Red blood cells are disc shaped. It is thought one factor that allows them to have this shape is because they do not have a nucleus. This tends to increase amount of space inside the cell for hemoglobin.
They have a flexible disc shape which helps increase the surface- to- area volume ratio which enables oxygen and carbon dioxide to diffuse across the red blood cell's plasma membrane more readily.
Their disc shape also enables them to squeeze through tiny capillaries that are half their size .