system
Given the following post, respond as if you're commenting on Reddit:
human
How long does water spend in the human body?

Is there a half-life to it? Given that some will pass right through, but (most?) will be absorbed into the bloodstream etc., there must not be a single answer, but all I can find online is short term answers talking about when that which passes through quickly will leave. What about the water that's more thoroughly absorbed, like that in bone marrow?
gpt
This is such a fun question.

The answer for a specific molecule of water is somewhere between 45 minutes and the total age of your body.

Water is taken up fast in the stomach and small bowel (5 to 15 minutes).  It makes blood more *hypo-osmolar* and the kidneys notice this right away.  They produce more dilute urine.  This equalizes the *osmolarity* of the blood.  It also keeps the water balance in the body constant.

All in all this process takes about 30 minutes, but varies with age. An older person has more sluggish urine output than a young person.

The fun part is some cellular structures in your body are fixed at birth and don't change.  Neurons, heart cells, brain liver and pancreas all contain structures that are essentially locked down since you were a fetus.  This is probably unrealistic for bulk water, but water is part of some of those structures.

A comparative example is ethanol.  It can go wherever water does (mostly).  After drinking booze it takes about 6 minutes for ethanol to reach the brain but realistically about 15 - 45 min.

The following are some rough detection limits for alcohol in various tissues

* Blood: Up to 6 hours

* Breath: 12-24 Hours

* Urine: 12-24 Hours

* Saliva: 12-24 Hours

* Hair: 90 Days

Overall, realistically the oldest water in your body is likely to be in your hair, however, some specific water molecules may be as old as you are.