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 be used for blood transfusions. Several blood tests involve red blood cells. These include a "RBC count" (the number of red blood cells per volume of blood), calculation of the hematocrit (percentage of blood volume occupied by red blood cells), and the erythrocyte sedimentation rate. The blood type needs to be determined to prepare for a blood transfusion or an organ transplantation. Many diseases involving red blood cells are diagnosed with a blood film (or peripheral blood smear), where a thin layer of blood is smeared on a microscope slide. This may reveal abnormalities of red blood cell shape and