Which cell type is primarily responsible for carrying oxygen in the bloodstream?

• A. Adipocytes
• B. Erythrocytes
• C. Osteocytes
• D. Myocytes

Answer: (B)

Red blood cells carry oxygen in the bloodstream. They contain hemoglobin, a protein with iron that binds oxygen in the lungs where oxygen levels are high, forming oxyhemoglobin. As blood travels to tissues with lower oxygen, hemoglobin releases oxygen to cells that need it, aided by factors such as higher carbon dioxide, lower pH, and warmer temperature, which lower hemoglobin’s affinity for oxygen. The binding of oxygen to hemoglobin is cooperative, so once one molecule binds, it becomes easier for others to attach. The biconcave shape of red blood cells increases surface area for gas exchange and allows them to squeeze through tiny capillaries. In most mammals, mature red blood cells lack a nucleus and mitochondria to maximize space for hemoglobin, obtaining energy via glycolysis. Most oxygen is carried bound to hemoglobin; only a small amount is dissolved directly in plasma. Other cell types—like adipocytes that store fat, osteocytes in bone, and myocytes in muscle—have roles unrelated to transporting oxygen.