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Red blood cells. Coloured scanning electron micrograph (SEM) of freeze-fractured red blood cells (erythrocytes). An unfractured red blood cell is seen at centre. The fractured cells around it show the various cross-sectional shapes of the cells. The cells are biconcave (seen at bottom), and disc-shaped (seen at centre). They transport oxygen from the lungs to body cells, and remove carbon dioxide back to the lungs. The oxygen and carbon dioxide bind with haemoglobin, the protein that gives blood its red colour. Red blood cells are the most abundant cell in human blood.
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