What do red blood cells primarily carry?

Red blood cells (RBCs), also known as erythrocytes, primarily carry oxygen from the lungs to the tissues throughout the body. This function is facilitated by a protein called hemoglobin, which binds to oxygen molecules. Hemoglobin's structure allows it to pick up oxygen as blood flows through the lungs and release it in tissues where it is needed for cellular respiration.

While red blood cells also play a role in transporting a small amount of carbon dioxide back to the lungs to be exhaled, their primary and most critical function is the delivery of oxygen. This oxygen transport is vital for maintaining cellular function and overall metabolism in the body.