The question of cell renewal is one that all of us have intuitive daily experience with. We all notice that our hair falls out regularly, yet we don’t get bald (at least not until males reach a certain age!).

Similarly, we have all had the experience of cutting ourselves only to see how new cells replaced their damaged predecessors. And we donate blood or give blood samples without gradually draining our circulatory system.

All of these examples point to a replacement rate of cells, that is characteristic of different tissues and in different conditions, but which makes it abundantly clear that for many cell types renewal is a part of their story.

To be more concrete, our skin cells are known to constantly be shed and then renewed. Red blood cells make their repetitive journey through our bloodstream with a lifetime of about 4 months (BNID 107875, 102526).

We can connect this lifetime to the fact calculated in the vignette on “How many cells are there in an organism?” that there are about 3×1013 red blood cells to infer that about 100 million new red blood cells are being formed in our body every minute!

Replacement of our cells also occurs in most of the other tissues in our body, though the cells in the lenses of our eyes and most neurons of our central nervous system are thought to be special counterexamples. A collection of the replacement rates of different cells in our body is given in Table 1.