Half a litre leaves you every day, whatever you do about it.

There’s a paper from February that opens by admitting the field hasn’t agreed on something for about seventy-five years.

Not some small thing off in a corner. The question is how your kidneys hold on to water. Which they did last night, and are doing while you read this.

Hi, it’s Lily.

Before the argument, the part that is settled. It’s also the useful part.

Everything your body has to get rid of each day — leftover salt, what’s left of the protein you ate — can only leave dissolved in water. It can’t leave dry. And a kidney can squeeze that water down only so far: about four times more concentrated than blood, and no further. Do the arithmetic on a normal day of eating and it comes to roughly half a litre. That much has to go, whatever else happens.

So there’s a floor under how little water can leave you. And the floor doesn’t move with how much you drink. It moves with how much there is to carry out.

In the upper part there’s a pump. Certain tube walls push salt out into the tissue around them, burning energy to do it, and that’s what makes the ground salty up there. Not in dispute.

Go further down and the pumps stop. In the thin tubes at the deep end, no meaningful active transport of salt has ever been found. Nothing is pushing. And that stretch is the saltiest part of the whole organ. It pulls hardest, at the far end, without the machinery that explains everything above it.

One of the candidates is urea. Waste. The thing a kidney is there to throw out in the first place.

Rather than send all of it away, the kidney lets a share soak back into the deep tissue, where it adds to the pull. Some drifts off into the blood, gets carried around, and is delivered back in. In the February paper the deep gradient is held up by two things fed inward: salt from the ascending tubes, urea from the collecting ducts. Whether that’s the whole answer is exactly what hasn’t been agreed.

That stretch has never had a break. It started before you were born and it hasn’t stopped since, and it kept going last night while three anatomists were still arguing about how.

There’s a stack of books on my studio table I only half follow, and I’ve always taken the not-following to be my own shortfall. Not this time. For the plain description of what kidneys handle across a day, NIDDK sets it out, and none of this argument appears there.