What are red blood cells and what does hemoglobin do?

• A. Red blood cells carry carbon dioxide; hemoglobin transports oxygen and nitrogen.
• B. Red blood cells carry oxygen; hemoglobin binds and transports oxygen and some CO2.
• C. Red blood cells transport nutrients; hemoglobin stores vitamins.
• D. Red blood cells regulate blood flow; hemoglobin dissolves gases.

Answer: (B)

Red blood cells are built to move gases between the lungs and the body's tissues. Inside each cell is hemoglobin, a protein with iron-containing heme groups that grabs oxygen in the lungs, then releases it where cells need it. Hemoglobin also helps remove carbon dioxide, picking it up from tissues and bringing it back toward the lungs. Most CO2 travels as bicarbonate in the plasma, but some CO2 binds directly to hemoglobin for transport. So the statement that best fits is that red blood cells carry oxygen, and hemoglobin binds and transports oxygen and some CO2. The other options misstate what red blood cells and hemoglobin primarily do (nutrient transport, regulation of blood flow, or dissolving gases).