The structure of red blood cells
Red blood cells (erythrocytes) are the most abundant cells in the human body, on average there are approximately 25 trillion or 25000000000000 (2.5x1013) per person. They are also the simplest cells in the human body and are little more than cell membranes filled with hemoglobin. The characteristic biconcave shape of red blood cells ensures that each of the 270 million hemoglobin molecules are close to the surface enabling rapid gas exchange.
Play with a applet below and look at the usual shape of red blood cells
If the movement is too slow on your machine then you can try to use a `coarser' - less smooth and thus simpler - model by clicking on the coarser button. If you want to make the shape smoother, but slower, click on the finer button.