What substance is primarily involved in the transportation of carbon dioxide?

• A. Platelets
• B. Hemoglobin
• C. Fluid cytoplasm
• D. Bicarbonate ions and plasma

Answer: (D)

The substance primarily responsible for the transportation of carbon dioxide in the bloodstream is bicarbonate ions and plasma. When carbon dioxide is produced by cells as a byproduct of metabolism, it is transported in several ways. A significant portion (about 70%) of carbon dioxide enters the bloodstream and is converted into bicarbonate ions (HCO3-) through a reaction with water, catalyzed by the enzyme carbonic anhydrase. This reaction occurs predominantly within red blood cells. The bicarbonate ions are then released into the plasma, which is the liquid component of blood, where they can be transported to the lungs to be exhaled.

The remaining carbon dioxide is transported in two other forms: a smaller fraction is physically dissolved in the plasma, while another part is bound to hemoglobin, the protein responsible for oxygen transport. However, the predominant means of transporting carbon dioxide back to the lungs is indeed in the form of bicarbonate ions in the plasma. This allows for efficient transport and regulation of acid-base balance in the body.