Red blood cells are perhaps the most specialized cells in the human body. They are a biconcave (donut) shape with a thin central disc. This shape is important because the disc increases the surface-area-to-volume ratio for faster exchange of gases and it allows red blood cells to stack, one on another, as they flow through very narrow vessels. Also, since some capillaries are as narrow as 0.00015748 inches (0.004 millimeters), red blood cells can literally squeeze through narrow vessels by changing shape.
