Red blood cells or erythrocytes, primarily carry oxygen and collect carbon dioxide through the use of haemoglobin. Haemoglobin is an iron-containing protein that gives red blood cells their color and facilitates transportation of oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs to be exhaled. Red blood cells are the most abundant cell in the blood, accounting for about 40% to 45% of its volume. Red blood cells are disk-shaped and deformable to allow them to squeeze through narrow capillaries. Red blood cells are much smaller than most other human cells.