Human erythrocytes or red blood cells (RBCs) have a shape of a disk that appears to be "caved in" or almost flattened in the middle as shown in . This bi-concave shape allows RBCs to bend and flow smoothly through the narrowest blood vessels in the body. For instance, it takes about 20 seconds, on average, for a single RBC to complete one cycle of circulation. This shape also facilitates oxygen transport.
A typical human RBC has a disk diameter of 6–8 µm and a thickness of 2 µm, being much smaller than most other human cells. These cells have an average volume of about 90 fL with a surface of about 136 μm2. They can swell up to a sphere shape containing 150 fL, without membrane distension.
Although RBCs are considered cells, they lack a nucleus (and DNA) and organelles. RBCs, therefore, cannot divide or replicate like other cells of the body (e.g. skin cells); they also lack the machinery for protein biosynthesis. RBC's have a short life span of about 120 days.
The blood's red color is due to the spectral properties of the hemic iron ions in hemoglobin. Each human red blood cell contains approximately 270 million of these hemoglobin biomolecules, each carrying four heme groups; hemoglobin comprises about a third of the total cell volume. This protein is responsible for the transport of more than 98% of the oxygen (the remaining oxygen is carried dissolved in the blood plasma).
Adult humans have roughly 2–3 × 10^13 (20-30 trillion) RBCs at any given time, comprising approximately one quarter of the total human body cell number (women: ~4 to 5 million RBCs/uL of blood; men: ~5 to 6 million RBCs/uL of blood).