Red blood cells make up nearly half our body’s blood volume. A healthy human produces about 2 million new red blood cells every minute. But red blood cells aren’t true cells, which we talk about. Of all the cells in the body, red blood cells have not DNA. Why?
When red blood cells are developing — in the marrow of your bones — they do have DNA. They also have a nucleus, mitochondria and ribosomes. But before red blood cells develop fully, they eject most of these typical cell structures — including their DNA.
Red blood cells have one primary job — they transport oxygen from your lungs to your tissues. A red blood cell in the bloodstream isn’t a true cell. It’s just a tiny sack full of the oxygen-binding protein called hemoglobin. Without DNA, red blood cells can’t synthesize new proteins, repair themselves, or divide. Scientists don’t know for sure why they evolved not to perform these functions.
But maybe the body doesn’t want them to. Consider that as a red blood cell ages, its outer membrane gets stiff. The red blood cell can’t squeeze anymore through tiny capillaries. Most cells could repair their own stiff membranes — but red blood cells can’t. Once a red blood cell begins to stick in the capillaries, the body knows to destroy and replace it — thus ensuring that all its red blood cell are in top working order.
Red blood cells don’t live very long — about 120 days — and they are produced in massive quantities. Women have on average about 4.8 million cells per microliter of blood, while men have around 5.4 million. Some Peruvians living at extreme altitudes may have red cells upwards of 8.3 million cells per microliter.
When people get severely anemic, or when they have cancer or infection of the bone marrow, they actually will produce nucleated red blood cells. This is because the bone marrow is pumping out so many new cells that it’s releasing them before they’re fully mature. A reticulocyte count is basically a count of the new red blood cells — in healthy people, its usually around 1%, but in a severely anemic person it can go up to 7 or 8% — an indication that the bone marrow is working serious overtime.
Non-nucleated red blood cells are smaller than nucleated cells, and they have a special shape, like a donut with an indentation instead of a hole. This shape has a high surface-to-volume ratio that’s good for gas exchange, and it’s also very flexible, so it can deform and squeeze through small capillaries.