Why is the shape of a red blood cell donut shape?

When a red blood cell matures it pushes out its nucleus. Its internal structures go through a shape change, giving the cell a discoid form, bi-concave, depressed centrally on both sides but not so much that the depressions meet to form a hole.

The technical term for a doughnut shape is 'torus'. A red blood cell is 'toroid' or 'a toroid' or 'toroidal'. Perhaps one could say it was a 'closed torus'.

This shape allows for more surface area to bind oxygen and carbon dioxide during gas exchange. The cells distinct form also makes the cell more flexible so that it can get through the smaller blood vessels.