Red blood cells change in shape as they pass along the capillaries, then assume into their original shapes. What are the advantages of this change?

The advantage of this change is that it allows the red blood cells to pass through the capillaries that are very tiny blood vessels.  Normally blood cells are disc -shaped (like a Frisbee) and are about 8 microns in diameter.  Capillaries have a diameter as small as 2 microns.  The blood cells stretch from a disc shape into a bullet shape and become almost fluid like.  This happens over and over during the life of a blood cell as it is circulated through the human body traveling from large blood vessels to small vessels (capillaries).  The disease sickle cell anemia occurs because some of the red blood cells are not able to change shape and they, therefore, become stuck in the capillaries.  This causes the blood flow to stop or slow down in the capillaries and means that the capillaries cannot deliver enough oxygen to the tissues they serve.