About two million of your red blood cells died while you read that line. About two million were finished in your marrow to replace them. Twenty trillion or so are in circulation, each lasting about four months, and the count barely moves for decades. This is No. 91’s arithmetic wearing marrow — stock equals rate times lifespan: two hundred billion a day, held a hundred and twenty days, gives the twenty trillion actually there. But the checkout line had nobody steering it. This one is steered — and the strangest thing about the steering is that nothing in your body has ever counted a red blood cell.
Be exact about what is standing. No. 12’s bone is remodelled in place — the same skeleton, dissolved and rebuilt around you. No. 67’s photoreceptor renews its own parts, one cell rebuilding its own disc stack from underneath. This is neither: whole cells, entire and separate, made in one organ, spending four months elsewhere, eaten in a third. The population is the pattern, and nothing in it persists at all.
So how is a number this large held by a body that cannot read it?
It is not read. It is inferred, from oxygen. Scattered between the tubules of your kidneys are interstitial fibroblasts — spindly cells that took most of a century to identify, since a filter is not an obvious place to keep a hormone about blood. They tally nothing. They sit in the tissue, respond to the oxygen tension around them, and when it drops they transcribe the gene for erythropoietin.
Why the kidney? The answer is the loveliest engineering in the loop, and it is an accident of plumbing. In most organs, tissue oxygen tension reports how much blood is arriving — useless, if what you need to know is what the blood contains. The kidney is built the other way round. Its oxygen consumption is a consequence of its blood flow: more flow means more filtrate, more filtrate means more sodium to pump back, and pumping sodium is what kidneys spend oxygen on. Supply and demand rise together and cancel. Cortical tissue is therefore nearly deaf to how much blood shows up, and sensitive almost only to how much oxygen each litre carries — which is the red cell mass itself. It also draws off only seven per cent or so of the oxygen delivered to it, idling far from starvation, where small changes are easy to hear. The body did not pick the kidney because it is central. It picked it because it is quiet.
The measurement itself is an enzyme that cannot work without the thing it measures. A family of prolyl hydroxylases tags a protein called HIF-2α for destruction, and to do the tagging they must consume molecular oxygen. Plenty of oxygen, and the protein is destroyed minutes after it is made. Short of oxygen, tagging slows, HIF-2α survives, reaches the nucleus and switches the erythropoietin gene on. No dial, no comparator, no stored target — only a reaction running at the speed its substrate allows. Kaelin, Ratcliffe and Semenza took the 2019 Nobel for working it out.
And the kidney does not turn the volume up. It turns more cells on. Output rises because the number of fibroblasts transcribing the gene rises — silent cells begin, and the smooth hormone curve in the blood is a vote count — near-binary switches recruited one at a time. A population regulated by a population.
What reaches the marrow is not an order to hurry. Erythroid progenitors are built to die; apoptosis is the default, and the marrow overbuilds against it daily. Erythropoietin’s main action is to keep those cells alive — a stay of execution granted to some fraction of a doomed cohort. The loop does not control manufacture. It controls how much of its own overproduction it spares.
Then the natural experiment. In 1890 François-Gilbert Viault carried the question into the Peruvian Andes and found red cell counts far above sea-level values — and, crucially, that the rise was acquired rather than inherited. Push the loop hard enough and it overshoots into chronic mountain sickness, an excess of red cells thick enough to do real harm, afflicting perhaps five to fifteen per cent of the Altiplano population. But native Tibetans, whose ancestors have held that altitude several times longer, carry lower haemoglobin than Andeans, not higher — along with variants in the oxygen-sensing genes themselves. Given long enough, selection did not turn the dial up. It moved the set point down.
Which is the last thing the loop has to teach, and medicine learned it expensively. Recombinant erythropoietin arrived in 1989, transformative for people whose kidneys no longer made their own. Then trials tried to push their counts back to a normal number — and found no survival benefit, and enough signal of cardiovascular harm that warnings were added and prescribing changed. The number was never the target. Oxygen delivery was. The count is only where a very slow loop comes to rest while doing something else, and a level nobody set can still be the wrong thing to aim at.
Twenty trillion cells, not one of which will see the spring, held for a lifetime at a value no organ knows, by a tissue that has never met one of them and is only checking whether it can breathe.
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One loop I’m watching
Next: the same kidneys, doing the other job. Every day they strain about a hundred and eighty litres of water out of your blood — your entire blood volume, thirty-odd times over — then take back some ninety-nine per cent of it, with the salt and sugar worth keeping. It reads as the body’s most wasteful design until you see what the waste buys: throw everything away, and you can adjust anything by declining to take it back. Next time.