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Red blood cells are perhaps one of the most specialized cells in the human body. They are a biconcave (doughnut) shape with a thin central disk. This shape is important because the disk increases the surface-area-to-volume ratio for faster exchange of gases and it allows the cells to stack on top of each other during their movement through tight blood vessels.

Also, since some capillaries are as narrow as 0.00015748 inches  (0.004 millimeters, red blood cells can literally squeeze through these vessels by changing their shape.