What is hemoglobin primarily known for in red blood cells?

Hemoglobin is primarily known for its function in red blood cells as the protein responsible for carrying oxygen from the lungs to the tissues throughout the body. It binds to oxygen molecules due to its iron-containing heme groups, allowing red blood cells to transport a significant amount of oxygen efficiently. This oxygen transport is critical for cellular respiration, where cells utilize oxygen to produce energy.

While red blood cells also play a role in carrying carbon dioxide back to the lungs for exhalation, the primary and most critical function of hemoglobin is still its ability to bind and transport oxygen. The other choices—carrying nutrients and hormones—do not pertain to hemoglobin's role but rather involve other components of the blood, such as plasma proteins and other cellular constituents.