The primary function of red blood cells (RBCs) is to transport oxygen from the lungs to tissues and organs throughout the body. They accomplish this using hemoglobin, a specialized protein that binds oxygen molecules and carries them through the bloodstream. As blood circulates, red blood cells also help return carbon dioxide — a waste product — from the tissues back to the lungs, where it is exhaled. Without red blood cells, your body wouldn't be able to deliver the oxygen necessary for cells to produce energy and function properly. A shortage of red blood cells, or hemoglobin, can lead to anemia, which causes fatigue, shortness of breath, and weakness. These cells make up about 40–45% of your blood volume and are continuously replenished by bone marrow. Their disk-like shape helps them move easily through blood vessels, ensuring every part of the body receives oxygen needed to survive.