Why do human red blood cells have no nucleus?

Answer

Dear Randy,

It is not true that all other mammals do have nucleated red blood cells. Other mammals, like us, have unnucleated red blood cells.

The red blood cells in all animal species come from cells with a nucleus. Stem cells with a nucleus are present in the bone marrow. They divide to form young red blood cells. These young cells also still have a nucleus, but continue to mature in the bone marrow. During this maturation process, the nucleus disappears, after which the cells enter the bloodstream. Because the mature red blood cells do not have a nucleus, they can no longer divide and they die after about 120 to 180 days.

The main function of a red blood cell is to transport oxygen to the cells. They do this by binding oxygen to a protein called hemoglobin. The more hemoglobin is present in a cell, the more oxygen can be bound and transported by that cell. Because there are no nucleus or other organelles present in the red blood cell, there is more room for hemoglobin and 1 red blood cell can therefore transport more oxygen, which means that in total fewer red blood cells are needed to transport sufficient oxygen than when the red blood cells and core. A second reason is that the smallest blood vessels in the mammalian body, called capillaries, have a very small diameter. Nucleated cells are smaller and more flexible and can more easily pass through these small blood vessels without getting trapped.

Some less evolved vertebrates, such as birds, amphibians or reptiles, do have red blood cells with a nucleus. Relative to their body size, these animal species require less oxygen than mammals, or have other adaptations that allow them to deliver sufficient oxygen to the tissues. For example, birds have lungs that allow them to breathe more efficiently and some amphibians can breathe through their skin.