How is carbon dioxide primarily transported from tissues to the lungs?

• A. As carbonic acid only
• B. Primarily as bicarbonate ions and dissolved in plasma
• C. Only within red blood cells
• D. Through lymphatic fluid

Answer: (B)

Carbon dioxide is primarily transported from tissues to the lungs in two main forms: as bicarbonate ions and dissolved in plasma. When carbon dioxide is produced in the tissues as a byproduct of metabolism, a significant portion (approximately 70% to 75%) diffuses into the blood and enters red blood cells, where it undergoes a reaction with water to form carbonic acid. This carbonic acid quickly dissociates into hydrogen ions and bicarbonate ions. The bicarbonate ions then enter the plasma, where they can be transported to the lungs.

This process is crucial for maintaining the body’s acid-base balance and facilitating the efficient removal of carbon dioxide from the body. The remaining portion of carbon dioxide is transported dissolved directly in the plasma and attached to hemoglobin in red blood cells, but these methods account for a smaller percentage compared to bicarbonate.

The other transport methods listed are less prominent in the overall transport of carbon dioxide. For instance, while carbonic acid plays a role in the transport process, it does not account for the majority of carbon dioxide transport. Transport solely within red blood cells or through lymphatic fluid is not how carbon dioxide is primarily moved to the lungs, emphasizing the importance of bicarbonate ions in this physiological process.