloride ions (Cl⁻), a process known as the chloride shift. This method of transport is crucial because bicarbonate is highly soluble in water and can easily diffuse in the bloodstream.
- Carbaminohemoglobin: Approximately 20% of carbon dioxide is carried by hemoglobin in a form known as carbaminohemoglobin. CO₂ binds to the amino groups of hemoglobin molecules in red blood cells. This binding reduces hemoglobin’s affinity for oxygen, promoting oxygen release at tissues where CO₂ concentrations are high. Conversely, in the lungs, the lower concentration of CO₂ causes carbaminohemoglobin to release CO₂, which is then expelled during exhalation.
- Dissolved in Plasma: A small portion (about 10%) of carbon dioxide is directly dissolved in the plasma. This dissolved CO₂ is in equilibrium with the bicarbonate ions and plays a role in the regulation of blood pH. Although this is a small percentage, it is important in the rapid exchange of CO₂ between blood and the alveoli in the lungs.
In summary, carbon dioxide is transported in the bloodstream primarily as bicarbonate ions, secondarily bound to hemoglobin, and to a lesser extent dissolved directly in plasma. These mechanisms work together to ensure efficient removal of CO₂ from tissues and its expulsion from the body.