# How much of you is really "yours"? The science of cellular renewal
Your body is a river in constant motion. Every second, millions of cells die and are replaced by new ones. In seven years, practically every atom in your body will have been replaced. However, this statistic is profoundly misleading, because different parts of your organism renew at radically different rates.This paradox, known as the Ship of Theseus Paradox, poses an ancient question: if you replace all the parts of something, is it still the same? In your case, it is a literal question. The atoms that make up your body today are not the same ones that were there 10 years ago, but you are still you.# Cellular Turnover: A Map of Your Dynamic Body
The speed of renewal varies dramatically by tissue:| Tissue | Cellular Average Life | Full Renewal | Description |
|---|---|---|---|
| Blood | 120 days | 4 months | Red blood cells are the fastest. Your bone marrow produces 200 billion daily. |
| Skin | 2-4 weeks | 1 month | Extremely fast renewal. You lose ~30,000 skin cells per minute. |
| Bone | 10 years | One decade | The skeleton is more conservative. Still, after 10 years, you will have replaced practically your entire skeleton. |
| Organs | 15 years | 1.5 decades | The liver renews in months. The heart, in years. A mosaic of rhythms. |
| Brain | 80+ years (neurons) | Almost never | Your cortical neurons from birth. But glia (support cells) do renew. |
# The Eye Lens: The Oldest Part of You
There is one structure in your body that is special: the eye lens. The cells that make up the lens are deposited during fetal development and are never replaced. If you live to be 100 years old, the central cells of your lens will still be the same ones you had in your mother's womb. They are, literally, the oldest part of you.# Total Renewal Calculation: The Paradox of Weights
This calculator uses a weighted average of different tissues:- 30% skin and blood: Almost complete renewal in younger years.
- 35% skeleton: Progressive renewal, reaching 100% at 10 years.
- 25% organs: Slower renewal, variable by organ.
- 10% brain: Minimal change in neurons, maximum in support structures.