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HUMAN · DNA & AGEING

WHY BODIES WEAR DOWN

ACT 01 · TURNOVER

330 billion cells a day — and the ones you never replace

Your body replaces about 330 billion cells every day — some 3.8 million a second, roughly 80 grams — and about 86% of them are red blood cells. But turnover is wildly uneven. The lining of your gut is renewed every few days; the neurons of your cerebral cortex and the core of your eye's lens are as old as you are. We know the long ages because of the Cold War: atmospheric bomb tests in the 1950s and early 60s nearly doubled the carbon-14 in the air, every living thing took it up, and a cell's DNA keeps the level it had on the day the cell was born. Measure it, and you can read the cell's birth year off the curve. Tap a tissue to find it in the body.

BodyParts3D scan, one adult · organs coloured by how long their cells live

sources

Education only, not medical advice. FACT = measured and replicated; THEORY = a mainstream model, or an association whose cause is open; SPECULATION = a hypothesis, or evidence only in animals or tiny uncontrolled studies.

  • Body and organs: BodyParts3D 4.0, © The Database Center for Life Science (DBCLS), CC BY-SA 2.1 Japan — re-shared under the same licence (see /anatomy/CREDIT.txt).
  • Molecules: RCSB Protein Data Bank, CC0 — 1KX5 nucleosome (Davey et al. 2002), 1BNA B-DNA (Drew et al. 1981; its base-pair geometry builds every longer strand here), 1EBM OGG1 with 8-oxoguanine (Bruner, Norman & Verdine 2000), 1W0T TRF1 and 1W0U TRF2 on telomeric DNA (Court et al. 2005), 1XJV POT1 on telomeric single strand (Lei, Podell & Cech 2004), 1BI7 p16INK4a–CDK6 (Russo et al. 1998).
  • Cells, organelles, chromosomes and tissue are drawn from typical measured sizes, not from one specimen.
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