Red blood cells are also known as erythrocytes (from Greek erythros for "red" and kytos for "hollow", with cyte nowadays translated as "cell"). Red blood cells are the most common type of blood cell and are the vertebrate body's principal means of delivering oxygen from the lungs to body tissues via the blood.
The diameter of a typical human erythrocyte disk is 6–8 µm; they are thus much smaller than most other human cells. A typical erythrocyte contains about 270 million hemoglobin molecules, with each carrying four heme groups.
Adult humans have roughly 2–3 × 1013 red blood cells at any given time (women have about 4 million to 5 million erythrocytes per cubic millimeter (microliter) of blood and men about 5 million to 6 million; people living at high altitudes with low oxygen tension will have more). Red blood cells are thus much more common than the other blood particles.
The blood types of humans are due to variations in surface glycoproteins of erythrocytes.