Which statement best describes hemoglobin's role in gas transport?

• A. Hemoglobin binds O2 in the lungs, transports it in blood, and releases it to tissues; CO2 binds to hemoglobin as well for transport.
• B. Hemoglobin binds O2 in the lungs, but does not transport CO2.
• C. Hemoglobin binds CO2 exclusively, not O2.
• D. Hemoglobin transports oxygen through plasma without binding it.

Answer: (A)

Hemoglobin carries gases by reversible binding: it picks up oxygen in the lungs, where its partial pressure is high, and releases it to tissues where the partial pressure is low. At the same time, it helps move carbon dioxide back to the lungs. Some CO2 binds to the amino groups of the globin chains to form carbaminohemoglobin, and most CO2 is converted to bicarbonate in red blood cells and transported in plasma. This means hemoglobin contributes to both O2 delivery and CO2 removal, though CO2 transport also relies on the bicarbonate system in plasma. The statement that best fits this is the one that mentions oxygen binding and release in the lungs and tissues, with CO2 also binding to hemoglobin for transport. The other ideas are incomplete or misleading because CO2 transport is indeed part of the system, oxygen is primarily carried by hemoglobin (not just dissolved in plasma), and CO2 is not transported exclusively by binding to hemoglobin.