What is hemoglobin's main function in the respiratory system?

• A. To filter out impurities from the air
• B. To assist in the electrical signaling of the lungs
• C. To carry oxygen to tissues and return carbon dioxide
• D. To help regulate blood pH levels

Answer: (C)

Hemoglobin's primary role is to transport gases between the lungs and the tissues. In the lungs, it binds oxygen, forming oxyhemoglobin, so red blood cells can carry it through the bloodstream. As blood reaches tissues where oxygen is needed, hemoglobin releases the oxygen to nourish cellular respiration. At the same time, carbon dioxide—a waste product of metabolism—diffuses into the blood from tissues. Much of it is carried back to the lungs: some binds to hemoglobin to form carbaminohemoglobin, and most is converted to bicarbonate inside red blood cells and then travels in the plasma. In the lungs, CO2 is released from hemoglobin and bicarbonate and exhaled.

Air filtration, neural electrical signaling, and pH regulation are separate processes that support respiration, but they are not the main function of hemoglobin. The key idea is that hemoglobin enables efficient gas exchange by loading oxygen for delivery to tissues and picking up carbon dioxide for removal.