Healthy red blood cells carry oxygen to all the other cells in your body. They are good oxygen carriers. They are also rounded and flexible, and they can slide easily through the smallest capillary.

In some people the red blood cells aren't shaped right. The cells are sickle- or crescent-shaped and rigid. Sickle-shaped cells don't carry as much oxygen as normal blood cells. Sickle-shaped red blood cells cause sickle cell anemia.