Red blood cell evolved a mode of red blood cell development substantially different from the mammalian norm. Overall, mammalian red blood cells are remarkably flexible and deformable so as to squeeze through tiny capillaries, as well as to maximize their apposing surface by assuming a cigar shape, where they efficiently release their oxygen load. Red blood cells in mammals are unique amongst vertebrates as they do not have nuclei when mature. Red blood cells of mammals cells have nuclei during early phases of erythropoiesis, but extrude them during development as they mature; this provides more space for hemoglobin. The red blood cells without
Red blood cell the centre of 0.8–1 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 μm, and can swell up to a sphere shape containing 150 fL, without membrane distension. Adult humans have roughly 20–30 trillion red blood cells at any given time, constituting approximately 70% of all cells by number. Women have about 4–5 million red blood cells per microliter (cubic millimeter) of blood and men about 5–6 million; people living at high altitudes with low oxygen tension will have more. Red blood cells are