Why Red Blood Cells look Like Donuts

I was looking at a slide of red blood cells under a microscope and I noticed something I never really caught, red blood cells look like donuts. Most cells do not have this shape to them so why do red blood cells look like donuts?
Well red blood cells don't really like donuts they actually look like "biconcave" disk. That means they are round but there side concave inwards.

The "biconcave" disk shape is created by a particular protein which is present in the outer membrane and holds it into shape. Red blood cells play the special role of carrying oxygen to all parts of your body. Red blood cell go through blood vessels to do so. The red blood cells have to be really flexible to squeeze through capillaries, capillaries can be the size of a strand of hair, this means that there smaller then the blood cells. Red blood cells have to "denuclei". They literally drop their nuclei and therefore the "biconcave" shape appears. Red blood cells don't need there nuclei to reproduce, they are reproduced in bone marrow after four months! They natural recycle!

Stay Curious xD!