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
Blood volume Blood volume Blood volume is the volume of blood (both red blood cells and plasma) in the circulatory system of any individual. A typical adult has a blood volume of approximately 5 liters, with females and males having approximately the same blood volume. Blood volume is regulated by the kidneys. Blood volume (BV) can be calculated given the hematocrit (HC; the fraction of blood that is red blood cells) and plasma volume (PV), with the hematocrit being regulated via the blood oxygen content regulator: Blood volume measurement may be used in people with congestive heart failure, chronic hypertension, renal failure