Under the microscope, red blood cells as round or oval biconcave anucleate discs that resemble a doughnut but with an indentation in the center instead of a hollow cut-out. 

Red blood cells get their bright red color from their rich hemoglobin content. 

Their biconcave disc shape increases the surface area to volume ratio, making it easier for gasses to diffuse in and out of the blood cells. 

The absence of nucleus makes it easier for red blood cells to alter their shape so they can squeeze through small capillaries and move throughout your body more efficiently.