Nadia, this is a really cool question. So you have many different types of blood cells, and all of them have a limited lifespan. For example, red blood cells live for about 100 days, and then they die and are replaced. Some of your white blood cells actually only live for hours to days. So where do the new ones come from? They come from cells in your bone marrow called stem cells.

Stem cells are very special cells, because they have the ability to turn into any cell type. For blood, it is called a blood-forming stem cell (also, and called a hematopoietic (a big word for blood!) stem cell).

If you looked at this cell, it looks pretty boring. But it starts to divide, and then some of those cells turn into red blood cells, while others turn into white blood cells. So a blood forming stem cell can make all of your blood. OK, so you are sitting at a computer reading this, right? Guess what, you replace about 3. 5 MILLION blood cells every SECOND. So how many blood cells have you made since you started reading this? Lots! And this goes on for your entire life. This is also true of other tissues, you are replacing your skin, and your intestine constantly, and there are skin stem cells and intestinal stem cells which are responsible.

Good question. When red blood cells mature, they lose the nucleus where all the DNA is. Without a nucleus, they can’t divide to make new red blood cells. But there are cells in places like the marrow of bones where stem cells do divide. Stem cells are immature, unspecialized cells. The cells they make mature and specialize into red blood cells and other blood cells. So the short answer is that there are cells that stay immature and divide because mature cells can’t.

Red blood cells usually live about 4 months, so replacement happens all the time. If we lose blood due to an injury, surgery, or donating blood, we make blood faster.

Why do you think red blood cells don’t have nuclei? Here’s a hint, they’re basically bags of hemoglobin. Hemoglobin carries oxygen and carbon dioxide.

[The question seems to assume that a damaged cell cannot be replaced, but I'm not sure why.]

Arguably, a damaged cell is exactly the kind of cell which must be repaired or replaced. Red blood cells are unlike most other cells in the body in that they lack most of the internal structures (such as the nucleus and mitochondria). This means they have no information on how to repair themselves. Instead, the body breaks them down and recycles the components. Researchers recently discovered that old, damaged, or "sick" cells are broken down by specialized cells in the liver. [Note - prior to this, the consensus was that macrophages in the spleen were primarily responsible for breaking down these blood cells. That appears to not be accurate.]

As the number of red blood cells falls, so does the oxygen level in the blood. Other cells in the body detect that drop and release protein molecules that make their way into the bone marrow. Bone marrow produces cells called stem cells. When a specific protein reaches these stem cells, they are induced to develop into red blood cells.

[Additional related information at these two questions on ScienceLine.]

The mammalian red blood cell (RBC) throws away its nucleus and most organelles upon maturation. Therefore they have very limited ability to repair damage.

The RBC has a lifetime about 100 to 120 days. As a RBC ages, its plasma membrane undergoes changes and allow itself to be destroyed by the immune system. This usually occur before the cell suffers enough damage to break apart. For RBC that did break apart before being recycled, the oxygen carrying protein, hemoglobin, is salvaged. In either case, the iron in hemoglobin is recycled.

It takes the stem cell in bone marrow 7 days to create new RBCs.

Bone marrow constantly makes more red blood cells to replace those that are lost.

Our bodies are always making new red blood cells [RBCs]. They only last about 100 days. They're made in the red bone marrow of large bones and are stored in the spleen.